Re: Where's a DOM builder that uses the Builder Pattern to ... build DOMs?

[Stefan Behnel]

Phlip, 04.01.2010 21:01:

Not Hyp:

I hope I'm wrong, but seems that DOMBuilder, found among the various
xml.dom packages, cannot build DOM like this:

var html = DomBuilder.apply();

var form = html.FORM(
   html.DIV(
 html.INPUT({type : 'text', name : 'email'}),
 html.INPUT({type : 'text', name : 'password'}),
 html.INPUT({type : 'submit'}),
   )
);

Do anyone know any good DOM builder packages that do build DOM good
like a DOM builder should?


You might be looking for something like this:

http://codespeak.net/lxml/lxmlhtml.html#creating-html-with-the-e-factory

Note that there are tons of ways to generate HTML with Python. A quick web 
search (or a quick read on PyPI or the Python Wiki) should get you started.


Stefan
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Re: [Python] Printing plain text with exact positioning on Windows

[Chris Gonnerman]

KvS wrote:

So now I'm looking for other ways to get this printing job done. I
know of Tim Goldens page about Windows printing:
http://timgolden.me.uk/python/win32_how_do_i/print.html and have been
googling a lot, but I'm still not convinced what the best way to go
is. E.g. I can't get clear whether the combination reportlab & Acrobat
Reader allows to print very close to the left and right edge of the
paper (without a user having to change something in a Print dialog
etc.).
  

I have a page, and a module, for Windows printing which gives a lot of
control.

http://newcenturycomputers.net/projects/pythonicwindowsprinting.html

But in your case, I think I'd consider ReportLab.  Also, it seems to me
that I've seen a simpler PDF generator module for Python.  Dunno where,
though.

-- Chris.


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Re: Exception as the primary error handling mechanism?

[Steven D'Aprano]

On Tue, 05 Jan 2010 02:31:34 +, r0g wrote:

> A pattern I have used a few times is that of returning an explicit
> success/failure code alongside whatever the function normally returns.

That doesn't work for languages that can only return a single result, 
e.g. C or Pascal. You can fake it by creating a struct that contains a 
flag and the result you want, but that means doubling the number of data 
types you deal with.


> While subsequent programmers might not intuit the need to test for
> (implicit) "magic" return values they ought to notice if they start
> getting tuples back where they expected scalars...

What if they're expecting tuples as the result?



> def foo(x)
> if x>0:
> return True, x*x
> else:
>   return False, "Bad value of x in foo:",str(x)
> 
> ok, value = foo(-1)

Oops, that gives:

ValueError: too many values to unpack


because you've returned three items instead of two. When an idiom is easy 
to get wrong, it's time to think hard about it.



> if ok:
> print "foo of x is", value
> else:
> print "ERROR:", value


Whenever I come across a function that returns a flag and a result, I 
never know whether the flag comes first or second. Should I write:

flag, result = foo(x)

or

result, flag = foo(x)



I've seen APIs that do both.

And I never know if the flag should be interpreted as a success or a 
failure. Should I write:

ok, result = foo(x)
if ok: process(result)
else: fail()

or


err, result = foo(x)
if err: fail()
else: process(result)


Again, I've seen APIs that do both.

And if the flag indicates failure, what should go into result? An error 
code? An error message? That's impossible for statically-typed languages, 
unless they have variant records or the function normally returns a 
string.

And even if you dismiss all those concerns, it still hurts readability by 
obfuscating the code. Consider somebody who wants to do this:

result = foo(bar(x))

but instead has to do this:


flag, result = bar(x)
if flag:  # I think this means success
flag, result = foo(x)  # oops, I meant result

Again, it's error-prone and messy. Imagine writing:


flag, a = sin(x)
if flag:
flag, b = sqrt(x)
if flag:
flag, c = cos(b)
if flag:
flag, d = exp(a + c)
if flag:
flag, e = log(x)
if flag:
# Finally, the result we want!!!
flag, y = d/e
if not flag:
fail(y)
else:
fail(e)
else:
fail(d)
else:
fail(c)
else:
fail(b)
else:
fail(a)



Compare that to the way with exceptions:

y = exp(sin(x) + cos(sqrt(x)))/log(x)


Which would you prefer?




-- 
Steven
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A null program - what is it doing?

[Gib Bogle]

No doubt a dumb question from a noob:

The following program (a cut down version of some test code) uses no CPU, and 
does not terminate:


import sys
from PyQt4.QtCore import *

if __name__=="__main__":
app = QCoreApplication(sys.argv)
sys.exit(app.exec_())

What is the program doing?  What is the correct way to terminate the execution?

Thanks in advance for educating me.
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Re: python xmlrpc client with ssl client certificates and standard modules

[News123]

Hi Martin,

Thanks a lot for your reply.
It helped me to find the correct solution.

Unfortunaltely xmlrpclib.ServerProxy does not allow a host tuple, but
just a uri.

So the simplest solution, that I found is to create a custom transport


import xmlrpclib

class SafeTransportWithCert(xmlrpclib.SafeTransport):
__cert_file = DFLT_CERTFILE
__key_file  = DFLT_KEYFILE
def make_connection(self,host):
host_with_cert = (host, {
  'key_file'  :  self.__key_file,
  'cert_file' :  self.__cert_file
} )
return  \
  xmlrpclib.SafeTransport.make_connection(  
   self,host_with_cert)


transport = SafeTransportWithCert()
server = xmlrpclib.ServerProxy(server_url,
transport = transport)

rslt = server.mymethod(args)


Perfect.
Now the server can ensure, that only certified clients connect.

My next task is how to find out at the client side, that the server
certificate is a properly signed one.

bye


N


 Martin v. Loewis wrote:
>> I can do xmlrpc over ssl WITHOUT certificates with following code:
>>
>> import xmlrpclib
>> server_url = 'https://myserver'
>> server = xmlrpclib.Server(server_url);
>>
>>
>> and I can perform a https get request WITH certificates with below snippet:
>>
>> import httplib
>> conn = httplib.HTTPSConnection(
>>  HOSTNAME,
>>  key_file = KEYFILE,
>>  cert_file = CERTFILE
>> )
>> conn.putrequest('GET', '/')
>> conn.endheaders()
>> response = conn.getresponse()
>> print response.read()
>>
>>
>> I'm just lost of how to 'combine' both.
> 
> In this case, read through the source of xmlrpclib:
> 
> a) SafeTransport receives x509 parameters from get_host_info
> b) get_host_info supports a case where host is a tuple host, x509
> 
> So, without testing:
> 
> server = xmlrpclib.Server((server_url, {'key_file': KEYFILE,
> 'cert_file': CERTFILE}))
> 
> Please do read the code before trying this out.
> 
> HTH,
> Martin
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Re: Exception as the primary error handling mechanism?

[r0g]

Lie Ryan wrote:
> On 1/5/2010 1:31 PM, r0g wrote:
>> Michi wrote:
>>> On Jan 4, 1:30 pm, Steven D'Aprano
>>>   wrote:

>> A pattern I have used a few times is that of returning an explicit
>> success/failure code alongside whatever the function normally returns.
>> While subsequent programmers might not intuit the need to test for
>> (implicit) "magic" return values they ought to notice if they start
>> getting tuples back where they expected scalars...
>>
>> def foo(x)
>>  if x>0:
>>  return True, x*x
>>  else:
>> return False, "Bad value of x in foo:",str(x)
>>
>> ok, value = foo(-1)
>> if ok:
>>  print "foo of x is", value
>> else:
>>  print "ERROR:", value
> 
> Except that that is a reinvention of try-wheel:
> 


True, but there's more than one way to skin a cat! Mine's faster if you
expect a high rate of failures (over 15%).




> def foo(x):
> if x > 0:
> return x*x
> else:
> raise MathError("Bad value of x in foo: %s" % x)
> 
> try:
> print foo(-1)
> except MathError, e:
> print "ERROR: System integrity is doubted"
> 
> or rather; that is perhaps a good example of when to use 'assert'. If
> the domain of foo() is positive integers, calling -1 on foo is a bug in
> the caller, not foo().


Maybe, although I recently learned on here that one can't rely on assert
 statements in production code, their intended use is to aid debugging
and testing really.

Besides, that was just a toy example.



> 
> I have been looking at Haskell recently and the way the pure functional
> language handled exceptions and I/O gives me a new distinct "insight"
> that exceptions can be thought of as a special return value that is
> implicitly wrapped and unwrapped up the call stack until it is
> explicitly handled.



