I think I found 2 undocumented string format codes.
My google-fu has failed me in finding info on %h and %l string formatting codes. >>> '%h' %'hello' exceptions.ValueError: incomplete format >>> '%l' %'hello' exceptions.ValueError: incomplete format Does anyone know what doing a "complete format" means? -- http://mail.python.org/mailman/listinfo/python-list
Re: CodeAcademy Python Tip Calculator
Cai Gengyang wrote: So, I’m on section (3. The Tip) … Instructions 1. Set the variable tip to decimal value of 15% on line 5. This was my input: You’re almost there! Assign the tip variable on line 5. meal = 44.50 tax = 6.75 / 100 tip = 15.0 But, when I tried to run the program, I don’t get any output at all. Nada, nothing zilch. Nothing happens, how come ? Not a big enough tip? You must be running the program as a script if no output is being printed to the screen. -- https://mail.python.org/mailman/listinfo/python-list
Re: version of Python to install Windows 7 professional 64 bit, intel core i5
Zubair Shaikh wrote: What version of Python to install Windows 7 professional 64 bit, intel core i5 You're joking, right? What is your budget? -- https://mail.python.org/mailman/listinfo/python-list
Re: The Incredible Growth of Python (stackoverflow.blog)
Paul Rubin wrote: Ben Finney writes: I've never seen one. who has told you... they are working on a Python 3 code base. Just because they've told me about it doesn't mean I saw it personally. The ones I've seen, including new ones, are Python 2. Some people here use Py3 but I haven't heard (or don't remember) enough about what they're working on, to know if those are py3 codebases of any size. If they say yes, I'll take their word for it, but this is a self-selected group of course. That simply isn't true, unless you think it more likely everyone who discusses their Python 3 code base is lying. People discuss Python language issues here a lot, but don't discuss as much about code bases. I know when I install a new OS (currently Debian 9 which was released a month or so ago) and type "python" on the command line, I get Py2. Try typing python3. It's in some distributions of Linux. -- https://mail.python.org/mailman/listinfo/python-list
Re: the core values of the Python "platform"
Steven D'Aprano wrote:
On Wed, 13 Sep 2017 09:08:41 -0400, Darin Gordon wrote:
Bryan Cantrill gave an interesting talk recently at a Node conference
about "platform values" [1].
For those of us who don't have the time or inclination to watch a video,
or who are unable to, could you summarise these platform values?
The talk lead me to think about what the
core values of the Python "platform" are and I thought it would be good
to ask this question of the community. What would you consider the top
(<= 5) core values?
In no particular order:
- The Zen of Python ("import this" at the interactive interpreter).
- "We're all adults here." We tend to be more relaxed about theoretical
errors, and push some of the responsibility for defensive programming
onto the caller, not just the callee. If the caller breaks my function's
contract by providing bad arguments, or messes with my class' internals,
they deserve whatever bad things happen.
- "We're all adults here." The flip side of that is that if I choose to
mess with your private methods or functions, you shouldn't take
extraordinary steps to try to prevent me. I'm an adult, and if I've got
good reason to call your private method (say, I'm debugging, or I'm
simply exploring the capabilities at the interactive interpreter) then I
should be allowed. If I want to shoot myself in the foot, it isn't your
responsibility to prevent me.
- All else being equal, it is better to ask forgiveness than permission.
In general, we prefer try...except and catching exceptions than to look
before you leap. It is often faster, but more importantly, it avoids
"Time Of Check To Time Of Use" bugs.
- Python is in one sense a pure Object Oriented language (all values,
even integers, are objects)
Even classes are objects.
but in another sense Python is a multiparadigm
language. Not everything needs to be a class. Top-level functions are
often better. We can, and do, write code using four of the big five
programming paradigms: procedural, OOP, functional, imperative. And mix
and match them within a single code base.
(Only deductive/logic programming has little support in Python.)
And if I may be allowed a sixth:
- Not every two line function needs to be a built-in.
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Re: String to Dictionary conversion in python
santosh.yelamar...@gmail.com wrote: Hi, Can anyone help me in the below issue. I need to convert string to dictionary string = " 'msisdn': '7382432382', 'action': 'select', 'sessionId': '123', 'recipient': '7382432382', 'language': 'english'" Can anyone help me with the code I'm new to Python too, but it looks like you need to "split" it about the commas. Right? -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Rick Johnson wrote:
I think for most languages an intuitive syntax is not
important -- C is such a language, Lisp is such a language,
Perl is such a language, and there are many more -- but
for Python, intuitiveness is very important.
I guess it depends on what you mean by "important" ("important to the
compiler", is a legitimate concern, for instance). From an intuition
perspective, C++ allowing you to separate the interface of a class from
it's implementation, I would say that C++ has it all over Python from
the point of view of "intuitiveness". It's much easier to tell what's
going on, at a glance, in a C++ program. I am much newer to Python, but
I have provided at least one concrete feature supporting my opinion. By
the way, I am not talking about "toy" programs.
--
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Re: How do I check all variables returned buy the functions exists
Robin Becker wrote: On 16/09/2017 01:58, Steve D'Aprano wrote: If you want to test for None specifically: if any(v is None for v in values): print "at least one value was None" ... for some reason that seems slow on my machine when compared with if None in values: . This does not seem particularly surprising. "None in values" is known as soon as None is found. In "any(v is None for v in values)", "any" probably isn't called until its argument is (fully) known. Of course the results would depend on the implementation. It would be interesting to compare the results if you used the optimize option (it's either -o or -O). Bill C:\usr\share\robin\pythonDoc>python -m timeit -s"values=(1,2,None)" "any(v is None for v in values)" 100 loops, best of 3: 0.62 usec per loop C:\usr\share\robin\pythonDoc>python -m timeit -s"values=(None,2,None)" "any(v is None for v in values)" 100 loops, best of 3: 0.504 usec per loop C:\usr\share\robin\pythonDoc>python -m timeit -s"values=(None,2,None)" "None in values" 1000 loops, best of 3: 0.0309 usec per loop C:\usr\share\robin\pythonDoc>python -m timeit -s"values=(1,2,None)" "None in values" 1000 loops, best of 3: 0.097 usec per loop it also seems a bit less obvious -- https://mail.python.org/mailman/listinfo/python-list
Re: How do I check all variables returned buy the functions exists
Steve D'Aprano wrote: In "any(v is None for v in values)", "any" probably isn't called until its argument is (fully) known. No, its a generator expression, so it provides the values one at a time, as needed. Okay, thank you for setting me straight. I'm only about 2 weeks down this road so far! : ) -- https://mail.python.org/mailman/listinfo/python-list
Re: Fwd: Issues with python commands in windows powershell
Joey Steward wrote: -- Forwarded message -- From: Joey Steward Date: Tue, Sep 19, 2017 at 10:30 PM Subject: Issues with python commands in windows powershell To: python-list@python.org Hello, I've been having issues using basic python commands in windows powershell. Did you try the same ones in an environment besides powershell? -- https://mail.python.org/mailman/listinfo/python-list
Re: errors with json.loads
Interesting problem, John.
I have probably even less experience with json than you do, so I'm
taking this as an opportunity to learn with you.
Suggestions:
1. Try your example with Python 2 rather than Python 3.
2. Take your file and make it into a string literal in your program, and
try calling json.loads with that as an argument.
Share with us what happens!
Good luck,
Bill
john polo wrote:
Greetings,
I am using IPython 6.1.0 with Python 3.6.2 on a Windows 7 machine. I
am not a programmer. I am using a book called Python Data Analytics to
try to learn some of Python. I am at a section for reading and writing
JSON data. The example JSON file is:
Listing 5-13. books.json
[{"writer": "Mark Ross",
"nationality": "USA",
"books": [
{"title": "XML Cookbook", "price": 23.56},
{"title": "Python Fundamentals", "price": 50.70},
{"title": "The NumPy library", "price": 12.30}
]
},
{"writer": "Barbara Bracket",
"nationality": "UK",
"books": [
{"title": "Java Enterprise", "price": 28.60},
{"title": "HTML5", "price": 31.35},
{"title": "Python for Dummies", "price": 28.00}
]
}]
and the example code for reading the file is:
>>> file = open('books.json','r')
>>> text = file.read()
>>> text = json.loads(text)
When I use those 3 lines, I get the following:
JSONDecodeError Traceback (most recent call
last)
in ()
> 1 text = json.loads(text)
c:\users\..\python\python36\lib\json\__init__.py in loads(s, encoding,
cls, object_hook, parse_float, parse_int, parse_constant,
object_pairs_hook, **kw)
352 parse_int is None and parse_float is None and
353 parse_constant is None and object_pairs_hook is
None and not kw):
--> 354 return _default_decoder.decode(s)
355 if cls is None:
356 cls = JSONDecoder
c:\users\..\python\python36\lib\json\decoder.py in decode(self, s, _w)
337
338 """
--> 339 obj, end = self.raw_decode(s, idx=_w(s, 0).end())
340 end = _w(s, end).end()
341 if end != len(s):
c:\users\..\python\python36\lib\json\decoder.py in raw_decode(self, s,
idx)
353 """
354 try:
--> 355 obj, end = self.scan_once(s, idx)
356 except StopIteration as err:
357 raise JSONDecodeError("Expecting value", s,
err.value) from None
JSONDecodeError: Expecting ':' delimiter: line 5 column 50 (char 161)
?json.loads says that the method is for deserializing "s", with "s"
being a string, bytes, or bytearray.
In [24]: type(text)
Out[24]: str
So "text" seems to be a string. Why does json.loads return an error?
John
--
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Re: errors with json.loads
john polo wrote:
Greetings,
I am using IPython 6.1.0 with Python 3.6.2 on a Windows 7 machine. I
am not a programmer. I am using a book called Python Data Analytics to
try to learn some of Python. I am at a section for reading and writing
JSON data. The example JSON file is:
Listing 5-13. books.json
[{"writer": "Mark Ross",
"nationality": "USA",
"books": [
{"title": "XML Cookbook", "price": 23.56},
{"title": "Python Fundamentals", "price": 50.70},
{"title": "The NumPy library", "price": 12.30}
]
},
{"writer": "Barbara Bracket",
"nationality": "UK",
"books": [
{"title": "Java Enterprise", "price": 28.60},
{"title": "HTML5", "price": 31.35},
{"title": "Python for Dummies", "price": 28.00}
]
}]
and the example code for reading the file is:
>>> file = open('books.json','r')
>>> text = file.read()
>>> text = json.loads(text)
Hmm... are you sure you want "text" on the left hand side. I'm not sure
whether this is okay or not. It looks suspicious (but, like I said, I'm
a Python newbie).
--
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Re: errors with json.loads
Stefan Ram wrote: Dennis Lee Bieber writes: After removing all the \xa0 bytes and trying to decode it I get... Apparenty an \xa0 byte corresponds to a "non-breaking space". What sort of white space characters are allowed in a json file ( tabs and newlines?)? Just curious. Bill I did the same here, before I read your post. I got the same results, but did not post them. Someone has posted programs with \xA0 (NBSP IIRC) at the start of lines of the soure here before, in: From: Christopher Reimer Newsgroups: comp.lang.python Subject: Setting property for current class from property in an different class... Date: Wed, 6 Sep 2017 19:24:40 -0700 Message-ID: , and when I removed them, I was able to execute the source from his post from 2017-Sep-06 without errors. So I was feeling to lazy to bother this time. -- https://mail.python.org/mailman/listinfo/python-list
Re: errors with json.loads
Ned Batchelder wrote: On 9/20/17 8:22 PM, Bill wrote: Apparenty an \xa0 byte corresponds to a "non-breaking space". What sort of white space characters are allowed in a json file ( tabs and newlines?)? Just curious. These things can be looked up. From RFC 7159 (https://tools.ietf.org/html/rfc7159): Thank you. So what is most likely the root cause of the original poster's problem (assuming he typed out the text with a text editor)? -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Stefan Ram wrote:
bartc writes:
On 20/09/2017 02:31, Bill wrote:
it's implementation, I would say that C++ has it all over Python from
the point of view of "intuitiveness". It's much easier to tell what's
going on, at a glance, in a C++ program.
You're being serious, aren't you?
For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T( &f )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?
I'll reveal that I'm not Bjarne Stroustrup. decltype was introduced in
C++11, which I haven't explored, and I have never written any code that
resembles your template. But it appears that an object of type S may
behave like a function, which in this case would invoke Example.f(). As
has already been pointed out, one can write "obfuscating code" in any
language, with little effort. I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.
--
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Re: [Tutor] beginning to code
Stefan Ram wrote:
Bill writes:
Stefan Ram wrote:
bartc writes:
On 20/09/2017 02:31, Bill wrote:
it's implementation, I would say that C++ has it all over Python from
the point of view of "intuitiveness". It's much easier to tell what's
going on, at a glance, in a C++ program.