Yes there's some very interesting paradigms coming out of functional
programming but, unless you're a maths major, functional languages are a
long way off being productivity tools! Elegant: yes, provable: maybe,
practical for everyday coding: not by a long shot!


Roger.
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Re: Exception as the primary error handling mechanism?

[Chris Rebert]

On Tue, Jan 5, 2010 at 1:07 AM, r0g  wrote:
> Lie Ryan wrote:
>> I have been looking at Haskell recently and the way the pure functional
>> language handled exceptions and I/O gives me a new distinct "insight"
>> that exceptions can be thought of as a special return value that is
>> implicitly wrapped and unwrapped up the call stack until it is
>> explicitly handled.
>
> Yes there's some very interesting paradigms coming out of functional
> programming but, unless you're a maths major, functional languages are a
> long way off being productivity tools! Elegant: yes, provable: maybe,
> practical for everyday coding: not by a long shot!

Methinks the authors of Real World Haskell (excellent read btw) have a
bone to pick with you.

Cheers,
Chris
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lxml 2.2.4 on python3.1, Windows XP gives importerror

[VYAS ASHISH M-NTB837]

Dear All
 
I have Python 3.1 installed on Windows XP and Works nice.
I downloaded lxml 2.2.4 (lxml-2.2.4.win32-py3.1.exe) from pypi.
 
 
When I try:
from lxml import etree
I get:
ImportError: DLL load failed: This application has failed to start
because the application configuration is incorrect. Reinstalling the
application may fix this problem.
 
For information: 'import lxml' works fine.
 
After reinstalling python3.1 also the error message is the same. Any
help is appreciated!
 
 
Regards,
Ashish Vyas
 
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Re: Efficiently determine where documents differ

[Richard]

On Jan 5, 9:46 am, "Gabriel Genellina"  wrote:
> En Mon, 04 Jan 2010 19:04:12 -0300, Richard  escribió:
>
> > I have been using the difflib library to find where 2 large HTML
> > documents differ. The Differ().compare() method does this, but it is
> > very slow - atleast 100x slower than the unix diff command.
>
> Differ compares sequences of lines *and* lines as sequences of characters  
> to provide intra-line differences. The diff command only processes lines.
> If you aren't interested in intra-line differences, use a SequenceMatcher  
> instead. Or, invoke the diff command using   subprocess.Popen +  
> communicate.
>
> --
> Gabriel Genellina


thank you very much Gabriel! Passing a list of the document lines
makes the efficiency comparable to the diff command.
Richard
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Re: Exception as the primary error handling mechanism?

[r0g]

Steven D'Aprano wrote:
> On Tue, 05 Jan 2010 02:31:34 +, r0g wrote:
> 
>> A pattern I have used a few times is that of returning an explicit
>> success/failure code alongside whatever the function normally returns.
> 
> That doesn't work for languages that can only return a single result, 
> e.g. C or Pascal. You can fake it by creating a struct that contains a 
> flag and the result you want, but that means doubling the number of data 
> types you deal with.


No, but that's why I try not to use languages where you can only return
a single result, I always found that an arbitrary and annoying
constraint to have. I leads to ugly practices like "magic" return values
 in C or explicitly packing things into hashtables like PHP, yuk!


> 
> 
>> While subsequent programmers might not intuit the need to test for
>> (implicit) "magic" return values they ought to notice if they start
>> getting tuples back where they expected scalars...
> 
> What if they're expecting tuples as the result?
> 
> 
> 
>> def foo(x)
>> if x>0:
>> return True, x*x
>> else:
>>  return False, "Bad value of x in foo:",str(x)
>>
>> ok, value = foo(-1)
> 
> Oops, that gives:
> 
> ValueError: too many values to unpack
> 
> 
> because you've returned three items instead of two. When an idiom is easy 
> to get wrong, it's time to think hard about it.
> 


That seems pretty clear to me, "too many values to unpack", either I've
not given it enough variables to unpack the result into or I've returned
too many things. That would take a couple of seconds to notice and fix.
In fact I was trying to make the point that it would be quite noticable
if a function returned more things than the programmer was expecting,
this illustrates that quite well :)




> 
> 
>> if ok:
>> print "foo of x is", value
>> else:
>> print "ERROR:", value
> 
> 
> Whenever I come across a function that returns a flag and a result, I 
> never know whether the flag comes first or second. Should I write:
> 


Flag then result, isn't it obvious? The whole point of returning a flag
AND a result is so you can test the flag so you know what to do with the
result so that implies a natural order. Of course it doesn't matter
technically which way you do it, make a convention and stick to it. If
you get perpetually confused as to the order of parameters then you'd
better avoid this kind of thing, can't say as I've ever had a problem
with it though.




> And I never know if the flag should be interpreted as a success or a 
> failure. Should I write:
> 
> ok, result = foo(x)
> if ok: process(result)
> else: fail()



Yes. That would be my strong preference anyway. Naturally you can do it
the other way round if you like, as long as you document it properly in
your API. As you say different APIs do it differently... Unix has a
convention of returning 0 on no-error but unix has to encapsulate a lot
in that "error code" which is a bit of an anachronism these days. I'd
argue in favour of remaining positive and using names like ok or
success, this is closer to the familiar paradigm of checking a result
does not evaluate to false before using it...

name = ""
if name:
print name





> And if the flag indicates failure, what should go into result? An error 
> code? An error message? That's impossible for statically-typed languages, 
> unless they have variant records or the function normally returns a 
> string.


Yeah, in my example it's an error message. Maybe I shouldn't have used
the word "pattern" above though as it has overtones of "universally
applicable" which it clearly isn't.



> Again, it's error-prone and messy. Imagine writing:
> 
> 
> flag, a = sin(x)
> if flag:
> flag, b = sqrt(x)
> if flag:

> Compare that to the way with exceptions:
> 
> y = exp(sin(x) + cos(sqrt(x)))/log(x)
> 
> 
> Which would you prefer?
> 

LOL, straw man is straw!

You know full well I'm not suggesting every function return a flag, that
would be silly. There's no reason returning flag and a value shouldn't
be quite readable and there may be times when it's preferable to raising
an exception.

I use exceptions a lot as they're often the right tool for the job and
they seem pleasingly pythonic but from time to time they can be too slow
or verbose, where's the sense in forcing yourself to use them then?

Roger.
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Re: Exception as the primary error handling mechanism?

[Paul Rudin]

r0g  writes:

> Steven D'Aprano wrote:
>> On Tue, 05 Jan 2010 02:31:34 +, r0g wrote:
>> 
>>> A pattern I have used a few times is that of returning an explicit
>>> success/failure code alongside whatever the function normally returns.
>> 
>> That doesn't work for languages that can only return a single result, 
>> e.g. C or Pascal. You can fake it by creating a struct that contains a 
>> flag and the result you want, but that means doubling the number of data 
>> types you deal with.
>
>
> No, but that's why I try not to use languages where you can only return
> a single result, I always found that an arbitrary and annoying
> constraint to have. I leads to ugly practices like "magic" return values
>  in C or explicitly packing things into hashtables like PHP, yuk!

Doesn't python just return a single result? (I know it can be a tuple and
assignment statements will unpack a tuple for you.)
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Re: A null program - what is it doing?

[r0g]

Gib Bogle wrote:
> No doubt a dumb question from a noob:
> 
> The following program (a cut down version of some test code) uses no
> CPU, and does not terminate:
> 
> import sys
> from PyQt4.QtCore import *
> 
> if __name__=="__main__":
> app = QCoreApplication(sys.argv)
> sys.exit(app.exec_())
> 
> What is the program doing?  What is the correct way to terminate the
> execution?
> 
> Thanks in advance for educating me.



I've never used QT but other graphical toolkits I have used all start
their own "main loop" which is a loop that cycles round receiving,
queuing and dispatching "events". You probably need to call the
QCoreApplication's quit method to break out of this e.g.

app.exit() or something similar, have a look at some complete PyQt4
examples or google for PyQt4 mainloop.


Roger.
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Re: A null program - what is it doing?