You're being serious, aren't you?
For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T( &f )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?As
has already been pointed out, one can write "obfuscating code" in any
language, with little effort. I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.
From the point of view of a C++ programmer,
Oh, we "all look the same", huh? I know that we've only just met, but I
will caution that you are coming across as something of a "know-it-all".
Bill
the above
is not obfuscated, but it is readable and simple C++.
It is of course not readable for readers who do not know
C++. Just as Python's »string[::-1]« appears "obfuscated"
to readers who don't know Python.
It was the answer to the question "How can I express the
class I'm in in, when I can't write that classes name
literally? So, »S« is »example«.
It works like this: The type of »&f« is »void ( example::*
)()«. So, the function-declaration template »T« infers »C«
to be »example«, and the type of »T( &f )« is »example«,
which then is transferred to the name »S« using typedef.
This is obvious for C++ programmers, but it takes a lot
of time to become a C++ programmer, maybe more than it
takes to become a Python programmer.
--
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Re: [Tutor] beginning to code
Stefan Ram wrote: Just as Python's »string[::-1]« appears "obfuscated" to readers who don't know Python. I understand string[::-1] after only studying python for a day or two (I've only been studying it for 2 weeks at this point). A student could study C++ for a semester or more and not encounter templates until they studied data structures. So in short, I don't believe that the example you chose from Python and the one you chose from C++, were at similar levels (not even close). It was the answer to the question "How can I express the class I'm in in, when I can't write that classes name literally? So, »S« is »example«. It works like this: The type of »&f« is »void ( example::* )()«. So, the function-declaration template »T« infers »C« to be »example«, and the type of »T( &f )« is »example«, which then is transferred to the name »S« using typedef. This is obvious for C++ programmers, but it takes a lot of time to become a C++ programmer, maybe more than it takes to become a Python programmer. -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Stefan Ram wrote:
Bill writes:
I understand string[::-1] after only studying python for a day or two
(I've only been studying it for 2 weeks at this point). A student could
study C++ for a semester or more and not encounter templates until they
studied data structures. So in short, I don't believe that the example
you chose from Python and the one you chose from C++, were at similar
levels (not even close).
I was responding to your assertion:
|It's much easier to tell what's going on, at a glance, in a
|C++ program.
and I showed a simple counterexample.
But it was *not* a simple counterexample.
And above, now, it seems that you /agree/ that one can learn
Python in a short time, but needs a lot longer to learn C++.
Learning to develop code, in either language, involves much more than
"learning C++" or "learning Python". I have been reading Beazley's
"Essential Reference", and I would say that Python is definitely a
bigger, and more complicated language than C++. In some aspects it has
simpler syntax. But consider all of the ways that you can pass
arguments to a function, for instance. There are definitely alot more
options than in C/C++. I like the way that both of these languages
(unlike Java) allow you to stick with the procedural paradigm if you
wish to (which I think is a natural way for a true beginner to start).
From my perspective, Python's simplicity lies in the fact that an
entire program does not have to be recompiled if a module is changed.
Since I was programming in C (and Fortran) before you were born, it's
syntax does not generally pose a hindrance to me.
BTW: templates have little to do with data structures. One can
show resonable examples for function templates that do not use
any data structure:
You are sounding like a "know-it-all" again. I am familiar with such
examples. One could code in C++ for a long time without a definitive
need for templates. It just depends on the application. By trade, I am
more of a mathematician than a programmer but sometimes my needs and/or
interests overlap.
template< typename T >
T maximum( T const a, T const b ){ return a > b ? a : b; }
. The corresponding Python def would be (untested):
def maximum( a, b ): return a if a > b else b
, that is, because Python is not statically typed, one does
not need a template for a corresponding definition in Python.
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Re: [Tutor] beginning to code
Stefan Ram wrote: Bill writes: "Essential Reference", and I would say that Python is definitely a bigger, and more complicated language than C++. In some aspects it has simpler syntax. But consider all of the ways that you can pass arguments to a function, for instance. There are definitely alot more options than in C/C++. I like the way that both of these languages In C++, you can pass: I used the word *arguments*, not parameters. FWIW, *const* seems to correspond to immutable, pretty well. I find Python to be more more like Java, with regard to "passing objects by reference". - by value (with a const or non-const parameter) (where the value can be a pointer or a non-pointer or a pointer-to-member or a unique_ptr or a shared_ptr) - by reference - by const reference - by rvalue reference - by universal reference - by name (in macros) - you can overload functions with the same name but different numbers of parameter or different types for their parameters - you can write templates for functions with ranges of types for a parameter - default values can be declared for parameters In Python, you pass - by value The section 6.3.4 on calls "in The Python Language Reference, Release 3.6.0" encompasses 1½ pages. You are sounding like a "know-it-all" again. I am familiar with such examples. One could code in C++ for a long time without a definitive need for templates. Ok, so if we exclude from C++ everything that you have not learned yet, than, what is left over, is "much easier" for you to read "at a glance". It seems that you want C++ to be adjusted to your knowledge of it and your assertions about it. I think that one should instead adjust one's assertions to the reality. The Python Language Reference, Release 3.6.0 146 pages latest ISO-C++ draft (without the library part) 464 pages Bear in mind that you don't have to know every feature of a language to make good use of it, unless, perhaps, that is your job. I don't believe reading the latest "ISO-C++ draft" would be a good use of my time; it sounds like a document geared more to compiler designers. I don't even try to memorize . If I need a function, I search for it. There is a reason that C++, Java and Python all coexist. It might be helpful for you to try to appreciate that. Regards, Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Steve D'Aprano wrote: On Fri, 22 Sep 2017 02:57 pm, Bill wrote: I find Python to be more more like Java, with regard to "passing objects by reference". Which is not a surprise, since both Python and Java use the same value passing style: pass by object reference, or pass by sharing if you prefer. Java people don't call it that. They call it pass by value, and categorically deny that it is pass by reference. (They're right about the second point.) I figure that, internally, an address, a pointer, is being passed by value to implement pass by reference. Why do you say "they are right" above? Are you saying it's not pass by reference? What really screws the terminology up is that it's not parameters that are being passed, it's arguments! %-) It also the exact same passing style as used by Swift, except Swift calls it pass by reference, and denies it is pass by value. (They're right about the second point.) -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Stefan Ram wrote:
Bill writes:
Stefan Ram wrote:
bartc writes:
On 20/09/2017 02:31, Bill wrote:
it's implementation, I would say that C++ has it all over Python from
the point of view of "intuitiveness". It's much easier to tell what's
going on, at a glance, in a C++ program.
You're being serious, aren't you?
For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T( &f )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?As
has already been pointed out, one can write "obfuscating code" in any
language, with little effort. I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.
From the point of view of a C++ programmer, the above
is not obfuscated, but it is readable and simple C++.
It is of course not readable for readers who do not know
C++. Just as Python's »string[::-1]« appears "obfuscated"
to readers who don't know Python.
It was the answer to the question "How can I express the
class I'm in in, when I can't write that classes name
literally?
I would try to use virtual cast in place of the *&%, I mean code, you
wrote. "Clever code" is a losing game--just look at your explanation
below. Simple==Good.
So, »S« is »example«.
It works like this: The type of »&f« is »void ( example::*
)()«. So, the function-declaration template »T« infers »C«
to be »example«, and the type of »T( &f )« is »example«,
which then is transferred to the name »S« using typedef.
This is obvious for C++ programmers, but it takes a lot
of time to become a C++ programmer, maybe more than it
takes to become a Python programmer.
--
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Re: [Tutor] beginning to code
Bill wrote:
Stefan Ram wrote:
Bill writes:
Stefan Ram wrote:
bartc writes:
On 20/09/2017 02:31, Bill wrote:
it's implementation, I would say that C++ has it all over Python
from
the point of view of "intuitiveness". It's much easier to tell
what's
going on, at a glance, in a C++ program.
You're being serious, aren't you?
For one example, this is a part of a C++ program:
template< typename C >C T( void ( C::* )() );
. It defines a template T, that can be used in a
class as follows:
struct example { void f(); typedef decltype( T( &f )) S; };
. The type »S« now has a certain property, that can
be useful sometimes. What is this property (asking Bill)?As
has already been pointed out, one can write "obfuscating code" in any
language, with little effort. I strive to write code which is easily
understandable--and I document it. I don't wish to debate whether I
could make more of a mess in Python, or not.
From the point of view of a C++ programmer, the above
is not obfuscated, but it is readable and simple C++.
It is of course not readable for readers who do not know
C++. Just as Python's »string[::-1]« appears "obfuscated"
to readers who don't know Python.
It was the answer to the question "How can I express the
class I'm in in, when I can't write that classes name
literally?
I would try to use virtual cast in place of the *&%, I mean code, you
wrote.
Sorry, I mean dynamic_cast().
"Clever code" is a losing game--just look at your explanation below.
Simple==Good.
So, »S« is »example«.
It works like this: The type of »&f« is »void ( example::*
)()«. So, the function-declaration template »T« infers »C«
to be »example«, and the type of »T( &f )« is »example«,
which then is transferred to the name »S« using typedef.
This is obvious for C++ programmers, but it takes a lot
of time to become a C++ programmer, maybe more than it
takes to become a Python programmer.
--
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Re: [Tutor] beginning to code
Marko Rauhamaa wrote:
Bill :
I figure that, internally, an address, a pointer, is being passed by
value to implement pass by reference. Why do you say "they are right"
above? Are you saying it's not pass by reference?
Thank you for your examples. I studied them carefully (and I'm not
through with them yet).
I just wanted to mention that my comment was made in the context that
Python is implemented by an interpreter written in C. I realize that
this may not always be the case. However, I haven't heard anyone
mention a Python interpreter written in Python yet.
"Pass by reference" could be "pass by reference to object" (Python,
Java, JavaScript, Lisp) or "pass by reference to memory slot" (available
to Pascal and C++).
Memory slots (or lvalues, as they are known in C) are not first class
objects in Python, which makes "pass by reference to memory slot" a bit
tricky in Python. Python *could* add memory slots to its sanctioned
collection of object types, and it *could* add special syntax to express
a memory slot reference and dereference ("&" and "*" in C).
However, Python doesn't need any language changes to implement memory
slots. A memory slot could be defined as any object that implements
"get()" and "set(value)" methods:
C: &x
Python:
class Xref:
def get(self): return x
def set(self, value): nonlocal x; x = value
ref_x = Xref()
C: &x->y[3]
Python:
class XY3ref:
def get(self): return x.y[3]
def set(self, value): x.y[3] = value
ref_xy3 = XY3ref()
The examples could be simplified:
ref_x = slot_ref(locals(), "x")
ref_xy3 = slot_ref(x.y, 3)
by defining:
def slot_ref(dict_or_array, key_or_index):
class SlotRef:
def get(self): return dict_or_array[key_or_index]
def set(self, value): dict_or_array[key_or_index] = value
return SlotRef()
Marko
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Re: [Tutor] beginning to code
Mark Lawrence wrote:
On 22/09/2017 08:01, Bill wrote:
Steve D'Aprano wrote:
On Fri, 22 Sep 2017 02:57 pm, Bill wrote:
I find Python to be more more
like Java, with regard to "passing objects by reference".
Which is not a surprise, since both Python and Java use the same
value passing
style: pass by object reference, or pass by sharing if you prefer.
Java people don't call it that. They call it pass by value, and
categorically
deny that it is pass by reference. (They're right about the second
point.)
I figure that, internally, an address, a pointer, is being passed by
value to implement pass by reference. Why do you say "they are
right" above? Are you saying it's not pass by reference?
Please see
http://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/
and http://effbot.org/zone/call-by-object.htm
I would would agree with the description provided for the C++ example
provided
string some_guy = "fred";
is replaced by
char* some_guy="fred";
To see that this is correct, note the some_guy may subsequently be
assigned to a character string much longer then "fred". An additional
note: A character string literal, like "cat", never occurs more than
once in compiled C++ program unit. This also shows that the provided
description can't be completely correct. One last thing,
string some_guy = "fred"
is really the same thing as
string some_guy("fred");
and both equivalently call the string constructor.
The data type of "fred" is const char*, not (class) string.
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Re: [Tutor] beginning to code
Bill wrote:
Mark Lawrence wrote:
On 22/09/2017 08:01, Bill wrote:
Steve D'Aprano wrote:
On Fri, 22 Sep 2017 02:57 pm, Bill wrote:
I find Python to be more more
like Java, with regard to "passing objects by reference".
Which is not a surprise, since both Python and Java use the same
value passing
style: pass by object reference, or pass by sharing if you prefer.
Java people don't call it that. They call it pass by value, and
categorically
deny that it is pass by reference. (They're right about the second
point.)
I figure that, internally, an address, a pointer, is being passed by
value to implement pass by reference. Why do you say "they are
right" above? Are you saying it's not pass by reference?