[Almar Klein]

2010/1/5 r0g 

> Gib Bogle wrote:
> > No doubt a dumb question from a noob:
> >
> > The following program (a cut down version of some test code) uses no
> > CPU, and does not terminate:
> >
> > import sys
> > from PyQt4.QtCore import *
> >
> > if __name__=="__main__":
> > app = QCoreApplication(sys.argv)
> > sys.exit(app.exec_())
> >
> > What is the program doing?  What is the correct way to terminate the
> > execution?
> >
> > Thanks in advance for educating me.
>
>
>
> I've never used QT but other graphical toolkits I have used all start
> their own "main loop" which is a loop that cycles round receiving,
> queuing and dispatching "events". You probably need to call the
> QCoreApplication's quit method to break out of this e.g.
>
> app.exit() or something similar, have a look at some complete PyQt4
> examples or google for PyQt4 mainloop.
>

app.exec_()  starts the QT mainloop. If you would have created a window or
app before starting the loop, you app would now be responsive and wait for
you to do things with it. When you would then close the app, the function
returns the error code with which the app closed, and then subsequently the
Python process exits with that same error code. But since no widgets were
created before starting the mainloop, I don't think you can stop the process
in any way other than killing it.

By the way, QT has excellent documentation:
http://doc.trolltech.com/4.4/qapplication.html#exec

Cheers,
  Almar
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Re: Exception as the primary error handling mechanism?

[r0g]

Chris Rebert wrote:
> 
> On Tue, Jan 5, 2010 at 1:07 AM, r0g  wrote:
>> Lie Ryan wrote:
>>> I have been looking at Haskell recently and the way the pure functional
>>> language handled exceptions and I/O gives me a new distinct "insight"
>>> that exceptions can be thought of as a special return value that is
>>> implicitly wrapped and unwrapped up the call stack until it is
>>> explicitly handled.
>> Yes there's some very interesting paradigms coming out of functional
>> programming but, unless you're a maths major, functional languages are a
>> long way off being productivity tools! Elegant: yes, provable: maybe,
>> practical for everyday coding: not by a long shot!
> 
> Methinks the authors of Real World Haskell (excellent read btw) have a
> bone to pick with you.
> 
> Cheers,
> Chris
> --
> http://blog.rebertia.com


LOL, it seems things have come a long way since ML! I'm impressed how
many useful libraries Haskell has, and that they've included
IF-THEN-ELSE in the syntax! :) For all its advantages I still think you
need to be fundamentally cleverer to write the same programs in a
functional language than an old fashioned "English like" language.

Maybe I'm just mistrusting of the new school though and you'll see me on
comp.lang.haskell in a few years having to eat my own monads!

Roger.
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Re: Exception as the primary error handling mechanism?

[r0g]

Paul Rudin wrote:
> r0g  writes:
> 
>> Steven D'Aprano wrote:
>>> On Tue, 05 Jan 2010 02:31:34 +, r0g wrote:
>>>
 A pattern I have used a few times is that of returning an explicit
 success/failure code alongside whatever the function normally returns.
>>> That doesn't work for languages that can only return a single result, 
>>> e.g. C or Pascal. You can fake it by creating a struct that contains a 
>>> flag and the result you want, but that means doubling the number of data 
>>> types you deal with.
>>
>> No, but that's why I try not to use languages where you can only return
>> a single result, I always found that an arbitrary and annoying
>> constraint to have. I leads to ugly practices like "magic" return values
>>  in C or explicitly packing things into hashtables like PHP, yuk!
> 
> Doesn't python just return a single result? (I know it can be a tuple and
> assignment statements will unpack a tuple for you.)


Yes, it returns a tuple if you return more than one value, it just has a
 lovely syntax for it. In static languages you'd need to manually create
an new array or struct, pack your return vars into it and unpack them on
the other side. That's something I'd be happy never to see again, sadly
I have to write in PHP sometimes :(

Roger.
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ctypes: How to call unexported functions in a dll

[Coert Klaver (DT)]

Hi,

I am using ctypes in python 3 on a WXP machine

Loading a dll and using its exported functions works fine.

Now I want to use a function in the dll that is not exported.

In C this can be done by just casting the address in the dll of that
function to an apropriate function pointer and call it (you need to be
sure about the address). Can a similar thing be done directly with
ctypes?

A work around I see is writing in wrapper dll in c that loads the main
dll, exports a function that calls the unexported function in the main
dll, but I don't find that an elegant solution.

Thanks for any hint in the right direction.

BR

Coert
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Re: Exception as the primary error handling mechanism?

[Dave Angel]

r0g wrote:



Maybe, although I recently learned on here that one can't rely on assert
 statements in production code, their intended use is to aid debugging
and testing really.

  
Hopefully, what you learned is that you can't use assert() in production 
code to validate user data.  It's fine to use it to validate program 
logic, because that shouldn't still need testing in production.




DaveA
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Re: A null program - what is it doing?

[Dave Angel]

r0g wrote:

Gib Bogle wrote:
  

No doubt a dumb question from a noob:

The following program (a cut down version of some test code) uses no
CPU, and does not terminate:

import sys
from PyQt4.QtCore import *

if __name__=="__main__":
app = QCoreApplication(sys.argv)
sys.exit(app.exec_())

What is the program doing?  What is the correct way to terminate the
execution?

Thanks in advance for educating me.





I've never used QT but other graphical toolkits I have used all start
their own "main loop" which is a loop that cycles round receiving,
queuing and dispatching "events". You probably need to call the
QCoreApplication's quit method to break out of this e.g.

app.exit() or something similar, have a look at some complete PyQt4
examples or google for PyQt4 mainloop.


Roger.

  

Likewise, I haven't used QT, but have used others.

Gib:

It looks like app.exec() is the mainloop for QT, and it won't terminate 
till it gets a terminate event.  That might be the user clicking the X 
in the corner, or it might be a special keystroke like Alt-F4.  Or it 
might be some other event for which your event handler makes an explicit 
call to terminate.  Once one of these things happens, the app.exec() 
will return, and the sys.exit() will execute.


DaveA

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embedded python on mac - linking problem

[Krzysztof Kobus]

Hi,

I have a problem with linking python module with my application on mac in order 
to make the module available in "embedded python".

My python module is contained in  j3kmodule.cxx file and module initialization 
function is exported in j3kmodule.h

j3kmodule.h:

PyMODINIT_FUNC PyInit_j3k(void);


j3kmodule.cxx:
--
PyMODINIT_FUNC
PyInit_j3k(void)
  {
  PyObject *m = NULL;

  if ((m = PyModule_Create(&j3k_module)) == NULL)
return NULL;

  return m;
  }


Then in my application in KkPython.cxx file I have:

KkPython.cxx:
-

#include "j3kmodule.h"

/* Add a builtin module, before Py_Initialize */
PyImport_AppendInittab("j3k", PyInit_j3k);


I link my application with the module and get following linking error on mac 
although on open suse linux exactly the same procedure works fine:

g++ -headerpad_max_install_names -o ../../bin/render.app/Contents/MacOS/render 
main.o KkPython.o -lkkbase 
-L/Users/kk/dev/J3K/trunk/j3ksrc/examples/render/../../lib/ 
-lQtSolutions_PropertyBrowser-2.5 -lpython3.1 
/Users/kk/dev/J3K/trunk/j3ksrc/python/modules/build/lib.macosx-10.3-i386-3.1/j3k.so
 -framework OpenGL -framework AGL
ld: warning in 
/Users/kk/dev/J3K/trunk/j3ksrc/python/modules/build/lib.macosx-10.3-i386-3.1/j3k.so,
 file is not of required architecture
Undefined symbols:
  "_PyInit_j3k", referenced from:
  _PyInit_j3k$non_lazy_ptr in KkPython.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [../../bin/render.app/Contents/MacOS/render] Error 1


I appreciate any hints,

best greetings,

Krzysztof Kobus


  
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Re: Printing plain text with exact positioning on Windows

[KvS]

On Jan 5, 9:21 am, "alejandro"  wrote:
> Did you mean borderless printing?
> Every printer needs his margins, some more some less. Some printers have the
> ability to do borderless printing but usualy they can do it only on special
> or photo paper. So you can adjust the pdf as you wish, even with no margins,
> and then try to find under printer options "borderless printing". That is
> why I didn't understand :-)) it is a printer thing not pdf!

As much as possible "borderless", yes. Of course the printer will
still apply some small margin, but that's ok. A margin of say <0.5 cm.
is fine. So it's not a printer thing, I accept the (physical)
limitations of the printer, but I want to avoid any extra margins due
to software settings. So I need the create a pdf without/less as
possible margins, and then I need Acrobat Reader to (silently) print
it without any/less as possible extra margins added. I believe the
first step is possible, but I am wondering if the second step is
possible as well, i.e. can I adjust the options normally appearing in
the Printing Dialog through Python?
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Re: Exception as the primary error handling mechanism?