Please see
http://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/
and http://effbot.org/zone/call-by-object.htm
I would would agree with the description provided for the C++ example
provided
string some_guy = "fred";
is replaced by
char* some_guy="fred";
On second thought, so that the description is correct (matches the
semantics), replace it by
char some_guy[10]="fred";
But then you need to use std::strcpy to reassign some_guy
to "george".
To see that this is correct, note the some_guy may subsequently be
assigned to a character string much longer then "fred". An additional
note: A character string literal, like "cat", never occurs more than
once in compiled C++ program unit. This also shows that the provided
description can't be completely correct. One last thing,
string some_guy = "fred"
is really the same thing as
string some_guy("fred");
and both equivalently call the string constructor.
The data type of "fred" is const char*, not (class) string.
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Re: Python Boolean Logic
Cai Gengyang wrote:
Hey guys, I'm testing this on CodeAcademy, but I cant get the program to output
a result even after pressing the run button. Just wanted to check if my logic
is correct. Thanks alot
Your answers appear correct, but you could write Python statements to
test them (or any you encounter in the future). For instance,
if (20 - 10) > 15 :
print("true")
else:
print("false");
Or,
s='(20 - 10) > 15'
b=(20 - 10) > 15
print(s, " is ", ("true" if b else "false") ); ## inside parentheses
may be removed.
I am new to Python. Maybe someone here is familiar with an elegant way
to get the the value of b directly from the string s? Hmm... It appears
that eval() would work (see "Python: Essential Reference", p. 115). I
just read about that for the first time last night! I may try that, for
practice, after I post this.
Bill
# Assign True or False as appropriate on the lines below!
# (20 - 10) > 15
bool_one = False# We did this one for you!
# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about 43 million.
bool_two = False
# 1**2 <= -1
bool_three = False
# 40 * 4 >= -4
bool_four = True
# 100 != 10**2
bool_five = False
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Re: Python Boolean Logic
Bill wrote:
Cai Gengyang wrote:
Hey guys, I'm testing this on CodeAcademy, but I cant get the program
to output a result even after pressing the run button. Just wanted to
check if my logic is correct. Thanks alot
Your answers appear correct, but you could write Python statements to
test them (or any you encounter in the future). For instance,
if (20 - 10) > 15 :
print("true")
else:
print("false");
Or,
s='(20 - 10) > 15'
b=(20 - 10) > 15
print(s, " is ", ("true" if b else "false") ); ## inside parentheses
may be removed.
I am new to Python. Maybe someone here is familiar with an elegant
way to get the the value of b directly from the string s? Hmm... It
appears that eval() would work (see "Python: Essential Reference", p.
115). I just read about that for the first time last night! I may try
that, for practice, after I post this.
Update: Yes,
b=(20 - 10) > 15
may be replaced by eval(s).
We can write:
print(s, " is ", ("true" if eval(s) else "false") )
# Assign True or False as appropriate on the lines below!
# (20 - 10) > 15
bool_one = False# We did this one for you!
# (10 + 17) == 3**16
# Remember that ** can be read as 'to the power of'. 3**16 is about
43 million.
bool_two = False
# 1**2 <= -1
bool_three = False
# 40 * 4 >= -4
bool_four = True
# 100 != 10**2
bool_five = False
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Re: Python Boolean Logic
Steve D'Aprano wrote:
On Sat, 23 Sep 2017 03:01 pm, Bill wrote:
s='(20 - 10) > 15'
b=(20 - 10) > 15
print(s, " is ", ("true" if b else "false") ); ## inside parentheses
may be removed.
I am new to Python. Maybe someone here is familiar with an elegant way
to get the the value of b directly from the string s? Hmm... It appears
that eval() would work
Indeed it will, but don't get into the habit of using eval willy-nilly. While it
is absolutely fine to use it with data you provide yourself, it is a HUGE
security risk to eval strings that came from an untrusted user.
eval("__import__('os').system('echo """rm-rf /"""')")
Thank you. Studying that was a nice little lesson in itself! I
recognize that this technique can be used for 'good' as well as 'evil'! : )
Bill
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Re: search and replace first amount of strings instances with one thing and a second amount of instances with another thing-
validationma...@gmail.com wrote:
i have a code in python to search and replace what i need though is to replace
the first say 10 instances of the number 1 with 2 and the second 10 instances
with the number 3. anybody knows how to do that?
Do you mean the (integer) number 1 or the character '1'? For instance,
if the first line of the file is:
"There were 100 cats in the yard."
Do you want to change this to
"There were 200 cats in the yard."?
Remember that string objects are "immutable" so your code probably
wouldn't work exactly like that.
fin = open(r'F:\1\xxx.txt')
fout = open(r'F:\1\xxx2.txt', "wt")
for line in fin:
fout.write( line.replace('1', '2') )
fin.close()
fout.close()
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Re: search and replace first amount of strings instances with one thing and a second amount of instances with another thing-
Bill wrote:
validationma...@gmail.com wrote:
i have a code in python to search and replace what i need though is
to replace the first say 10 instances of the number 1 with 2 and the
second 10 instances with the number 3. anybody knows how to do that?
Do you mean the (integer) number 1 or the character '1'? For instance,
if the first line of the file is:
"There were 100 cats in the yard."
Do you want to change this to
"There were 200 cats in the yard."?
Remember that string objects are "immutable" so your code probably
wouldn't work exactly like that.
I now realize that my last line does not apply. I suggest you start by
just thinking about changing the first ten ones to twos. Once you do
that, you should have little trouble finishing the job. Being a Python
newbie myself, your example helped motivate me to try my own example. I
used "type" to learn the data type of line (in your example), about
which I was curious. My first concern was that is might only be
"interable" object, but I learned that more is true.
fin = open(r'F:\1\xxx.txt')
fout = open(r'F:\1\xxx2.txt', "wt")
for line in fin:
fout.write( line.replace('1', '2') )
fin.close()
fout.close()
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Re: Python Boolean Logic
Chris Warrick wrote:
This outputs "False is false", because you used the variable in your
expression. You can just do this:
print("s is", s)
This will print "s is False".
Ah, good point! But I do like "self-documenting" output (so I don't
mind seeing s)---if you had 5 or more statements a in a row like that,
you would "miss" seeing the string s! : )
Bill
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Re: [Tutor] beginning to code
Stephan Houben wrote: Op 2017-09-23, Rick Johnson schreef : These pissing contests over how values are passed in Python are totally irrelevant. What does it matter? Nothing will be gained or lost by arguing over which is true, or not. Unless the distinction is preventing you from doing something that you'd like to do, or unless you want to argue that one "value passing method" would bring X, Y or Z benefits over the other, what does it matter? Amen. All one has to do, I think, is consider (1) that passing objects by "making copies" of them, would be prohibitively expensive and consider that something else has to happen as an alternative, and (2) understand that in Python, objects don't have names, they have references (which have names). The rest could be "implementation dependent" (no?) To be amusing, how did the chicken pass an egg to the_other_side_of_the_road(e)? Could the egg get crushed (stay tuned)? -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Chris Angelico wrote: On Sun, Sep 24, 2017 at 8:18 AM, Bill wrote: Stephan Houben wrote: Op 2017-09-23, Rick Johnson schreef : These pissing contests over how values are passed in Python are totally irrelevant. What does it matter? Nothing will be gained or lost by arguing over which is true, or not. Unless the distinction is preventing you from doing something that you'd like to do, or unless you want to argue that one "value passing method" would bring X, Y or Z benefits over the other, what does it matter? Amen. All one has to do, I think, is consider (1) that passing objects by "making copies" of them, would be prohibitively expensive and consider that something else has to happen as an alternative, and (2) understand that in Python, objects don't have names, they have references (which have names). The rest could be "implementation dependent" (no?) To be amusing, how did the chicken pass an egg to the_other_side_of_the_road(e)? Could the egg get crushed (stay tuned)? Actually they don't "have" references in any real sense of possession. I agree (I was a bit hasty in my choice of words); but if they didn't "have" these references, it would be difficult, though not impossible, to refer to them. Also keep in mind that the garbage collector keeps track, generally, of how many there are for each object! So from the gc's point of view, objects (definitely) "have" references. Next, what will Egg.__del__() do? : ) An object "has" things like its type, its attributes, etc etc; if you have a reference to an object, you can query it for its type. But you can't ask an object if there's a reference to it over here or there. (Yes, I know that sys.getrefcount exists in CPython, but this isn't part of the language's definition. Also, even that is just a counter - you can't find specific references.) An object may have a reference to other objects (eg a list's contents), but it's a one-way thing - there's no way to find all the references to this object. So more accurate would be to say that objects don't have names, but names refer to objects. When you assign to a simple name, you cause that name to refer to the object you gave it. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: [Tutor] beginning to code
Steve D'Aprano wrote: On Sun, 24 Sep 2017 08:18 am, Bill wrote: All one has to do, I think, is consider (1) that passing objects by "making copies" of them, would be prohibitively expensive Swift passes certain values (but not others!) by value and makes a copy. That includes many potentially large data types including strings, dictionaries and arrays, but using copy-on-write so the data isn't physically copied until you actually mutate it. From the Swift documentation: The description above refers to the “copying” of strings, arrays, and dictionaries. The behavior you see in your code will always be as if a copy took place. However, Swift only performs an actual copy behind the scenes when it is absolutely necessary to do so. Swift manages all value copying to ensure optimal performance, and you should not avoid assignment to try to preempt this optimization. https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/ClassesAndStructures.html So I maintain that one could design a language similar to Python except that objects are assigned and passed by value, making copies only when actually needed using copy-on-write. Swift is *almost* that language: the difference is that Swift distinguishes between "structs" that are copied, and "objects" which are not. and consider that something else has to happen as an alternative, and (2) understand that in Python, objects don't have names, they have references (which have names). The rest could be "implementation dependent" (no?) No. There are many different alternatives for "something else", and while some of them may be behave the same in some circumstances, they do not behave the same in all circumstances. Any interpreter calling itself Python would be expected to match the behaviour of the reference implementation, CPython. For example, if I made "Pass-By-Reference Python" where all argument passing was done by reference, my language would differ from real Python: function(x, y) # allowed function(namespace.x, module.y) # allowed function(x + 1, 2) # FORBIDDEN: can't pass expressions or constants This would be okay as long as x + 1 evaluates to an object, no? Obviously that's not Python! On the other hand, "Pass-By-Name Python" would allow passing expressions and constants, but will differ in other ways. Assignment by reference would mean that name binding was an *alias* operation: module.y = 1 x = module.y # x is an alias for the name "module.y" x = 2 # binds 2 to module.y assert module.y == 2 -- https://mail.python.org/mailman/listinfo/python-list
Re: Call by binding [was Re: [Tutor] beginning to code]
Chris Angelico wrote: On Tue, Sep 26, 2017 at 5:35 AM, Marko Rauhamaa wrote: Chris Angelico : On Tue, Sep 26, 2017 at 12:26 AM, Marko Rauhamaa wrote: Sorry, that was my bad in the terminology. But where do you get that all Python expressions evaluate to pointers? What do they evaluate to if not pointers? Anton's "identities" would work, too. "Address" would do, as well. I have previously proposed the term "leash." Call it "link" or "handle" or "arrow" if you want to. The term used isn't important. What's important is to understand that each expression and subexpression produces and operates on pointers. They evaluate to objects. Not to pointers to objects. Not to references to objects. To objects. Expressions evaluate to actual objects, and when you assign "name = value", you bind the name to the object that value evaluates to. ChrisA And when you pass a reference r to a function, a copy of the reference is passed. So even if you reassign *that* copy of r to refer to another object, upon return, r still still refers to whatever it did originally. ::: case closed :::, I think. -- https://mail.python.org/mailman/listinfo/python-list
Re: Call by binding [was Re: [Tutor] beginning to code]
Tim Golden wrote: On 25/09/2017 20:40, Marko Rauhamaa wrote: Rhodri James : On 25/09/17 15:26, Marko Rauhamaa wrote: That's not what I said. I said all expressions *evaluate to* pointers. This may well be true in particular implementations, but it is an implementation detail so Chris' point still stands. Another implementation could evaluate expressions to indices (as BCPL used to treat its globals), pieces of string or cheese, who knows? Those are all pointers. A pointer is something that points to a data object. (Slight sigh). Can I step in as a list owner and ask the people who are so very fond of debating this topic again and again: please let it drop. Threads such as this can remain interesting when the discussion generates a better understanding of the subject matter as the contributors are forced to articulate their positions leading to some kind of convergence of understanding. However, even if this were the only iteration of this debate, it would be clear by now that there's no chance of a simple "I see what you mean" moment. And this is far from the only iteration, even in the past few months. I feel for your need to demonstrate the validity of your respective positions. But it has moved on from being an enlightening debate and into the realms of mere tedium. Thanks. TJG Perhaps it would be better to amass examples and let them speak for themselves? -- https://mail.python.org/mailman/listinfo/python-list
Re: Aliasing [was Re: [Tutor] beginning to code]
Steve D'Aprano wrote:
On Wed, 27 Sep 2017 02:03 am, Stefan Ram wrote:
Steve D'Aprano writes:
On Tue, 26 Sep 2017 03:26 am, Antoon Pardon wrote:
at that moment, but it still needed correction. If the assignment is
an alias operator then after the statements
Here's some C++ code that demonstrates it. Apologies in advance if it isn't
the most idiomatic C++ code.
In C++, assignments and initializations are different
concepts.
int& b = a; // reference variable or alias
This is an initialization, not an assignment.
A pedantic difference that makes no difference to my argument.
I see that you ignored the later assignment:
b = 2;
which also assigned to a. *That** is the fundamental point: b is certainly an
alias for a, and assigning to b assigns to a.
That's how aliases work in C++. That's how var parameters in Pascal work, and
out parameters in Ada. That is what it means to say that "b is an alias to a".
b is another name for the *variable* a, not just whatever value a happens to
hold now.
I say that assignment in Python is NOT an aliasing operation. Antoon claims I'm
wrong, and his evidence is:
a = []
b = a # Antoon says this is an alias operation
b.append(1)
assert a == [1]
But that's not enough for the variable b to be an alias for the variable a.
Antoon is correct that a and b are two different names for the same list, but
the two variables are not aliases to each other because assignments to b do not
affect a, and vice versa.
A good test for aliasing is to take the source code and mechanically replace
every occurrence of the alias (in the same scope of course) with the original,
or vice versa, and see whether the meaning of the code changes.
In C++, apart from the initial binding:
int& b = a;
("initialisation") you could now randomly swap a for b or b for a and the
meaning of the code will not change.
But in Python, if we try the same trick, the code *does* change:
a = 1
b = a
b = 2
*is not* the same as:
a = 1
b = a
a = 2
(1) In Pascal, Ada, C++ etc using a reference variable (or var parameter, or
whatever terminology they use) makes the two names aliases to EACH OTHER, not
to the value they are bound to.
(2) In Python, Javascript, Ruby etc assignment can give you two names for the
same object, but the names do not alias each other.
The critical distinction here is whether the names refer to each other:
a <---> b
or whether they merely refer to the same value:
a ---> [ value ] <--- b
Python uses the second model. Var parameters in Pascal and references in C++ use
the first. Since the term "aliasing" is well-established for the first, using
it in Python *without making the difference clear* is wrong.
That is a very nice argument! : )
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Spacing conventions
Ever since I download the MyCharm IDE a few days ago, I've been noticing all sort of "spacing conventions (from PEP) that are suggested. How do folks regard these in general? For instance, the conventions suggest that if x>y : pass should be written if x > y: pass Personally, I like seeing a space before the colon (:). And then in my_list = [ i for i in range(0, 10) ] it complains about my extra space inside of the brackets. If you are teaching beginning students, do you expect them to try to follow these sorts of conventions? Is it perfectly fine to let "taste" guide you (I'm just trying to get a feel for the philosophy here)? I also notice "break" and exception handling is used much more in Python than in C++, for instance. I was taught "break" and "continue" led to "unstructured code"--but that was a while back. I can still see their use causing potential trouble in (really-long) real-world code. Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Spacing conventions
Thank you for all of the feedback provided! It was just what I was looking for. : ) I'm going to go back and read some of the links more carefully. Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Spacing conventions
Steve D'Aprano wrote: Similarly for break and continue. I can still see their use causing potential trouble in (really-long) real-world code. How so? Besides, if your code is "really long", you probably should factorise it into smaller, meaningful chunks. I worked in maintenance programming. You got the hand you were dealt! And you weren't allowed to "improve" the code unless the customer contracted you to do so. I maintained for-loops (containing for-loops)... hundreds of lines long. Would you be searching for break or continue? : ) -- https://mail.python.org/mailman/listinfo/python-list
Re: Beginners and experts (Batchelder blog post)
Paul Moore wrote: On 27 September 2017 at 17:41, leam hall wrote: Hehe...I've been trying to figure out how to phrase a question. Knowing I'm not the only one who gets frustrated really helps. I'm trying to learn to be a programmer. I can look at a book and read basic code in a few languages but it would be unfair to hire myself out as a programmer. I'm just not yet worth what it costs to pay my bills. You're already ahead of the game in wanting to be useful, rather than just knowing the jargon :-) However, I've always found that the biggest asset a programmer can have is the simple willingness to learn. I basically agree with what has been posted. I just wanted to mention a couple things that separates beginners and non-beginners. One is "how long it takes to identify and fix an error"--even a syntax error. And that is a skill which is acquired with some practice, maybe more "some" than anyone likes. Another critical skill is the ability to write good documentation--from program requirements, on down. Another is to know what is means to "test". Another is to have some familiarity with the UML. Skills in 3 of these 4 area might be assisted by reading about software engineering. So after you have those skills, then, perhaps, you can think about "interviewing"--of course a degree will help. As always, your mileage may vary... It IS True that you don't have to wait until you "know everything"--most of use will never get there. -- https://mail.python.org/mailman/listinfo/python-list
Re: Beginners and experts (Batchelder blog post)
Chris Angelico wrote: On Fri, Sep 29, 2017 at 5:45 AM, Bill wrote: Paul Moore wrote: On 27 September 2017 at 17:41, leam hall wrote: Hehe...I've been trying to figure out how to phrase a question. Knowing I'm not the only one who gets frustrated really helps. I'm trying to learn to be a programmer. I can look at a book and read basic code in a few languages but it would be unfair to hire myself out as a programmer. I'm just not yet worth what it costs to pay my bills. You're already ahead of the game in wanting to be useful, rather than just knowing the jargon :-) However, I've always found that the biggest asset a programmer can have is the simple willingness to learn. I basically agree with what has been posted. I just wanted to mention a couple things that separates beginners and non-beginners. One is "how long it takes to identify and fix an error"--even a syntax error. And that is a skill which is acquired with some practice, maybe more "some" than anyone likes. Be careful with this one. For anything other than trivial errors (and even for some trivial errors), finding the bug is basically searching through a problem space of all things that could potentially cause this symptom. A novice could accidentally stumble onto the right solution to a tricky bug, or an expert could search a thousand other things and only get to the true cause after a long time. some "expert"! ; ) So while you're partly correct in saying "how long", you can't just put someone on the clock and say "if you find the bug in less than five minutes, you're hired". Ultimately, the only person who can truly evaluate a programmer's skill is another programmer, usually by watching the candidate go through this sort of debugging work. But yeah, broadly speaking, an experienced programmer can usually debug something more quickly than a novice can. On average. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Beginners and experts (Batchelder blog post)
Chris Angelico wrote: On Fri, Sep 29, 2017 at 6:59 AM, Bill wrote: Chris Angelico wrote: Be careful with this one. For anything other than trivial errors (and even for some trivial errors), finding the bug is basically searching through a problem space of all things that could potentially cause this symptom. A novice could accidentally stumble onto the right solution to a tricky bug, or an expert could search a thousand other things and only get to the true cause after a long time. some "expert"! ; ) Yep. Pick anyone on this list that you believe is an expert, and ask him/her for a story of a long debug session that ended up finding a tiny problem. I can pretty much guarantee that every expert programmer will have multiple such experiences, and it's just a matter of remembering one with enough detail to share the story. ChrisA I won't claim to be any sort of "expert". But one memorable problem, for me, was ultimately accounted for by the "inherent problem" of the floating point variables x0 and xo coexisting in the same module. It's sort of funny if you think about it just right. FWIW, my job was to fix the problem, I didn't create it! Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Spacing conventions
Steve D'Aprano wrote: On Thu, 28 Sep 2017 03:56 pm, Bill wrote: I worked in maintenance programming. You got the hand you were dealt! And you weren't allowed to "improve" the code unless the customer contracted you to do so. How do you tell the difference between a bug fix and an code improvement, if the behaviour of the program remains the same? They ran the Unix command "diff" to double-check that nothing unaccounted for got added to the source code as they did new builds. The "rule" was you could "fix" the code if you were already there, in the immediate vicinity. In retrospect, this certainly helped to keep "human error" from sneaking in. I maintained for-loops (containing for-loops)... hundreds of lines long. Would you be searching for break or continue? : ) It depends on whether it was relevant to the bug or not. I've never felt the urge to search for some arbitrary keyword when debugging. Why search for break or continue? How do you know the bug has anything to do with one of them? I would start with narrowing down where the bug occurs. Hopefully the stacktrace or error gives a line number. Then work backwards from there. I certainly wouldn't say "Oh, the code crashed on line 587. I'll do a quick search for the closest break statement and start working from there." What do you do? -- https://mail.python.org/mailman/listinfo/python-list
Re: Beginners and experts (Batchelder blog post)
Steve D'Aprano wrote: (say). Reading error messages is a skill that must be learned, even in Python. Let alone (say) gcc error messages, which are baroque to an extreme. The other day I was getting an error like: /tmp/ccchKJVU.o: In function `__static_initialization_and_destruction_0(int, int)': foo.cpp:(.text+0x7c): undefined reference to `std::ios_base::Init::Init()' foo.cpp:(.text+0x91): undefined reference to `std::ios_base::Init::~Init()' collect2: error: ld returned 1 exit status Yes, that's the sort of character-building that I was referring to (that a beginner needs to learn!)They have to learn that if it "breaks", then there must be a simpler way to break it! Hopefully one which will satisfy Log_2 (n).: ) -- https://mail.python.org/mailman/listinfo/python-list
Re: Refactoring
Stefan Ram wrote: The customer pays for the solution. The software manufacturer does the refactoring for it's own sake, because when it's a longer running project, the refactorings will pay for themself. The customer owns the source code (at least where I was). YMMV -- https://mail.python.org/mailman/listinfo/python-list
Re: Beginners and experts (Batchelder blog post)
Chris Angelico wrote: On Sat, Sep 30, 2017 at 2:42 AM, Steve D'Aprano wrote: Oh, and I'd like to make a (moderate) defense of a kind of "bug fixing by random perturbation". Obviously making unrelated, arbitrary changes to code is bad. But making non-arbitrary but not fully understood changes to relevant code sections can be useful in (at least) two scenarios. (1) I know there's a bug in a specific chunk of code, but I'm having trouble working out where. When everything else fails, if I perturb the code a bit (reorder lines, calculate things in a different order, rename variables, etc) it may change the nature of the bug enough for me to understand what's happening. That's not *random* or *arbitrary* changes, but they are changes not directed at any specific outcome other than "make the code a bit different, and see if the error changes". I'd like to say it is the debugging technique of last resort, except its perhaps not quite as *last* resort as I'd like, especially in code I'm not familiar with. Its an experiment, but not really "carefully designed". I'll write for the possible benefit of any beginners who may be reading. I guess by definition, if one still has a "bug" it's because one doesn't quite understand what the code is doing. And I would say you should lose your license if you "fix something", and don't understand why it works (within reason of course--some mystery's of library functions should probably remain so forever). So ADT (Any Damn Thing--I just made that up that acronym) you can do to understand your code better is fair game! : )In fact, in my experience, the sooner you start getting a little bit angry, the sooner you'll get to the heart of matter. Usually, what looks like a long route, isn't, in the end. Don't be afraid to write *really descriptive* output statements, and do so even though you "don't need to". Besides for making you more productive, it will help soothe you : ) Beginners almost never need to... I think that getting out of the beginner phase requires developing a certain amount of humility. Just wait 5 or 10 years, any look back, and see if what I've written isn't more true than false. The only part I am unsure of is whether you are supposed to get a little big angry or not (YMMV). I find 2 cups of coffee about right. That is, 2 before and 2 after lunch. Of course, that does not include "meetings". -- https://mail.python.org/mailman/listinfo/python-list
newb question about @property
I spent a few hours experimenting with @property. To my mind it seems like it would be preferable to just define (override) instance methods __get__(), __set__(), and possibly __del__(), as desired, as I could easily provide them with "ideal" customization. Am I overlooking something? Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Ned Batchelder wrote: On 9/30/17 5:47 PM, Bill wrote: I spent a few hours experimenting with @property. To my mind it seems like it would be preferable to just define (override) instance methods __get__(), __set__(), and possibly __del__(), as desired, as I could easily provide them with "ideal" customization. Am I overlooking something? It would be easier to comment if you showed the two options. One with @property, and one with __get__ etc. A downside to __get__ is that you need to create a class with those methods, and then instantiate that class as an attribute in your real class, whereas @property can be used without a lot of rigamarole. --Ned. I am basically mimmicking what I see at (the very bottom of) this page: https://www.programiz.com/python-programming/property Thanks, Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Steve D'Aprano wrote: On Sun, 1 Oct 2017 08:47 am, Bill wrote: I spent a few hours experimenting with @property. To my mind it seems like it would be preferable to just define (override) instance methods __get__(), __set__(), and possibly __del__(), as desired, as I could easily provide them with "ideal" customization. Am I overlooking something? Probably. You and Ned are both right. Up until a few minutes ago, I wasn't thinking about a class having more than 1 attribute that I wanted to change. And now I realize that __get__ doesn't really make sense in that context (in the back of my mind was the notion that @property defined __get__, __set__ and __del__) and I allowed that to obscure my vision. I was on the verge of giving up anything to do with computers, forever. : ) BTW, your example (below) is very nice! I may have seen something similar before, but I am starting to appreciate it better now. I think all of this would have made a bit more sense (to me), if instead of just "@property", the syntax was "@property.getter". Now I am forced to ask the question, why did they use the underscore (on temperature) in the example on the bottom of this page? Is one forced to introduce new identifiers in order to define a setter? https://www.programiz.com/python-programming/property Thanks! -Bill This is a particularly simple example, with only getters. How would you write it by overriding __get__? class Circle(object): def __init__(self, centre, radius): self.centre = centre self.radius = radius @property def diameter(self): return 2*self.radius @property def area(self): return pi*self.radius**2 @property def circumference(self): return pi*self.diameter -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Ned Batchelder wrote:
On 9/30/17 7:18 PM, Bill wrote:
Ned Batchelder wrote:
On 9/30/17 5:47 PM, Bill wrote:
I spent a few hours experimenting with @property. To my mind it
seems like it would be preferable to just define (override)
instance methods __get__(), __set__(), and possibly __del__(), as
desired, as I could easily provide them with "ideal" customization.