[Ben Finney]

r0g  writes:

> Paul Rudin wrote:
> > Doesn't python just return a single result? (I know it can be a
> > tuple and assignment statements will unpack a tuple for you.)
>
> Yes, it returns a tuple if you return more than one value, it just has
> a lovely syntax for it.

No, there is nothing inherent to the ‘return’ statement for dealing with
multiple values.

The ‘return’ statement *always* returns a single value: whatever value
you specify as the argument to the statement (or the value ‘None’ if no
argument is specified. If you specify a tuple — and Python has a nice
syntax for creating a literal tuple — that's the single value the
statement will use.

-- 
 \   “There's no excuse to be bored. Sad, yes. Angry, yes. |
  `\Depressed, yes. Crazy, yes. But there's no excuse for boredom, |
_o__)  ever.” —Viggo Mortensen |
Ben Finney
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Re: [Python] Re: Printing plain text with exact positioning on Windows

[Chris Gonnerman]

KvS wrote:

... can I adjust the options normally appearing in
the Printing Dialog through Python?
  
Yes, if you use my method or my module, as I gave in my previous post.  
If you use Adobe Reader to print, I'm not sure how to automate the print 
settings.

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Re: Exception as the primary error handling mechanism?

[r0g]

Dave Angel wrote:
> 
> 
> r0g wrote:
>> 
>>
>> Maybe, although I recently learned on here that one can't rely on assert
>>  statements in production code, their intended use is to aid debugging
>> and testing really.
>>
>>   
> Hopefully, what you learned is that you can't use assert() in production
> code to validate user data.  It's fine to use it to validate program
> logic, because that shouldn't still need testing in production.
> 
> 
> 
> DaveA



Well maybe I didn't quite get it then, could you explain a bit further?

My understanding was that asserts aren't executed at all if python is
started with the -O or -OO option, or run through an optimizer. If
that's the case how can you expect it to validate anything at all in
production? Do you mean for debugging in situ or something? Could you
maybe give me an example scenario to illustrate your point?

Cheers,

Roger.
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Re: Exception as the primary error handling mechanism?

[Roy Smith]

In article ,
 r0g  wrote:

> No, but that's why I try not to use languages where you can only return
> a single result, I always found that an arbitrary and annoying
> constraint to have. I leads to ugly practices like "magic" return values
>  in C or explicitly packing things into hashtables like PHP, yuk!

Python only lets you return a single result, just like C or C++.

The difference is that in Python it's trivial to build tuples on the fly 
and return them.  About the closest you get to that in C++ is std::pair, 
and that's about 5 bucks short and a week late.
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Re: Printing plain text with exact positioning on Windows

[alejandro]

Did you mean borderless printing?
Every printer needs his margins, some more some less. Some printers have the 
ability to do borderless printing but usualy they can do it only on special 
or photo paper. So you can adjust the pdf as you wish, even with no margins, 
and then try to find under printer options "borderless printing". That is 
why I didn't understand :-)) it is a printer thing not pdf! 


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Re: Exception as the primary error handling mechanism?

[Steven D'Aprano]

On Tue, 05 Jan 2010 13:06:20 +, r0g wrote:

> Dave Angel wrote:
>> 
>> 
>> r0g wrote:
>>> 
>>>
>>> Maybe, although I recently learned on here that one can't rely on
>>> assert
>>>  statements in production code, their intended use is to aid debugging
>>> and testing really.
>>>
>>>
>> Hopefully, what you learned is that you can't use assert() in
>> production code to validate user data.  It's fine to use it to validate
>> program logic, because that shouldn't still need testing in production.
>> 
>> 
>> 
>> DaveA
> 
> 
> 
> Well maybe I didn't quite get it then, could you explain a bit further?
> 
> My understanding was that asserts aren't executed at all if python is
> started with the -O or -OO option, 

Correct.


> or run through an optimizer. 

I don't know what you mean by that.


> If
> that's the case how can you expect it to validate anything at all in
> production? 

The asserts still operate so long as you don't use the -O switch.

> Do you mean for debugging in situ or something? Could you
> maybe give me an example scenario to illustrate your point?


There are at least two sorts of validation that you will generally need 
to perform: validating user data, and validating your program logic.

You *always* need to validate user data (contents of user-editable config 
files, command line arguments, data files, text they type into fields, 
etc.) because you have no control over what they put into that. So you 
shouldn't use assert for validating user data except for quick-and-dirty 
scripts you intend to use once and throw away.

Program logic, on the other hand, theoretically shouldn't need to be 
validated at all, because we, the programmers, are very clever and 
naturally never make mistakes. Since we never make mistakes, any logic 
validation we do is pointless and a waste of time, and therefore we 
should be able to optimise it away to save time.

*cough*

Since in reality we're not that clever and do make mistakes, we actually 
do want to do some such program validation, but with the option to 
optimise it away. Hence the assert statement.

So, a totally made-up example:


def function(x, y):
if x < 0:
raise ValueError("x must be zero or positive")
if y > 0:
raise ValueError("y must be zero or negative")
z = x*y
assert z < 0, "expected product of +ve and -ve number to be -ve"
return 1.0/(z-1)



This example cunningly demonstrates:

(1) Using explicit test-and-raise for ensuring that user-supplied 
arguments are always validated;

(2) Using an assertion to test your program logic;

(3) That the assertion in fact will catch an error in the program logic, 
since if you pass x or y equal to zero, the assertion will fail.


Any time you are tempted to write a comment saying "This can't happen, 
but we check for it just in case", that is a perfect candidate for an 
assertion. Since it can't happen, it doesn't matter if it goes away with 
the -O flag; but since we're imperfect, and we want to cover ourselves 
just in case it does happen, we perform the test when not optimized.

>From my own code, I have a global constant:

UNICODE_NUMERALS = u'\uff10\uff11\uff12\uff13\uff14\uff15\uff16\uff17
\uff18\uff19'


And then to make sure I haven't missed any:

assert len(UNICODE_NUMERALS) == 10


In another function, I validate a mapping {key:value} to ensure that all 
the values are unique:

seen_values = set()
for k,v in mapping.items():
if v in seen_values:
raise ValueError('duplicate value %s' % k)
seen_values.add(v)
# If we get here without error, then the mapping contains no 
# duplicate values.
assert len(seen_values) == len(mapping)


The assertion acts as a double-check on my logic, not the data. If my 
logic is wrong (perhaps there is a way to get past the for-loop while 
there is a duplicate?) then the assertion will catch it.


-- 
Steven
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Talking with ebay using easyBay

[starglider develop]

Hi,
I made an application to manage auctions with
easyBay
my problem is that easybay site is down and there is any other source of
information in the net
regarding the packadge.
Any one has the docs? or know how to  make a search?

Thank you in advance.
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Re: Exception as the primary error handling mechanism?

[D'Arcy J.M. Cain]

On 05 Jan 2010 14:02:50 GMT
Steven D'Aprano  wrote:
> shouldn't use assert for validating user data except for quick-and-dirty 
> scripts you intend to use once and throw away.

A mythcial beast that has yet to be spotted in the wild.

-- 
D'Arcy J.M. Cain  |  Democracy is three wolves
http://www.druid.net/darcy/|  and a sheep voting on
+1 416 425 1212 (DoD#0082)(eNTP)   |  what's for dinner.
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Re: Exception as the primary error handling mechanism?

[r0g]

Ben Finney wrote:
> r0g  writes:
> 
>> Paul Rudin wrote:
>>> Doesn't python just return a single result? (I know it can be a
>>> tuple and assignment statements will unpack a tuple for you.)
>> Yes, it returns a tuple if you return more than one value, it just has
>> a lovely syntax for it.
> 
> No, there is nothing inherent to the ‘return’ statement for dealing with
> multiple values.
> 
> The ‘return’ statement *always* returns a single value: whatever value
> you specify as the argument to the statement (or the value ‘None’ if no
> argument is specified. If you specify a tuple — and Python has a nice
> syntax for creating a literal tuple — that's the single value the
> statement will use.
> 



That's what I said Ben...

>>> Doesn't python just return a single result?

>> Yes,


See how I agree that "The ‘return’ statement *always* returns a single
value"?



>> it returns a tuple if you return more than one value, it just has
>> a lovely syntax for it.