Am I overlooking something?
It would be easier to comment if you showed the two options. One
with @property, and one with __get__ etc.
A downside to __get__ is that you need to create a class with those
methods, and then instantiate that class as an attribute in your
real class, whereas @property can be used without a lot of rigamarole.
--Ned.
I am basically mimmicking what I see at (the very bottom of) this page:
https://www.programiz.com/python-programming/property
Can you show us the code you are using it to mimic that?
--Ned.
Here it is, Ned. It's my first attempt at using classes in Python.
I still have to learn how to incorporate datetime appropriately! :)
import datetime
# object oriented example class Employee(object):
''' This class will abstract an employee. Class date members name, a
string birthday, a date object address, a string position It also has a
static data member for the number of employees. ''' num_employees =0 # class data item @classmethod def get_num_employees(cls):
return Employee.num_employees
def __init__(self, name, birthdate, address, position):
Employee.num_employees +=1 self.name = name
self.birthdate = birthdate
self.address = address
self.position = position
@property def address(self):
print("**Hi from address-getter**")
return self._address
@address.setter def address(self, value):
print("*Hi, from address setter()!")
self._address = value
def __del__(self):
print("*** Hi, from __del__()")
##Employee.num_employees -= 1 def __str__(self):
return 'Name: {}, Born: {} \nAddress: {} \nPosition: {} \n'.\
format(self.name,self.birthdate,self.address,self.position)
class SalesPerson(Employee):
def __init__(self, name, bdate, addr):
super().__init__(name, bdate, addr,"Salesperson")
def main():
emp1 = Employee("Sam","4/30/1970","212 Elm","Programmer")
emp2 = SalesPerson("Gene","5/1/79","414 Maple")
## Note: learn to use datetime.date--> str print(emp1)
print(emp2)
emp1.address ="230 Main Street" # call setter? print(emp1)
del(emp1)
print("Number of employees", Employee.get_num_employees())
print('*'*30)
main()#call main()
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Re: newb question about @property
Steve D'Aprano wrote:
Circle
didn't use any setters, but I could have let you set the diameter, which in
turn would set the radius:
circle.radius = 2
assert circle.diameter == 4
circle.diameter == 2 # requires a setter
assert circle.radius == 1
Getting that to work is left as an exercise :-)
I may start that exercise in a few minutes!
But most commonly, computed attributes need to store some data aside, somewhere.
You could use a global variable, or write it to a file, or stick it in a list.
All of these things have obvious problems, so the most sensible approach it to
stick the data in a private attribute.
The interpreter doesn't enforce notions of private/public when it comes to
Python classes, but there's a very strong convention that anything starting
with a single underscore is private.
[1] Technically, the interpreter knows nothing about properties. What it cares
about is *descriptors*. Properties are just one kind of descriptor, as are
methods. But I'm intentionally not talking about the gory details of
descriptors. Feel free to ask if you care, but honestly, you don't need to care
unless you are writing your own descriptor class.
Thank you, and everyone else who has contributed to this thread, for
helping me. Each contribution I read helped me to get further ahead!
I watched an example on YouTube where someone wrote a simple descriptor
("@Time_it) to output the amount of time that it took ordinary functions
to complete.To be honest, I AM interested in descriptors. I may
reexamine whether David Beazley has more to say about them in his book
"Python: Essential Reference", which I have been reading. Obviously, I
still have some gaps in my understanding after my first reading.
If you were going to show non-Python users, say science undergraduates
and faculty, that Python is an interesting tool (in 45 minutes), would
one delve into descriptors? I am thinking maybe. Here is what I am
thinking at this moment: trivial applications (probably), list
comprehensions (definitely), generators (maybe briefly). Whatever I
would discuss, I think ending with descriptors could be a strong finish.
But I'm certainly not merely interested for the sake of my talk, I
obtain some satisfaction in learning how things work. If you can
suggest any references for descriptors which you think are good, I would
be interested.
Thanks,
Bill
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Re: newb question about @property
Steve D'Aprano wrote: [1] Technically, the interpreter knows nothing about properties. What it cares about is *descriptors*. Properties are just one kind of descriptor, as are methods. But I'm intentionally not talking about the gory details of descriptors. Feel free to ask if you care, but honestly, you don't need to care unless you are writing your own descriptor class. I found the following page to be a very easily accessible discussion about descriptors (it represents the state of my knowledge about descriptors). You got more? : ) https://www.programiz.com/python-programming/decorator -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Steve D'Aprano wrote: On Sun, 1 Oct 2017 05:46 pm, Bill wrote: If you were going to show non-Python users, say science undergraduates and faculty, that Python is an interesting tool (in 45 minutes), would one delve into descriptors? Hell no :-) Oops, I see I used the word "descriptor", where I meant "decorator" (at least newb is still in the subject line). I don't even know what a descriptor is yet--I know a bit more about meta-classes! %-) So, on your list, I'm basically up to the leading edge of (5), writing decorators. But my previous programming experience helped me to blast through (1) to (4). Going forward, it appears the walkway will be a little steeper. I do appreciate your list as it improves my perspective. From the point of view of getting others to be interested, I'm not sure *classes* and object oriented design/thinking do it. I think they are more of an acquired taste... Functions, on the other hand, are easy to like, I think--especially in Python. I think there's a hierarchy of difficulty/complexity/mind-bogglingness in Python. From least complex to most: - Using Python in an imperative fashion, as in simple scripts. - Writing your own functions. - Writing your own classes. - Writing generators. - Using decorators, including property. - Writing your own decorators. - Writing your own descriptors. - Writing your own metaclasses (a.k.a. "the killer joke"). I wouldn't touch the last three in a beginner's class, not unless they already had a significant amount of programming experience. I am thinking maybe. Here is what I am thinking at this moment: trivial applications (probably), list comprehensions (definitely), generators (maybe briefly). Whatever I would discuss, I think ending with descriptors could be a strong finish. That depends on whether your aim is to confuse them or not :-) I don't think the descriptor protocol is something you'll be able to explain in five or ten minutes. *Using* descriptors like property, sure, that's fine. But I'm certainly not merely interested for the sake of my talk, I obtain some satisfaction in learning how things work. If you can suggest any references for descriptors which you think are good, I would be interested. The definitive explanation of descriptors is here: https://docs.python.org/3/howto/descriptor.html Thanks! Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Stephan Houben wrote:
Op 2017-10-01, Bill schreef :
I watched an example on YouTube where someone wrote a simple descriptor
("@Time_it) to output the amount of time that it took ordinary functions
to complete.To be honest, I AM interested in descriptors.
Are you sure you are not confusing deSCRIPTtors and deCORAtors here?
Yet, you are absolutely correct! Thank you for clarifying! From your
description, I can see that it was *decorators*, which drew my
interest. It appears that *property* is perhaps both a decorator and a
descriptor, at least when used as in the examples we have been
discussing. According to the language grammar, which all newbys like me
should have handy (remember my ADT acronym from another thread?),
decorators can be applied to a classdef, a funcdef, or a async_funcdef
(the latter I assume is a "callback" function definition). Surely the
difference in syntax between funcdef and async_funcdef will be revealed
to me by looking closer at the grammar! : )
Bill
@Time_it
is decorator syntax.
Despite the similarity in the words, there are totally different things.
Descriptors are objects with __get__, and optionally __set__ and
__delete__ methods (i.e. they implement the descriptor protocols).
Decorators aren't really an official type, but loosely speaking these
are any functions which can be applied meaningfully with a single
function or class as argument. Some very mundane functions can be
(ab)used as decorators.
In [1]: @repr
...: def hello():
...: pass
...:
In [2]: hello
Out[2]: ''
Stephan
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Re: newb question about @property
Steve D'Aprano wrote: The definitive explanation of descriptors is here: https://docs.python.org/3/howto/descriptor.html Thank you! It is next on my list. Then I'll try that Circle problem you mentioned as an exercise last night! I don't expect run into any difficulties. : ) -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Steve D'Aprano wrote:
Circle didn't use any setters, but I could have let you set the
diameter, which in
turn would set the radius:
circle.radius = 2
assert circle.diameter == 4
circle.diameter == 2 # requires a setter
assert circle.radius == 1
Getting that to work is left as an exercise :-)
It WAS a good exercise!! I was concerned about "infinite recursion"
between my two property setters.. Thanks! Next? :)
Bill
import math
class Circle(object):
""" Define a circle class with radius and diameter""" def __init__(self,
radius):
self.radius = radius
self.diameter =2*radius
@property def radius(self):
return self._radius
@radius.setter def radius(self, value):
self._radius = value
self._diameter=2*value
@property def diameter(self):
return self._diameter
@diameter.setter def diameter(self, value):
self._diameter = value
self._radius = value /2 @property def area(self):
return math.pi *self.radius **2 @property def circumference(self):
return 2 * math.pi *self.radius
## Start here ## circle = Circle(1 / math.pi)
print("Area = {:.2f}".format(circle.area))
print("Circumference = {:.2f}".format(circle.circumference))
circle.radius =2 assert circle.diameter ==4 print("Area =
{:.2f}".format(circle.area))
print("Circumference = {:.2f}".format(circle.circumference))
circle.diameter =2 # requires a setter assert circle.radius ==1 print("Area =
{:.2f}".format(circle.area))
print("Circumference = {:.2f}".format(circle.circumference))
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Re: newb question about @property
Chris Angelico wrote: On Tue, Oct 3, 2017 at 6:23 AM, Larry Hudson via Python-list wrote: On 10/01/2017 03:52 PM, Bill wrote: Steve D'Aprano wrote: The definitive explanation of descriptors is here: https://docs.python.org/3/howto/descriptor.html Thank you! It is next on my list. Then I'll try that Circle problem you mentioned as an exercise last night! I don't expect run into any difficulties. : ) Except perhaps for your sense of time... "I'll try" implies the future, "last night" is the past.:-) :-) (Couldn't resist...) Yes, but "you mentioned" implies the past. I think you have an operator precedence issue. Kappa ChrisA Reading the "definitive explanation of descriptors" took me longer than expected.. I am as overly optimistic as any programming person.. ;) Can you inspire me with a good decorator problem (standard homework exercise-level will be fine)? Otherwise I will go create one which will prints a row of asterisks before and after the call to the original function (which I think I should do). But maybe you have something more interesting? Or maybe you can refer me to a good source of Python problems, so I can bug you less? Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Steve D'Aprano wrote: There's no need to set the radius and the diameter, as one is completely derived from the other Good point; I'm glad I submitted my code for grading. Sort of a "trick question" to ask me to add diameter and then take off points because I used it! ; ) Bill and the transformation is cheap enough to perform on the fly as needed. Consider what happens if, due to a bug or an accident, you end up with a Circle instance that says the radius is 5 and the diameter is 20. They can't *both* be right, and -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Chris Angelico wrote: Decorators are fairly straight-forward if you understand higher-order functions. ChrisA I was just minding my own business, and thought to write my first decorator for a simple *recursive* function f. The decorator WORKS if f does not make a call to itself.Otherwise, f seems to have "difficulty" calling itself (I get a typerror, f(n) has value "NoneType"). What is the explanation for this? Does f have a new name because it has a decorator on it now? Note: I am not using functools.wraps since I don't yet understand the reason I might do that yet (first things first... ). Thanks! Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: newb question about @property
Bill wrote:
Chris Angelico wrote:
Decorators are fairly straight-forward if you understand higher-order
functions.