You're confusing literal and conceptual returns. You can tell there are
two senses of return at play because the two returns have different
possessives within the same sentence. Look...

"Yes, IT returns a tuple" - Speaking literally about Python...

return 1, 2, 3   # Python returned one value

"YOU return more than one value" - Speaking conceptually from a
programmers perspective.

return 1, 2, 3   # I returned three values

Both are valid in their own context. The second (conceptual) statement
MIGHT be ambiguous if it weren't for the first literal one. Both cannot
be literally true and the first one clearly IS a literal factual
statement about Python so the second cannot also be interpreted as
literal. So let's disgard it...

>> it returns a tuple, it just has a lovely syntax for it.

The first two "it"s are Python, the third could either be...

(a) The construction of tuples

or at a stretch...

(b) The act of returning tuples i.e. some special syntax for returning
tuples.


You seem to think I meant (b) - I actually meant (a)

Maybe I could have made that clearer but I don't think people want to
read legalese and I think it takes a particular pedantic, nitpicking
disposition to even spot such small ambiguities.

Of course I'm now guilty of pedantry too :/ I might have let it slip had
you not started your reply with the word "No", that just p* me off.

Having said that I find the mental image of you slamming your fist on
the table and shouting it out loud whenever you read something you
disagree with on usenet quite amusing!

Roger.
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Re: twenty years ago Guido created Python

[n00m]

Stick your English into your ass
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Re: Exception as the primary error handling mechanism?

[r0g]

Steven D'Aprano wrote:
> On Tue, 05 Jan 2010 13:06:20 +, r0g wrote:

>> Well maybe I didn't quite get it then, could you explain a bit further?
>>
>> My understanding was that asserts aren't executed at all if python is
>> started with the -O or -OO option, 
> 
> Correct.
> 
> 
>> or run through an optimizer. 
> 
> I don't know what you mean by that.


I've never used them but I heard there are optimizers for python
(psycho?). I assumed these would do everythin -O does and more,
including losing the asserts.



> 
>> If
>> that's the case how can you expect it to validate anything at all in
>> production? 
> 
> The asserts still operate so long as you don't use the -O switch.
> 
>> Do you mean for debugging in situ or something? Could you
>> maybe give me an example scenario to illustrate your point?
> 
> 
> There are at least two sorts of validation that you will generally need 
> to perform: validating user data, and validating your program logic.
> 



Cool, that's what I thought i.e. you can't rely on asserts being there
so don't use them for anything critical but it's still a good idea to
use them for logic/consistency checking in production code as, should
you be running your production code unoptimised, it might catch
something you'd otherwise miss.

Thanks for responding is such detail :)

Roger.
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Re: ctypes: How to call unexported functions in a dll

[Thomas Heller]

Am 05.01.2010 12:19, schrieb Coert Klaver (DT):
> Hi,
> 
> I am using ctypes in python 3 on a WXP machine
> 
> Loading a dll and using its exported functions works fine.
> 
> Now I want to use a function in the dll that is not exported.
> 
> In C this can be done by just casting the address in the dll of that
> function to an apropriate function pointer and call it (you need to be
> sure about the address). Can a similar thing be done directly with
> ctypes?
> 
> A work around I see is writing in wrapper dll in c that loads the main
> dll, exports a function that calls the unexported function in the main
> dll, but I don't find that an elegant solution.

No need for a workaround.

One solution is to first create a prototype for the function by calling 
WINFUNCTYPE
or CFUNCTYPE, depending on the calling convention: stdcall or cdecl, then call 
the
prototype with the address of the dll function which will return a Python 
callable.
Demonstrated by the following script using GetProcAddress to get the address of
the GetModuleHandleA win32 api function:

>>> from ctypes import *
>>> dll = windll.kernel32
>>> addr = dll.GetProcAddress(dll._handle, "GetModuleHandleA")
>>> print hex(addr)
0x7c80b741
>>> proto = WINFUNCTYPE(c_int, c_char_p)
>>> func = proto(addr)
>>> func

>>> func(None)
486539264
>>> hex(func(None))
'0x1d00'
>>> hex(func("python24.dll"))
'0x1e00'
>>> hex(func("python.exe"))
'0x1d00'
>>>

Thomas
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Re: Dynamic text color

[Dave McCormick]

John Posner wrote:
On Fri, 01 Jan 2010 21:01:04 -0500, Cousin Stanley 
 wrote:





I was not familiar with the re.finditer method
for searching strings ...


Stanley and Dave --

So far, we've just been using finditer() to perform standard-string 
searches (e.g. on the word "red"). Since Dave now wants to color 
multiple words the same color (e.g. the words in redList), we can use 
a single regular-expression search to locate *all* the words in a 
list. This eliminates the need to use a "for" loop to handle the list. 
Here's what I mean:


 >>> import re
 >>> s = "it is neither red nor crimson, but scarlet, you see"

## individual searches

 >>> [matchobj.span() for matchobj in re.finditer("red", s)]
 [(14, 17)]
 >>> [matchobj.span() for matchobj in re.finditer("crimson", s)]
 [(22, 29)]
 >>> [matchobj.span() for matchobj in re.finditer("scarlet", s)]
 [(35, 42)]

## one "swell foop"

 >>> redList = "red crimson scarlet".split()
 >>> redList_regexp = "|".join(redList)
 >>> redList_regexp
 'red|crimson|scarlet'
 >>> [matchobj.span() for matchobj in re.finditer(redList_regexp, s)]
 [(14, 17), (22, 29), (35, 42)]

-John

Thanks again John,
This is fun!!!
I made a "red.text" file to hold the "red words", they are separated by 
a space in the file.
Still need to add the extra parameter "color" someplace. But this is 
what I have so far.

##
file = 'red.txt'
file = open("red.txt","r")
rList = file.readlines()
file.close()
redList = str(rList).split()
blueList = "blue ball".split()
greenList = "green grass".split()
def get_complete_text(event):
   complete_text = Tbox.get("1.0", END)
   Tbox.tag_remove("red", "1.0", END)
   Tbox.tag_remove("blue", "1.0", END)
   Tbox.tag_remove("green", "1.0", END)
RED
   redList_regexp = "|".join(redList)
   for matchobj in re.finditer(redList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("red", "1.0 + %d chars" % start,"1.0 + %d chars" % end)
   Tbox.tag_config("red", foreground="red")
BLUE###
   blueList_regexp = "|".join(blueList)
   for matchobj in re.finditer(blueList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("blue", "1.0 + %d chars" % start,"1.0 + %d chars" % 
end)

   Tbox.tag_config("blue", foreground="blue")
GREEN###
   greenList_regexp = "|".join(greenList)
   for matchobj in re.finditer(greenList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("green", "1.0 + %d chars" % start,"1.0 + %d chars" 
% end)

   Tbox.tag_config("green", foreground="green")
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Re: Exception as the primary error handling mechanism?

[Dave Angel]

r0g wrote:

Dave Angel wrote:
  

r0g wrote:




Maybe, although I recently learned on here that one can't rely on assert
 statements in production code, their intended use is to aid debugging
and testing really.

  
  

Hopefully, what you learned is that you can't use assert() in production
code to validate user data.  It's fine to use it to validate program
logic, because that shouldn't still need testing in production.



DaveA





Well maybe I didn't quite get it then, could you explain a bit further?

My understanding was that asserts aren't executed at all if python is
started with the -O or -OO option, or run through an optimizer. If
that's the case how can you expect it to validate anything at all in
production? Do you mean for debugging in situ or something? Could you
maybe give me an example scenario to illustrate your point?

Cheers,

Roger.

  
You understand the -O and -OO options fine.  But the point is that you 
should not use assert() for anything that will be properly debugged 
before going to the user.  You use if statements, and throw's to catch 
the error, and print to stderr, or GUI dialog boxes, or whatever 
mechanism you use to tell your user.  But those errors are ones caused 
by his data, not by your buggy code.  And the message tells him what's 
wrong with his data, not that you encountered a negative value for some 
low level function.


I agree with Steve's pessimistic view of the state of most released 
software.  But if you view a particular internal check as useful for 
production, then it should be coded in another mechanism, not in 
assert.  Go ahead and write one, with a UI that's appropriate for your 
particular application.  But it should do a lot more than assert does, 
including telling the user your contact information to call for support.


   def production_assert(expression, message):
if  not expression:
  dialog_box("Serious internal bug, 
call  NNN-NNN- immediately", message)



For an overly simplified example showing a user validation, and an assert :

import sys
def main():
   try:
   text = raw_input("Enter your age, between 1 and 22 ")
   age = int(text)
   except  ValueError, e:
   age = -1
   if not 1 <= age <= 22: #not an assert
   print "Age must be between 1 and 22"
   print "Run program again"
   sys.exit(2)
   grade = calc_grade(age)
   print "Your grade is probably", grade

table = [0, 0, 0, 0, 0, "K", "First", "2nd", 3]
def calc_grade(age):
   """ calculate a probable grade value, given an
   i2nteger age between 1 and 22, inclusive
   """
   assert(1 <= age <= len(table))
   grade = table[age]#assume I have a fixed-length table for this
   return grade

main()

Note a few things.  One I have a bug, in that the table isn't as big as 
the limit I'm checking for.  With defensive coding, I'd have another 
assert for that, or even have the table size be available as a global 
constant (all uppers) so that everyone's in synch on the upper limit.  
But in any case, the test suite would be checking to make sure the code 
worked for 1, for 22, for a couple of values in between, and that a 
proper error response happened when a non-integer was entered, or one 
outside of the range.  That all happens in separate code, not something 
in this file.  And the test suite is run after every change to the 
sources, and certainly before release to production.


Next, see the docstring.  It establishes a precondition for the 
function.  Since the function is called only by me (not the user), any 
preconditions can be checked with an assert.  An assert without a 
supporting comment (or docstring) is almost worthless.


And finally, notice that I check the user's input *before* passing it on 
to any uncontrolled code.  So any asserts after that cannot fire, unless 
I have a bug which was not caught during testing.


All opinions my own, of course.

DaveA

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Re: embedded python on mac - linking problem

[Benjamin Kaplan]

On Tue, Jan 5, 2010 at 7:33 AM, Krzysztof Kobus  wrote:
> Hi,
>
> I have a problem with linking python module with my application on mac in 
> order to make the module available in "embedded python".
>
> My python module is contained in  j3kmodule.cxx file and module 
> initialization function is exported in j3kmodule.h
>
> j3kmodule.h:
> 
> PyMODINIT_FUNC PyInit_j3k(void);
>
>
> j3kmodule.cxx:
> --
> PyMODINIT_FUNC
> PyInit_j3k(void)
>  {
>  PyObject *m = NULL;
>
>  if ((m = PyModule_Create(&j3k_module)) == NULL)
>    return NULL;
>
>  return m;
>  }
>
>
> Then in my application in KkPython.cxx file I have:
>
> KkPython.cxx:
> -
>
> #include "j3kmodule.h"
>
> /* Add a builtin module, before Py_Initialize */
> PyImport_AppendInittab("j3k", PyInit_j3k);
>
>
> I link my application with the module and get following linking error on mac 
> although on open suse linux exactly the same procedure works fine:
>
> g++ -headerpad_max_install_names -o 
> ../../bin/render.app/Contents/MacOS/render main.o KkPython.o -lkkbase 
> -L/Users/kk/dev/J3K/trunk/j3ksrc/examples/render/../../lib/ 
> -lQtSolutions_PropertyBrowser-2.5 -lpython3.1 
> /Users/kk/dev/J3K/trunk/j3ksrc/python/modules/build/lib.macosx-10.3-i386-3.1/j3k.so
>  -framework OpenGL -framework AGL
> ld: warning in 
> /Users/kk/dev/J3K/trunk/j3ksrc/python/modules/build/lib.macosx-10.3-i386-3.1/j3k.so,
>  file is not of required architecture
> Undefined symbols:
>  "_PyInit_j3k", referenced from:
>      _PyInit_j3k$non_lazy_ptr in KkPython.o
> ld: symbol(s) not found
> collect2: ld returned 1 exit status
> make: *** [../../bin/render.app/Contents/MacOS/render] Error 1
>
>
> I appreciate any hints,
>
> best greetings,
>
> Krzysztof Kobus
>

Well, it seems that one of your files is a different architecture than
the others. Based on the location, I'd say it's i386 while the rest of
it would be PowerPC. You can cross-compile but you can't link an i386
library to a PowerPC library.
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Re: Speeding up network access: threading?

[Jens Müller]

Hello,


The fairly obvious thing to do is use a queue.queue for tasks and another
for results and a pool of threads that read, fetch, and write.


Thanks, indeed.

Is a list thrad-safe or do I need to lock when adding the results of my
worker threads to a list? The order of the elements in the list does not
matter.

Jens

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Re: Speeding up network access: threading?

[Jens Müller]

Hello,

The fairly obvious thing to do is use a queue.queue for tasks and another 
for results and a pool of threads that read, fetch, and write.


Thanks, indeed.

Is a list thrad-safe or do I need to lock when adding the results of my 
worker threads to a list? The order of the elements in the list does not 
matter.


Jens 


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Re: Printing plain text with exact positioning on Windows

[KvS]

On Jan 5, 12:56 pm, Chris Gonnerman
 wrote:
> KvS wrote:
> > ... can I adjust the options normally appearing in
> > the Printing Dialog through Python?
>
> Yes, if you use my method or my module, as I gave in my previous post.  
> If you use Adobe Reader to print, I'm not sure how to automate the print
> settings.

Thanks Chris, I'll go on and have a look.
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Re: Printing plain text with exact positioning on Windows

[KvS]

On Jan 5, 12:56 pm, Chris Gonnerman
 wrote:
> KvS wrote:
> > ... can I adjust the options normally appearing in
> > the Printing Dialog through Python?
>
> Yes, if you use my method or my module, as I gave in my previous post.  
> If you use Adobe Reader to print, I'm not sure how to automate the print
> settings.

Ok, actually I quite like being able to print straightforward through
your code, i.e. without any extra modules installed. I understand that
sending text to the printer is in principle as simple as

dc.TextOut(scale_factor * 72,
-1 * scale_factor * 72,
"Testing...")

I didn't see you do anything with adjusting margins in the code. Does
that mean that if I would e.g. do

dc.TextOut(0,
0,
"Testing...")

the printout would appear in the upper left corner of the paper, as
close to the edges as the printer is capable of? (Sorry, but I only
have Ubuntu available at the moment, no Windows).
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Re: A null program - what is it doing?

[John Posner]

No doubt a dumb question from a noob:

The following program (a cut down version of some test code) uses no 
CPU, and does not terminate:


import sys
from PyQt4.QtCore import *

if __name__=="__main__":
app = QCoreApplication(sys.argv)
sys.exit(app.exec_())

What is the program doing?  What is the correct way to terminate the 
execution?




Are you trying to understand QCoreApplication()? I can't help you there, 
since I've never used it. If you're just trying to get started with 
PyQt, use QApplication() instead:


import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

if __name__=="__main__":
   app = QApplication(sys.argv)
   window = QLabel("I'm a PyQt window")
   window.show()
   sys.exit(app.exec_())

To terminate execution, just close the window by clicking the "X" in the 
window banner.


HTH,
John

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Re: Where's a DOM builder that uses the Builder Pattern to ... build DOMs?

[Phlip]

On Jan 5, 12:16 am, Stefan Behnel  wrote:

> Note that there are tons of ways to generate HTML with Python.

Forgot to note - I'm generating schematic XML, and I'm trying to find
a way better than the Django template I started with!
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Re: Speeding up network access: threading?

[MRAB]

Jens Müller wrote:

Hello,


The fairly obvious thing to do is use a queue.queue for tasks and another
for results and a pool of threads that read, fetch, and write.


Thanks, indeed.

Is a list thrad-safe or do I need to lock when adding the results of my
worker threads to a list? The order of the elements in the list does not
matter.


Terry said "queue". not "list". Use the Queue class (it's thread-safe)
in the "Queue" module (assuming you're using Python 2.x; in Python 3.x
it's called the "queue" module).
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Re: Exception as the primary error handling mechanism?

[Lie Ryan]

On 1/6/2010 1:48 AM, r0g wrote:

Steven D'Aprano wrote:

On Tue, 05 Jan 2010 13:06:20 +, r0g wrote:

If
that's the case how can you expect it to validate anything at all in
production?


The asserts still operate so long as you don't use the -O switch.


Do you mean for debugging in situ or something? Could you
maybe give me an example scenario to illustrate your point?



There are at least two sorts of validation that you will generally need
to perform: validating user data, and validating your program logic.





Cool, that's what I thought i.e. you can't rely on asserts being there
so don't use them for anything critical but it's still a good idea to
use them for logic/consistency checking in production code as, should
you be running your production code unoptimised, it might catch
something you'd otherwise miss.