ChrisA
I was just minding my own business, and thought to write my first
decorator for a simple *recursive* function f. The decorator WORKS if
f does not make a call to itself
(it really wasn't).
Using the (PyCharm) debugger, I determined that my inner function that
was calling my wrapped (Fibonacci sequence) function but wasn't
returning anything to the invoking environment. I fixed it for the sake
of a good, if costly, lesson. Not much of a wrapper, but it "works". A
version which merely prints a tab before the function call, instead of a
row of asterisks produces output which is more interesting to look at.
def wrap(func):
def inner(*args, **kwargs):
print('*'*20)
a= func(*args, **kwargs)
print(a)
print('*'*20)
return a
return inner
Bill
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Re: newb question about @property
Steve D'Aprano wrote:
On Tue, 3 Oct 2017 06:51 am, Bill wrote:
Can you inspire me with a good decorator problem (standard homework
exercise-level will be fine)?
Here is a nice even dozen problems for you. Please ask for clarification if any
are unclear.
Thank you for sharing the problems on decorators! I just finished
reading Bruce Eckels' note on decorators (as well as some of the
comments left by readers), which you shared a link to, and found the
whole discussion very informative. If I was to point to two details, the
analogy of decorator's with macros is helpful to bear in mind, as is the
remark that "@Wrapper(def f..)" reassigns f to the composition. With the
latter unstated, the matter can cause confusion! %-)
Bill
(1) Write a decorator which simply prints a descriptive message and the name of
the decorated function once, when the function is first decorated.
E.g. if you write:
@decorate
def spam(x):
return x + 1 # for example
print(spam(1))
print(spam(2))
Python should print:
Decorating function spam.
2
3
Note: "spam" must not be hard-coded, it must be taken from the function being
decorated. (Hint: all functions have their name available as func.__name__.)
(2) Modify the decorator from (1) so that calling the wrapped function also
print a descriptive message such as "Calling function spam". The expected
output will be:
Decorating function spam.
Calling function spam.
2
Calling function spam.
3
(3) Write a decorator that checks that the decorated function's first argument
is a non-empty string, raising an appropriate exception if it is not, and lets
through any other arguments unchanged.
(4) Same as above, except the first argument is automatically stripped of
leading and trailing whitespace and forced to uppercase.
(5) Write a decorator which injects the argument 10 into the list of arguments
received by the wrapped function. E.g. if you write:
@inject
def add(a, b):
return a + b
@inject
def sub(a, b):
return a - b
print(add(5), sub(5))
Python should print "15 5". (And *not* "15 -5".)
(6) [ADVANCED] Modify the decorator in (5) so that it takes an argument telling
it what value to inject into the list of arguments:
@inject(99)
def sub(a, b):
return a - b
print(sub(5))
will now print "94".
(7) Write a decorator which checks the decorated function's two arguments are
given smallest first, swapping them around if needed.
(8) Write a decorator which prints the name of the wrapped function, its
arguments, and the time, each time the wrapped function is called.
(9) [ADVANCED] Modify the decorator from (8) to take an argument specifying the
path to a file, and use the logging module to log the details to that file
instead of printing them.
(10) Write a decorator which adds an "cache" attribute initialised to an empty
dictionary to the decorated function.
(11) Write a decorator which wraps a class (not function!), and adds a "help"
method to the class which prints a message as shown below. For example:
@addhelp
class Spam:
pass
@addhelp
class Eggs:
pass
x = Spam()
x.help()
y = Eggs()
y.help()
will print:
See http://example.com/Spam
See http://example.com/Eggs
(Hint: classes also have a __name__ attribute.)
(12) [ADVANCED] Write a decorator which wraps a class, and applies the decorator
from (10) above to each non-dunder¹ method in the class. That is, after:
@addcaches
class MyClass:
def foo(self):
pass
def bar(self):
pass
print(MyClass.foo.cache, MyClass.bar.cache)
should print "{} {}".
¹ Remember that dunder methods are those that start with two leading and
trailing underscores: "Double UNDERscore" methods.
* * *
Bruce Eckel has an excellent introduction to Python decorators, from way back
when they were first introduced in 2008. His introduction is notable because:
- he points out explicitly that Python decorators are not the same as
the Decorator design pattern (I thought they were!);
- he recommends using a class as the decorator, and building the extra
functionality in object oriented fashion, rather than functional
programming fashion (this may give an easier introduction to those
who aren't familiar with functional idioms);
- and he correctly predicted that the introduction of the @ syntactic
sugar would have a big impact on the way people think about Python
code.
http://www.artima.com/weblogs/viewpost.jsp?thread=240808
Feel free to read his post before trying the problems I set.
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Re: The "loop and a half"
Stefan Ram wrote:
Is this the best way to write a "loop and a half" in Python?
Is your goal brevity or clarity, or something else (for instance, what
does the code written by the other members of your "team" look
like--woudn't it be nice if it matched)?
Bill
x = 1
while x:
x = int( input( "Number (enter 0 to terminate)? " ))
if x:
print( f'Square = { x**2 }' )
In a C-like language, one could write:
while x = int( input( "Number (enter 0 to terminate)? " ))
print( f'Square = { x**2 }' )
.
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Re: Python community "welcoming" feedback
Leam Hall wrote: A while back I pointed out some challenges for the Python community's intake of new coders. Mostly focusing on IRC and the Python e-mail list. What is the Python e-mail list? Thanks, Bill Several people have stepped up their "welcome" game and I've been very impressed with the way things are going. Great job! Leam -- https://mail.python.org/mailman/listinfo/python-list
Re: Creating a Dictionary
Stefan Ram wrote:
One might wish to implement a small language with these commands:
Explain why. What is the advantage?
F - move forward
B - move backward
L - larger stepsize
S - smaller stepsize
. One could start with the following pseudocode for a dictionary:
{ 'F': lambda: myturtle.forward( s ),
'B': lambda: myturtle.backward( s ),
'L': lambda: global s; s *= 2,
'S': lambda: global s; s /= 2 }
. But lambda-expressions cannot contain statements.
In real Python one could write something like (untested):
def f():
myturtle.forward( s )
def b():
myturtle.backward( s )
def l():
global siz
size *= 2
def s():
global siz
size /= 2
{ 'F': f,
'B': b,
'L': l,
'S': s }
. Is this more readable or less readable?
Any other suggestions?
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Re: Introducing the "for" loop
Stefan Ram wrote:
"ROGER GRAYDON CHRISTMAN" writes:
On Wed, Oct 4, 2017 22:42 Stefan Ram (r...@zedat.fu-berlin.de) wrote:
Steve D'Aprano writes:
So, "bottom-up" in this case means: iterators should be
taught before for-loops.
Why?
The easy answer here is to not use the range in the first for loop.
I never intended to use »range«. But I also will not use lists.
Very early in the course, I teach numeric and string literals:
1, 2.3, 'abc'
then come operators, functions and »if«, »while« and »try«.
But neither »range« nor lists have been shown so far.
As long as I have two teachers here, which textbooks are you using? I am
hoping to teach a college course in Python next fall.
Thanks,
Bill
The basic course may already and there after about 12 - 18 hours.
(This time includes many exercises in the classroom.)
But if I have time to introduce »for«, I'll do it as follows
at this point in the course:
We want to walk through (traverse) a string
character-by-character:
To do this we need a walker. A walker can be
obtained using »iter«:
|>>> walker = iter( 'abc' )
Now, we can get character after character from the walker
using »next« (transcript simplified):
|>>> next( walker )
|'a'
|>>> next( walker )
|'b'
|>>> next( walker )
|'c'
|>>> next( walker )
|StopIteration
We can use »while« to automate this:
def example():
walker = iter( 'abc' )
try:
while True:
print( next( walker ))
except StopIteration:
pass
A walker also is known as an /iterator/.
An object, one can get an iterator from
is called an /iterable object/.
Strings are iterable objects.
This for-loop does just what our previous
while-loop did:
def example():
for ch in 'abc':
print( ch )
It gets an iterator from »'abc'« and then does the suite
repeatedly with »ch« being the result of a each »next«
call until StopIteration.
No »range«, no list.
(Yes, »print('\n'.join('abc'))« can do the same and will
be shown later in the course if there is still time.)
--
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Re: Lies in education [was Re: The "loop and a half"]
Marko Rauhamaa wrote: That's good advice, but it's not all that dangerous to express off-topic statements in this newsgroup. It may not be "dangerous", but I find it a little annoying. I wasn't going to say anything, but now you are bringing it up explicitly. -- https://mail.python.org/mailman/listinfo/python-list
Re: why does memory consumption keep growing?
Fetchinson . wrote: Hi folks, I have a rather simple program which cycles through a bunch of files, does some operation on them, and then quits. There are 500 files involved and each operation takes about 5-10 MB of memory. As you'll see I tried to make every attempt at removing everything at the end of each cycle so that memory consumption doesn't grow as the for loop progresses, but it still does. import os for f in os.listdir( '.' ): x = [ ] for ( i, line ) in enumerate( open( f ) ): import mystuff x.append( mystuff.expensive_stuff( line ) ) del mystuff import mystuff mystuff.some_more_expensive_stuff( x ) del mystuff del x What can be the reason? I understand that mystuff might be leaky, but if I delete it, doesn't that mean that whatever memory was allocated is freed? Similary x is deleted so that can't possibly make the memory consumption go up. Any hint would be much appreciated, Daniel Try calling the garbage collector explicitly. -- https://mail.python.org/mailman/listinfo/python-list
Re: Lies in education [was Re: The "loop and a half"]
Rhodri James wrote: On 09/10/17 20:06, Stefan Ram wrote: r...@zedat.fu-berlin.de (Stefan Ram) writes: Steve D'Aprano writes: At various stages of education, we teach many lies-to-children, including: Many of those lies can be perfectly true in some sense. I pick some examples: Another noun phrase with "lie" is "white lie". In his book about programming, Bjarne Stroustrup writes: |We try hard to avoid "white lies"; that is, we refrain from |oversimplified explanations that are clear and easy to |understand, but not true in the context of real languages and |real problems. That would go a long way to explaining why I tried and failed to learn C++ three times from Stroustrup's books. He is surely one of the best authors in computer science (at least based upon "The C++ Programming Language", 3rd ed.). He indicates that he assumes that the reader has some experience developing serious software. Beazley's, "Python: Essential Reference" is one of the best books I've seen since. It doesn't have the same depth, but it (too) provides articulate answers, head-on. Both books have 5-star rankings on Amazon.com. That doesn't mean that either of them is right for everybody. Come back to Stroustrup's book "after" you learn C++ somewhere else, and maybe you'll enjoy it more. Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Lies in education [was Re: The "loop and a half"]
Steve D'Aprano wrote:
On Tue, 10 Oct 2017 06:06 am, Stefan Ram wrote:
In his book about programming, Bjarne Stroustrup writes:
|We try hard to avoid "white lies"; that is, we refrain from
|oversimplified explanations that are clear and easy to
|understand, but not true in the context of real languages and
|real problems.
Bjarne Stroustrup is famous for designing one of the most heavyweight,
baraque, hard-to-understand, difficult-to-use programming languages in common
use. While C++ has many excellent features, and is constrained by the need to
be compatible with C, I don't think many people believe that it is a
well-designed language.
It is a well-designed language. It is and was carefully thought out.
One could argue that there are perhaps "too many ways" to do a given
thing in Python (one could say it's features are "not orthogonal"). I'm
sure you are familiar with where the language drew its name. I'm not
here to "cast stones", I like Python. I just think that you shouldn't
cast stones at C/C++. People started programming in C in the late 70's,
and before that some were programming in B ("B Programming Language"),
if I recall correctly. Python generally runs "further ways from the
hardware" than these other languages, and in some sense, it still would,
*even if* you were able to statically compile it to machine language. To
me if feels like Python runs like an application. I don't wish to
debate this as I have other needs this week. But I felt compelled to try
to explain why maybe you shouldn't be casting stones at C/C++.
One thing you didn't bring up at all, is that the audiences for the
languages appears to be different. You still need folks who can encode
data structures and write device drivers, from scratch. And "woe" if
you need performance, such as applications involving AI.
Cheers,
Bill
But even if it were the best language in the world, and Stroustrup the
greatest language designer in the history of computing, what makes you think
that he knows anything about teaching?
--
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Re: Lies in education [was Re: The "loop and a half"]
Dennis Lee Bieber wrote:
On Tue, 10 Oct 2017 20:48:26 -0400, Bill
declaimed the following:
cast stones at C/C++. People started programming in C in the late 70's,
and before that some were programming in B ("B Programming Language"),
Preceded by BCPL (which leads to the joke that the language that
supplants C will be named P)
PL-I has already been taken. That is a pretty free-wheeling language
compared to Fortran, where, IIRC, you had to start in the 7th column.