Steven described the traditional approach to using assertions; another 
approach to when to use assertion is the one inspired by 
Design-by-Contract paradigm. DbC extends the traditional approach by 
focusing on writing a contract (instead of writing assertions) and 
generating assertions[1] to validate the contract. Just like assertions, 
these contracts are meant to be removed in production releases.


In Design-by-Contract, only codes that interacts with the outer-world 
(e.g. getting user/file/network input, etc) need to do any sort of 
validations. Codes that doesn't interact directly with outside world 
only need to have a "contract" and simplified by *not* needing argument 
checking, since the function relies on the caller obeying the 
contract[2] and never calling it with an invalid input.


DbC uses assertions[1] spuriously, unlike the traditional approach which 
is much more conservative when using assertions.


[1] or explicit language support which is just syntax sugar for assertions
[2] of course, on a debug release, the contract validation code will 
still be enforced to catch logic/consistency bugs that causes the violation

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Re: Speeding up network access: threading?

[Antoine Pitrou]

Le Tue, 05 Jan 2010 15:04:56 +0100, Jens Müller a écrit :
> 
> Is a list thrad-safe or do I need to lock when adding the results of my
> worker threads to a list? The order of the elements in the list does not
> matter.

The built-in list type is thread-safe, but is doesn't provide the waiting 
features that queue.Queue provides.

Regards

Antoine.

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Re: Exception as the primary error handling mechanism?

[Steve Holden]

D'Arcy J.M. Cain wrote:
> On 05 Jan 2010 14:02:50 GMT
> Steven D'Aprano  wrote:
>> shouldn't use assert for validating user data except for quick-and-dirty 
>> scripts you intend to use once and throw away.
> 
> A mythcial beast that has yet to be spotted in the wild.
> 
Not true (he wrote, picking nits). Such programs are written all the
time. The fact that they invariably get used more often than intended
doesn't negate the intentions of the author. ;-)

regards
 Steve
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Re: Dynamic text color

[John Posner]

On Tue, 05 Jan 2010 10:31:09 -0500, Dave McCormick   
wrote:



... But this is what I have so far.
##
file = 'red.txt'
file = open("red.txt","r")
rList = file.readlines()
file.close()
redList = str(rList).split()


Dave, you're doing exactly the right thing: gradually expanding your  
program, to provide more functionality and to learn more about the  
available programming tools. It's also very good that you take care to  
close() the file after processing it. Now for the bad news ...


1. Don't use "file" as a variable name -- it's a built-in object type.  
(Some people don't like the fact that Python allows you to redefine such  
"reserved words".)


2. It's probably not the best idea to use a single variable (you use  
"file") to do double-duty: to hold the name of a file, and to hold the  
open-file object returned by the open() function. It's perfectly legal,  
but it hides information that might be useful when you're debugging a  
program. This is better:


  fname = 'red.txt'
  inpf = open(fname, "r")

3. It might be better to use read() rather than readlines() to process the  
"red.txt" file. It depends on what that file is supposed to contain. For  
example, if you expect "red.txt" to contain exactly one line, which has  
one or more words, you can process the open-file object like this:


  file_contents = inpf.read()
  redList = file_contents.split()

   ... or ...

  redList = inpf.read().split()

It's certainly a mistake to use the expression "str(rList).split()". Using  
str() to convert the list "rList" into a string creates a mess that  
includes square-bracket characters. Did this actually work for you?


Best,
John

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parsing an Excel formula with the re module

[vsoler]

Hello,

I am acessing an Excel file by means of Win 32 COM technology.
For a given cell, I am able to read its formula. I want to make a map
of how cells reference one another, how different sheets reference one
another, how workbooks reference one another, etc.

Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?

I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?

For example"=Book1!A5+8" should be parsed into ["=","Book1", "!",
"A5","+","8"]

Can anybody help? Any suggestions?

Vicente Soler
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Re: Printing plain text with exact positioning on Windows

[KvS]

On Jan 5, 12:56 pm, Chris Gonnerman
 wrote:
> KvS wrote:
> > ... can I adjust the options normally appearing in
> > the Printing Dialog through Python?
>
> Yes, if you use my method or my module, as I gave in my previous post.  
> If you use Adobe Reader to print, I'm not sure how to automate the print
> settings.

Sorry, one more. I completely forgot it's not exactly plain text, but
occasionally also a limited number of non-ASCII characters (accents in
names etc.). Would this be possible through your method?
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it gets worse (was: How do you configure IDLE on a Mac...)

[Mensanator]

On Jan 5, 12:32 am, Ned Deily  wrote:
> In article
> <0d70cb54-3d77-4176-b621-e764ecf61...@26g2000yqo.googlegroups.com>,
>
>
>
>
>
>  Mensanator  wrote:
> > I assume I've been using the IDLE from macports. From the command
> > prompt I've
> > been typing "idle". This launches a "shell" window which appears to
> > have an X11
> > parent application for which there are no "preferences" applicable to
> > fonts.
>
> > However, if I use the quick launcher from the python.org, I get a
> > "shell" whose
> > parent is named "IDLE"! And that one has a completely different
> > preferences,
> > one similar the the Windows Configure which allows me to set the font!
>
> > Now, if I close this shell and start IDLE from the command line again,
> > I still
> > get a "shell" with an X11 parent, but, lo and behold, the font has
> > changed to
> > what I had previously set with the IDLE parent.
>
> > Course, I can't import gmpy, cause the python.org version can't
> > compile it, so I
> > still have to use the macports install of 3.1, but that's ok, once I
> > use
> > the IDLE application to set the preferences, I can switch back to the
> > X11 version and the preferences will follow.
>
> The prefs follow because all versions of IDLE use the same (unversioned)
> directory for configuration files, ~/.idlerc/.  In particular, the
> configuration file ~/.idlerc/config-main.cfg contains, among other
> things, any changes to the default font.  So, if you're successful at
> changing it in one version of IDLE, it will likely affect all versions
> you have.  Note the file is a simple ini format:
>
> [EditorWindow]
> font = monaco
>
> so you can edit it by hand.

Good to know. But, as the subject says...

>
> BTW, the python.org IDLEs and the Apple-supplied IDLEs use the
> system-supplied Aqua (aka Quartz) Tk not the X11 one that MacPorts
> builds by default.  The MacPorts Tk port does have a "quartz" variant
> but that doesn't yet work in 64-bit mode.

So, for all practical purposes, the macports install is broken also.

IDLE simply does not work in an X11 window (you think someone would
have noticed that). The missing preferences is just the beginning.
Apparently NONE of the menu item shortcuts work.

For example, the Cut, Copy, Paste shortcuts are given as Command-X,
Command-C and Command-V. But that doesn't work in an X11 window,
apperently only in an Aqua Tk (parent application appears as IDLE).

Of course, I can do Control-X, Control-C and Control-V to do Cut,
Copy and Paste. Don't know if this works for all shortcuts, but
I suppose I could just pick them from the menu (and I can bang
my head against the wall while I'm at it).

What do you think, suppose I copy the gmpy built with the macports
install over to the directory where the python.org version is? Would
it
import? If that'll work, I can switch back to using the python.org
install and use it's version of IDLE. I certainly won't be needing
distutils once I have a working version of gmpy.

>
> --
>  Ned Deily,
>  n...@acm.org- Hide quoted text -
>
> - Show quoted text -

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Re: parsing an Excel formula with the re module

[MRAB]

vsoler wrote:

Hello,

I am acessing an Excel file by means of Win 32 COM technology.
For a given cell, I am able to read its formula. I want to make a map
of how cells reference one another, how different sheets reference one
another, how workbooks reference one another, etc.

Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?

I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?

For example"=Book1!A5+8" should be parsed into ["=","Book1", "!",
"A5","+","8"]

Can anybody help? Any suggestions?


Do you mean "how" or do you really mean "whether", ie, get a list of the
other cells that are referred to by a certain cell, for example,
"=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5"]?
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Re: Speeding up network access: threading?

[Jens Müller]

Hi and sorry for double posting - had mailer problems,


Terry said "queue". not "list". Use the Queue class (it's thread-safe)
in the "Queue" module (assuming you're using Python 2.x; in Python 3.x
it's called the "queue" module).


Yes yes, I know. I use a queue to realize the thread pool queue, that works 
all right.