Oops, hand me another punch card... :)
--
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Re: Lies in education [was Re: The "loop and a half"]
Grant Edwards wrote: On 2017-10-11, Bill wrote: [...] I'm not here to "cast stones", I like Python. I just think that you shouldn't cast stones at C/C++. Not while PHP exists. There aren't enough stones in the world... PHP seems (seemed?) popular for laying out web pages. Are their vastly superior options? I'm a little naive in this matter, thus my question. -- https://mail.python.org/mailman/listinfo/python-list
Re: Lies in education [was Re: The "loop and a half"]
Mikhail V wrote: [...] I'm not here to "cast stones", I like Python. I just think that you shouldn't cast stones at C/C++. Not while PHP exists. There aren't enough stones in the world... PHP seems (seemed?) popular for laying out web pages. Are their vastly superior options? Python? Superior syntax for sure I believe that. What accounts for the popularity of PHP then? -- https://mail.python.org/mailman/listinfo/python-list
Re: Lies in education [was Re: The "loop and a half"]
Grant Edwards wrote: On 2017-10-11, Bill wrote: Grant Edwards wrote: On 2017-10-11, Bill wrote: [...] I'm not here to "cast stones", I like Python. I just think that you shouldn't cast stones at C/C++. Not while PHP exists. There aren't enough stones in the world... PHP seems (seemed?) popular for laying out web pages. It's not really used for "laying out" web pages. Thats what HTML and CSS do. PHP is for server-side generation of dynamic content and handling of submitted forms and uploaded data. Thank you. I DO appreciate learning about the correct terminology. -- https://mail.python.org/mailman/listinfo/python-list
Re: Lies in education [was Re: The "loop and a half"]
Marko Rauhamaa wrote: Grant Edwards : I like [const qualifiers] in C because it allows the linker to place them in ROM with the code. It also _sometimes_ provides useful diagnostics when you pass a pointer to something which shouldn't be modified to something that is going to try to modify it. Unfortunately, "const" is so tacky in practice that the language and the standard libraries have rendered it useless. One example is the surprising fact that string literals in C are "char *" and not "const char *". If not, you couldn't pass a string literal to a function having prototype void f(char *s); Of course, if f tries to modify *s, there will be a run time error as character string literals are stored in a "special place", and are not mutable. This improves performance. -- https://mail.python.org/mailman/listinfo/python-list
Re: I used list, def. why li += [100,200] , and li = li + [100,200] is different
Rob Gaddi wrote: On 10/23/2017 09:29 AM, 임현준 wrote: I am a Korean student, and I am a beginner in English and Python.;( I can't understand about this def If I want to print [1,2,3,4,5] [1,2,3,4,5,100,200] I will make code like this, and I can understand code. def modify(li): li += [100,200] list = [1,2,3,4,5] print(list) modify(list) print(list) BUT, when I make code like this. I will make code like this, and I can understand code. def modify(li): li = li + [100,200] list = [1,2,3,4,5] print(list) modify(list) print(list) python print [1,2,3,4,5] [1,2,3,4,5] why 'li+= [100,200]'and 'li = li + [100,200]' 's print is different please help me Lists are mutable, they can be changed. Your call to += is equivalent to a call to li.extend([100, 200]), which changes the single existing object pointed to by the "li" reference. The second time, however, you take the existing value that "li" refers to [1,2,3,4,5], create a new object that is ([1,2,3,4,5] + [100,200]), and reassign the local reference "li" to point to that new object. Then your function ends, "li" goes out of scope, nothing points to that newly created object and it gets lost. The problem and both solutions are great! Thanks for posting! Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Let's talk about debuggers!
Fabien wrote: On 10/25/2017 03:07 PM, Thomas Jollans wrote: What options are there for Python (that work)? PyCharm's debugger is fine (also available in the community edition) +1 Cheers, Fabien -- https://mail.python.org/mailman/listinfo/python-list
Re: Getting started with python
subhendu.pand...@gmail.com wrote: Hi, Could you please help me with the below if possible: Possible and reasonable are two different things. Why don't you try some web searches and try to answer some of your own questions. I offer this advice as a Python newbe myself. Bill 1. Best site to go ahead for python. 2. How python is different from other languages and future scope of it. 3. Tasks that are getting done using python in present. 4. Link where I can be able to get python videos, ebooks from basics to expert level free. Thanks Subhendu -- https://mail.python.org/mailman/listinfo/python-list
Re: Easiest way to access C module in Python
John Pote wrote: Hi all, I have successfully used Python to perform unit and integration tests in the past and I'd like to do the same for some C modules I'm working with at work. There seem to be a number of ways of doing this but being busy at work and home I looking for the approach with the least learning curve. I don't want to add the C modules into the CPython build itself as I've never done this and at work it's a real pain getting admin rights to do this, and when you do it lasts just 24 hours. The C modules are likely to change frequently as bugs are found and features added. The other option I'm considering is to use sockets and write a C wrapper round the C modules I want to test. This has the advantage for me that I know about sockets from Python & C points of view and I get complete control of the C compile process. This may be important as the C modules come from an embedded project and I don't want to change them in any way. Are there any other approachs to this problem? I'll be using Python 3.5 (work) and 3.6 (home). Feedback appriciated. Install Oracle's "Virtual Box" software on your computer. It's free. Then install a version of Linux on it (or something else, if you prefer). Whatever you do, if you don't like it, you can replace your installation in a matter of minutes. -- https://mail.python.org/mailman/listinfo/python-list
Re: Python homework
I think carelessness in choosing variable names may be at the root of the problem. nick.martin...@aol.com wrote: I have a question on my homework. My homework is to write a program in which the computer simulates the rolling of a die 50 times and then prints (i). the most frequent side of the die (ii). the average die value of all rolls. I wrote the program so it says the most frequent number out of all the rolls for example (12,4,6,14,10,4) and will print out "14" instead of 4 like I need. This is what I have so far: import random def rollDie(number): rolls = [0] * 6 for i in range(0, number): roll=int(random.randint(1,6)) rolls[roll - 1] += 1 return rolls if __name__ == "__main__": result = rollDie(50) print (result) print(max(result)) -- https://mail.python.org/mailman/listinfo/python-list
Re: Processing Game Help
Lauren Porter wrote:
Hello all! I've been trying to create a game in Python Processing where a
spaceship moves horizontally in order to miss a collision with an asteroid. I'm
having difficulty making it so that the game quits when an asteroid hits the
spaceship, could anybody help? Here is my code.
The code looks rather "naked" with any sort of documentation.
Do you want to ask us to figure out how it is supposed to work in order
to help you?
My debugging suggestion is "print, print, print!" so you can follow what
your code is doing.
I find that the sooner I start doing that, the sooner my debugging
session is over. Good luck!
Bill
As you can see, I tried using globals in order use variables from previous
classes, but nothing has worked. For this specific approach, no error message
popped up, but nothing happened either. Thanks so much!
COLORS = {"black":"#00", "white":"#FF",
"red":"#FF", "green":"#00FF00",
"blue":"#D1F5FF", "yellow":"#00",
"orange":"#FFA500", "hendrixorange":"#F58025",
"purple":"#9B30FF", "gray":"#808080", "lightgray": "#CACACA",
"darkgray":"#A9A9A9"}
import random
asteroid = []
spaceship = []
stars = []
justPressed = False
aX = 0
aY = 0
x = 0
y = 0
def setup():
global aX, aY, X, Y
size(640, 480)
for s in range(1):
X = 320
Y = 440
spaceship.append(Space(X, Y, COLORS["orange"]))
for a in range(4):
aX = random.randint(0, 640)
aY = 0
asteroid.append(Asteroid(aX, aY, COLORS["orange"]))
for d in range(100):
randX = random.randint(0, 640)
randY = random.randint(0, 480)
stars.append(Star(randX, randY, COLORS["orange"]))
class Space:
def __init__(self, x, y, color):
self.x = x
self.y = y
self.color = color
def spaceship(self):
fill(COLORS["blue"])
stroke(COLORS["white"])
ellipse(self.x, self.y, 75, 75) #body
fill(COLORS["green"])
fill(COLORS["gray"])
stroke(COLORS["green"])
ellipse(self.x, self.y + 20, 120, 35)
stroke(COLORS["orange"])
fill(COLORS["purple"])
ellipse(self.x, self.y + 20, 10, 10)
ellipse(self.x + 30, self.y + 20, 10, 10)
ellipse(self.x - 30, self.y + 20, 10, 10)
def move(self, dx, dy):
self.x += dx
self.y += dy
class Asteroid:
def __init__(self, x, y, color):
self.x = x
self.y = y
self.color = color
def update(self):
self.velx = 0
self.vely = random.randint(1, 5)
self.x += self.velx
self.y += self.vely
if self.y > 480:
self.y = 0
self.x = random.randint(1,640)
def asteroid(self):
fill(COLORS["lightgray"])
stroke(COLORS["white"])
ellipse(self.x, self.y, 50, 50)
fill(COLORS["gray"])
ellipse(self.x +15, self.y + 6, 15, 15)
ellipse(self.x -12, self.y + 7, 10, 10)
ellipse(self.x + 5, self.y - 13, 11, 11)
ellipse(self.x - 12, self.y - 9, 7, 7)
ellipse(self.x + 2, self.y + 18, 7, 7)
class Star:
def __init__(self, x, y, color):
self.x = x
self.y = y
self.color = color
def star(self):
fill(COLORS["white"])
ellipse(self.x, self.y, 3, 3)
def draw():
global justPressed
background(COLORS["black"])
for f in stars:
f.star()
for f in spaceship:
f.spaceship()
if f.x < 60:
f.move(3, 0)
elif f.x > 580:
f.move(-3, 0)
elif justPressed :
if key == 'd':
f.move(3, 0)
elif key == 'a':
f.move(-3, 0)
for f in asteroid:
f.asteroid()
f.update()
#collision detection 'for' loop
distance = sqrt(((aX - X)**2) + ((aY - Y)**2))
for f in asteroid:
if distance < 62.5:
background(COLORS["red"])
def keyTyped():
global justPressed
justPressed = True
print("typed", key)
def keyPressed():
global justPressed
justPressed = True
print("pressed", key)
--
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Re: Python Learning
Varun R wrote: Hi All, I'm new to programming, can anyone guide me, how to start learning python programming language,...plz suggest some books also. Thanks all Are you sure you want to learn Python first? Python does enough things "behind the scene" that it makes me question the wisdom of that. Other points of view are welcome, of course. Learning the primitives of C++ first, may make for an easier transition. Surely this has been discussed before? -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: On Sat, Dec 16, 2017 at 8:51 AM, Bill wrote: Varun R wrote: Hi All, I'm new to programming, can anyone guide me, how to start learning python programming language,...plz suggest some books also. Thanks all Are you sure you want to learn Python first? Python does enough things "behind the scene" that it makes me question the wisdom of that. Other points of view are welcome, of course. Learning the primitives of C++ first, may make for an easier transition. Surely this has been discussed before? On the contrary, that makes Python an *excellent* first language. We don't force people to learn about the chemistry of petrochemical combustion before letting them learn how to drive a car; we don't make people understand TCP/IP networking before they're allowed to type something into Google. And if you DO want people to start off with a lower-level language, why C++? Why not machine code (or at least assembly code), since that's what the CPU actually executes? Most decent introductions to C++ discuss machine language (it helps make sense of compilation). As you indirectly suggest, learning is something of a circular process, so it really doesn't make that much difference where one starts, just "Do It!". : ) -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Rustom Mody (Rustom Mody) wrote: On Saturday, December 16, 2017 at 9:45:17 AM UTC+5:30, Bill wrote: Chris Angelico wrote: On Sat, Dec 16, 2017 at 8:51 AM, Bill wrote: Varun R wrote: Hi All, I'm new to programming, can anyone guide me, how to start learning python programming language,...plz suggest some books also. Thanks all Are you sure you want to learn Python first? Python does enough things "behind the scene" that it makes me question the wisdom of that. Other points of view are welcome, of course. Learning the primitives of C++ first, may make for an easier transition. Surely this has been discussed before? On the contrary, that makes Python an *excellent* first language. We don't force people to learn about the chemistry of petrochemical combustion before letting them learn how to drive a car; we don't make people understand TCP/IP networking before they're allowed to type something into Google. And if you DO want people to start off with a lower-level language, why C++? Why not machine code (or at least assembly code), since that's what the CPU actually executes? Most decent introductions to C++ discuss machine language (it helps make sense of compilation). As you indirectly suggest, learning is something of a circular process, so it really doesn't make that much difference where one starts, just "Do It!". : ) Reallyâ¿? https://en.wikipedia.org/wiki/Principles_of_learning#Primacy You would give precedence to something written on a wikipedia page over your experience? In our current context, we are talking about self-learning where one is one's own teacher. To my mind, if one gets that part right, one doesn't need to worry about the other types. I learned a great deal from BYTE magazine and before that Popular Electronics. Computer technology is so ubiquitous today, that the interested person one need only grab hold of something and start learning. YouTube for instance, offers a plethora of places to "begin". I tell students, that if they have the math behind them, then the doors of science will be open to them. In my experience, if they do not have the basic (~pre-calc) math behind them, then learning from a textbook on a programming language, say, may be a bit beyond them. -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Gregory Ewing wrote: Bill wrote: In my experience, if they do not have the basic (~pre-calc) math behind them, then learning from a textbook on a programming language, say, may be a bit beyond them. Very little mathematical *knowledge* is needed to get started with programming. You can do a lot of useful things in Python without even using any arithmetic. What does seem to correlate is that if you're the sort of person who finds maths interesting and fun to play with, you're also likely to take to programming easily. The same kind of logical thought processes are involved. I think we are talking about the same people. But in college, the prerequisite of "at least co-enrolled in pre-calc", turned out to be the right one (based upon quite a lot of teaching experience). -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Rustom Mody wrote: In response to Rustom Mody wrote: On Saturday, December 16, 2017 at 9:45:17 AM UTC+5:30, Bill wrote: so it really doesn't make that much difference where one starts, just "Do It!". : ) Really ¿? https://en.wikipedia.org/wiki/Principles_of_learning#Primacy On Sunday, December 17, 2017 Bill wrote: You would give precedence to something written on a wikipedia page over your experience? Bill also wrote: …in college, the prerequisite of "at least co-enrolled in pre-calc", turned out to be the right one (based upon quite a lot of teaching experience). So… I dont understand where you are coming from: Is there such a thing as a “learning curve” or not? The point is that it takes a certain amount of what is referred to as "mathematical maturity" (not mathematical knowledge) to digest a book concerning computer programming. In my years of teaching experience, students who came to college without the equivalent of "college algebra" were under-prepared for what was expected of them. This is not just an opinion, it's a fact. Of course, you could argue that a student who arrives at college needing to take college algebra is of a certain category. In your words, they are not yet ready to face any sort of (steep) learning curve. Bill -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Larry Martell wrote: So, your experience is that the style of learning you offer is unsuitable to anyone who doesn't have some background in algebra. That's fine. For your course, you set the prereqs. But that's not the only way for someone to get into coding. You do NOT have to go to college before you start creating software. That is also not an opinion; it's a fact backed by a number of proven instances (myself included). You might benefit by a course in logic. I never said that was the only way to learn. I learned (BASIC) in 8th grade too. I started coding when I was 16, in 1975. I wrote a downhill skiing game in basic, which I had taught to myself. I went to collage when I was 17, and I had taught myself FORTRAN and I tested out of the class. -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Gregory Ewing wrote: Bill wrote: In my years of teaching experience, students who came to college without the equivalent of "college algebra" were under-prepared for what was expected of them. This could be simply because it weeds out people who aren't good at the required style of thinking. I think that's absolutely true. If someone gets out of high school and doesn't understand college algebra (~8th grade math), they come into a programming class with an inherent disadvantage. They "can't understand" the book, just like they "couldn't" understand their math book. If that's true, anything that exercises mathematical thinking should have the same effect. There just doesn't happen to be anything else in the mainstream education curriculum that does that. -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: On Mon, Dec 18, 2017 at 6:51 AM, Bill wrote: The point is that it takes a certain amount of what is referred to as "mathematical maturity" (not mathematical knowledge) to digest a book concerning computer programming. Emphasis on *a book*. In my years of teaching experience, students who came to college without the equivalent of "college algebra" were under-prepared for what was expected of them. This is not just an opinion, it's a fact. So, your experience is that the style of learning you offer is unsuitable to anyone who doesn't have some background in algebra. That's fine. For your course, you set the prereqs. I never said that they needed any knowledge of algebra. But if they don't understand the logic behind an inequality, for instance, then may run into some trouble. -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: On Mon, Dec 18, 2017 at 11:31 AM, Bill wrote: Larry Martell wrote: So, your experience is that the style of learning you offer is unsuitable to anyone who doesn't have some background in algebra. That's fine. For your course, you set the prereqs. But that's not the only way for someone to get into coding. You do NOT have to go to college before you start creating software. That is also not an opinion; it's a fact backed by a number of proven instances (myself included). You might benefit by a course in logic. I never said that was the only way to learn. I learned (BASIC) in 8th grade too. You said earlier: In my years of teaching experience, students who came to college without the equivalent of "college algebra" were under-prepared for what was expected of them. This is not just an opinion, it's a fact. If you want to qualify that by saying that coming to college is not the only way to learn programming, you'd better say so, because otherwise, the tone of what you're saying says that students NEED algebra prior to learning a programming language. I tried pretty hard not to say that. I said they needed some "mathematical sophistication"--not actual mathematics. My error was using that expression among an audience not so familiar with that terminology. That said, I think I would have a hard time explaining polymorphism and vtables to a student who had not grasped the distributive property of multiplication over addition. You can try, and let me know how it goes for you... I've been there. Bill Logic isn't the problem here. Clarity of language is. Watch your implications if you don't want to be misunderstood. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: I don't know about vtables as needing to be in ANY programming course. They're part of a "let's dive into the internals of C++" course. You certainly don't need them to understand how things work in Python, because they don't exist; and I'm doubtful that you need to explain them even to C++ programmers. I guess "need to" is a relative thing. You can drive a car without know how one works. You can use functions with little knowledge of a run-time stack. You can avoid recursion if you are scared of it. And you can totally avoid functions if you are scared of them. And who needs documentation...seems like a big waste of time! Programmers don't need to know how to write, do they? Ask some programmers that just taught themselves, and they will tell you! After reading, writing, and arithmetic, then perhaps programming. Polymorphism... actually, I could explain that to a three year old. "Eat your broccoli" "Eat your dessert" See? Polymorphism. The same operation being done to different things and having different results. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: I don't know about vtables as needing to be in ANY programming course. They're part of a "let's dive into the internals of C++" course. You certainly don't need them to understand how things work in Python, because they don't exist; and I'm doubtful that you need to explain them even to C++ programmers. Then how are you going to explain dynamic_cast? Polymorphism... actually, I could explain that to a three year old. "Eat your broccoli" "Eat your dessert" See? Polymorphism. The same operation being done to different things and having different results. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: I agree with some of that, but you then take it to absurdity. You most certainly CAN drive a car without knowing how one works; in fact, with this century's cars, I think that's very much the case. How many people REALLY know what happens when you push the accelerator pedal, and how that makes the car go faster? Originally, it would actually open a valve and allow more fuel into the engine; now, it's all managed by a computer. So how much can you really understand of how it works? You can understand the "systems" which comprise your car. Engine, Cooling, braking, transmission, exhaust, etc. And you can similarly understand the systems which comprise the workings of your computer program. As it happens, I've been studying electronics lately. In answer to your question, there is an "invisible line" between hardware and software which most people don't generally cross. As another response to your question, try to master all of the features of the emacs editor. Personally, I've never felt motivated enough to do so. You can certainly use functions without knowing details of the run-time stack, though you'll need at least SOME comprehension of it. Avoiding functions and documentation, though, now you're just being ridiculous. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: On Mon, Dec 18, 2017 at 4:04 PM, Chris Angelico wrote: On Mon, Dec 18, 2017 at 3:54 PM, Bill wrote: Chris Angelico wrote: I don't know about vtables as needing to be in ANY programming course. They're part of a "let's dive into the internals of C++" course. You certainly don't need them to understand how things work in Python, because they don't exist; and I'm doubtful that you need to explain them even to C++ programmers. Then how are you going to explain dynamic_cast? I wouldn't. I'd teach Python to beginning programmers. C++ is not a good first language. And even for C++ programners, you can go a pretty long way without dynamic_cast. You either know it exists, and what it can do for you, or you don't. If you want to teach a 2nd course in C++, and leave out vtables and dynamic_cast, I don't have a problem with that. If you always choose the lowest common denominator, your class may not be that interesting. I told my students that my goal was for them was to learn how to learn so that they "didn't need me". ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Chris Angelico wrote: On Sun, Dec 17, 2017 at 12:01 PM, Bill wrote: I think we are talking about the same people. But in college, the prerequisite of "at least co-enrolled in pre-calc", turned out to be the right one (based upon quite a lot of teaching experience). Fortunately for the programming world, college isn't when everyone starts. I started coding at six years old. Should I have waited till I had some pre-calc before getting into programming? It probably wouldn't have hurt if you had co-enrolled in it. ;) And add an English course too to help you to write suitable documentation! I would think a 6 year old would have priorities above coding--what's the hurry? Bill ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Python Learning
Christian Gollwitzer wrote:
You don't need to explain a vtable to explain dynamic_cast. Only if
you want to become a compiler writer. It is not even required, vtables
are just the most common implementation.
dynamic_cast simply checks if the actual object that the pointer
points to is an instance of a derived class, and then casts it into
that. You could "explain" it with the following pseudo-code
template
T* dynamic_cast(base *input) {
if (isinstanceof(base, *input)) { return (T*)input; }
else { return nullptr; }
}
How that works (particularly the "isinstanceof") is not the business
of the programmer - it is sufficient to know that the compiler
somewhere stores this information for every object.
Christian
With that, the students would "boo you off the stage!"... and maybe
accuse you of being a "know it all". ; )The point of college is
more about teaching students to think rather than in being efficient. I
have little doubt that a tech school could "get through everything" much
faster.
Bill
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Re: adding elif to for
bob gailer wrote: Has any thought been given to adding elif to the for statement? I don't think it is a good idea because it needlessly, from my point of view, embeds too much complexity into a single construct (making it more difficult to maintain, for instance). That's what language designer's do though--they kick ideas like that around. If you have a great idea, maybe propose a new language. for x in foo: if y: break elif a==b: something else: something else as a shortcut to: for x in foo: if y: break else: if a==b: something else: something else bob gailer -- https://mail.python.org/mailman/listinfo/python-list
Re: correctness proof for alpha-beta algorithm
namenobodywa...@gmail.com wrote: On Tuesday, December 19, 2017 at 3:28:39 PM UTC-8, Steve D'Aprano wrote: Does this have anything specifically to do with Python programming? i'm working on a game-playing script (ie: in python), i want to incorporate pruning into my search algorithm, and i'd like to understand why it works; i'm not sure if that counts Based upon your posts, I think you should just write your own. Then you can be sure that it will work. That's better than working with something that you don't understand. -- https://mail.python.org/mailman/listinfo/python-list
Re: correctness proof for alpha-beta algorithm
Steve D'Aprano wrote: On Thu, 21 Dec 2017 08:37 am, Bill wrote: namenobodywa...@gmail.com wrote: On Tuesday, December 19, 2017 at 3:28:39 PM UTC-8, Steve D'Aprano wrote: Does this have anything specifically to do with Python programming? i'm working on a game-playing script (ie: in python), i want to incorporate pruning into my search algorithm, and i'd like to understand why it works; i'm not sure if that counts Based upon your posts, I think you should just write your own. Then you can be sure that it will work. How long did you say you've been programming? Perhaps you've heard of something we in the programming world call "bugs". You're supposed to get rid those those. Use Raid! : ) -- https://mail.python.org/mailman/listinfo/python-list
Re: Are the critiques in "All the things I hate about Python" valid?
boB Stepp wrote: This article is written by Nathan Murthy, a staff software engineer at Tesla. The article is found at: https://medium.com/@natemurthy/all-the-things-i-hate-about-python-5c5ff5fda95e Apparently he chose his article title as "click bait". Apparently he does not really hate Python (So he says.). His leader paragraph is: "Python is viewed as a ubiquitous programming language; however, its design limits its potential as a reliable and high performance systems language. Python is simply not a high performance system language, but it has other virtues. You would not likely choose Python for a sophisticated distributed computing application, but not everyone needs to write a sophisticated distributed computing application. I believe those that do, will be conscientious enough to choose an appropriate tool. Unfortunately, not every developer is aware of its limitations." As I currently do not have the necessary technical knowledge to properly evaluate his claims, I thought I would ask those of you who do. I have neither the knowledge or boB-hours to write a large distributed system code base, but I am curious if Python is truly limited for doing these in the ways he claims. BTW, I am not trying to start (another) heated, emotional thread. You guys do sometimes get carried away! I honestly just what to know the truth of the matters out of my continuing to learn Python. I suspect there is probably some truth in his claims, but I am not sure if he is taking things out of their proper application contexts or not. Thanks! -- https://mail.python.org/mailman/listinfo/python-list