But each worker thread calculates a result and needs to make it avaialable 
to the application in the main thread again. Therefore, it appends its 
result to a common list. This seems works as well, but I was thinking of 
possible conflict situations that maybe could happen when two threads append 
their results to that same result list at the same moment.


Regards,
Jens 


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Re: IOError - cannot create file (linux daemon-invoked script)

[Nobody]

On Mon, 04 Jan 2010 21:30:31 -0800, cassiope wrote:

> One more tidbit observed: my last note, that it works when using
> seteuid/setegid?
> Well - that only applies if the daemon is running under strace (!).
> It fails
> if started directly by root, or if the strace session has ended,
> leaving the
> main body of the daemon running in its normal headless manner.
> 
> I wonder if running under "strace -f" - might setegid/seteuid be
> prevented from
> having their normal effect?

Possibly. The ptrace() syscall on which strace depends will fail if you
try to trace a "privileged" process and you aren't root, so it's possible
that a ptrace()d process will refuse to become privileged.

Here, "privileged" includes a process which has changed any of its UIDs or
GIDs (this prevents a normal user from tracing, killing, etc an otherwise
privileged process which has switched to the user's UID for the time being).

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Re: twenty years ago Guido created Python

[Phlip]

On Dec 31 2009, 2:06 pm, Steve Howell  wrote:

> Python is a truly awesome programming language.  Not only is Guido a
> genius language designer, but he is also a great project leader.  What
> an accomplishment.  Congratulations to everybody who has contributed
> to Python in the last two decades!

The more languages you learn before getting to Smalltalk, the more
awesome Smalltalk will be for you.

--
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Re: parsing an Excel formula with the re module

[vsoler]

On 5 ene, 19:35, MRAB  wrote:
> vsoler wrote:
> > Hello,
>
> > I am acessing an Excel file by means of Win 32 COM technology.
> > For a given cell, I am able to read its formula. I want to make a map
> > of how cells reference one another, how different sheets reference one
> > another, how workbooks reference one another, etc.
>
> > Hence, I need to parse Excel formulas. Can I do it by means only of re
> > (regular expressions)?
>
> > I know that for simple formulas such as "=3*A7+5" it is indeed
> > possible. What about complex for formulas that include functions,
> > sheet names and possibly other *.xls files?
>
> > For example    "=Book1!A5+8" should be parsed into ["=","Book1", "!",
> > "A5","+","8"]
>
> > Can anybody help? Any suggestions?
>
> Do you mean "how" or do you really mean "whether", ie, get a list of the
> other cells that are referred to by a certain cell, for example,
> "=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5"]?

I'd like to know how to do it, should it be possible.

Vicente
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Re: parsing an Excel formula with the re module

[John Posner]

On Tue, 05 Jan 2010 13:12:00 -0500, vsoler  wrote:


Hello,

I am acessing an Excel file by means of Win 32 COM technology.
For a given cell, I am able to read its formula. I want to make a map
of how cells reference one another, how different sheets reference one
another, how workbooks reference one another, etc.

Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?

I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?

For example"=Book1!A5+8" should be parsed into ["=","Book1", "!",
"A5","+","8"]

Can anybody help? Any suggestions?


It seems like you want to recreate data structures that Excel, itself,  
must maintain in order to recalculate cells in the correct order. As long  
as you're using COM, you might be able to tap into those data structures.  
My 15-year-old (!) "Using Excel Visual Basic for Applications" book wasn't  
any help. :-( After a short Google session, I came up with one possible  
lead: http://www.decisionmodels.com/


Good luck!
John
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Re: parsing an Excel formula with the re module

[Mensanator]

On Jan 5, 12:35 pm, MRAB  wrote:
> vsoler wrote:
> > Hello,
>
> > I am acessing an Excel file by means of Win 32 COM technology.
> > For a given cell, I am able to read its formula. I want to make a map
> > of how cells reference one another, how different sheets reference one
> > another, how workbooks reference one another, etc.
>
> > Hence, I need to parse Excel formulas. Can I do it by means only of re
> > (regular expressions)?
>
> > I know that for simple formulas such as "=3*A7+5" it is indeed
> > possible. What about complex for formulas that include functions,
> > sheet names and possibly other *.xls files?
>
> > For example    "=Book1!A5+8" should be parsed into ["=","Book1", "!",
> > "A5","+","8"]
>
> > Can anybody help? Any suggestions?
>
> Do you mean "how" or do you really mean "whether", ie, get a list of the
> other cells that are referred to by a certain cell, for example,
> "=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5]

Ok, although "Book1" would be the default name of a workbook, with
default
worksheets labeled "Sheet1". "Sheet2", etc.

If I had a worksheet named "Sheety" that wanted to reference a cell on
"Sheetx"
OF THE SAME WORKBOOK, it would be =Sheet2!A7. If the reference was to
a completely
different workbook (say Book1 with worksheets labeled "Sheet1",
"Sheet2") then
the cell might have =[Book1]Sheet1!A7.

And don't forget the $'s! You may see =[Book1]Sheet1!$A$7.
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Re: fsync() doesn't work as advertised?

[Nobody]

On Mon, 04 Jan 2010 08:09:56 -0800, Brian D wrote:

> If I'm running a process in a loop that runs for a long time, I
> occasionally would like to look at a log to see how it's going.
> 
> I know about the logging module, and may yet decide to use that.
> 
> Still, I'm troubled by how fsync() doesn't seem to work as advertised:
> 
> http://docs.python.org/library/os.html
> 
> "If you’re starting with a Python file object f, first do f.flush(),
> and then do os.fsync(f.fileno())"

The .flush() method (and the C fflush() function) causes the
contents of application buffers to be sent to the OS, which basically
copies the data into the OS-level buffers.

fsync() causes the OS-level buffers to be written to the physical drive.

File operations normally use the OS-level buffers; e.g. if one process
write()s to a file and another process read()s it, the latter will see
what the former has written regardless of whether the data has been
written to the drive.

The main reason for using fsync() is to prevent important data from being
lost in the event of an unexpected reboot or power-cycle (an expected
reboot via the "shutdown" or "halt" commands will flush all OS-level
buffers to the drive first). Other than that, fsync() is almost invisible
(I say "almost", as there are mechanisms to bypass the OS-level buffers,
e.g. the O_DIRECT open() flag).

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Re: What is the best data structure for a very simple spreadsheet?

[vsoler]

On 3 ene, 22:40, mdipierro  wrote:
> Perhaps this can be useful:http://www.web2py.com/examples/spreadsheet
>
> The code is in a single file with not dependencies and it does not
> require web2py to 
> run:http://code.google.com/p/web2py/source/browse/gluon/contrib/spreadshe...
>
> Here is a sample controller that shows you how to embed the
> spreadsheet in web 
> page:http://code.google.com/p/web2py/source/browse/applications/examples/c...
>
> Massimo
>
> On Jan 3, 5:27 am, vsoler  wrote:
>
> > Hi,
>
> > Not sure this is the best group to post, but I cannot think of any
> > other.
>
> > My application would contain a limited set of "cells" represented by
> > the instances of a Cell class:
>
> > class Cell:
> > ...
>
> > A1=Cell(7)
> > A2=Cell(2*A1)
> > A3=Cell(3*A1+A2)
> > A4=Cell(A3*4)
>
> > Of course, A1 = 7, A2 = 14, A3 = 35 and A4 = 140
>
> > Now, I somehow want to be able to show a dependency tree
>
> > 1 level dependency trees
> >   A1: None
> >   A2: A1
> >   A3: A1, A2
> >   A4: A3
>
> > All levels dependency trees
>
> >   A1: None
> >   A2: A1
> >   A3: A1, A2
> >   A4: A3, A2, A1
>
> > Leaf + values dependency trees:
>
> >   A1: 7
> >   A2: A1=7, 2
> >   A3: 3, A1=7, 2
> >   A4: 3, A1=7, 2, 4
>
> > What I'd like to know is:
>
> > 1) what are, in your opinion, the basic elements of the Cell class?
> > 2) Do I need a parser to evaluate the formulas like “3*A1+A2”? Can you
> > recommend one library that already contains one?
> > 3) Do I need a tree data structure to represent my data? would the
> > tree be an attribute of the class instance?
>
> > I imagine a lot can be said on these questions. What I am looking for
> > is some hints that help me get out of where I am now.
>
> > Any help is highly appreciated.
>
> > Vicente Soler
>
>

There is something that I appreciate in this group, and it is the high
degree of knowledge of all the participants that are helping with
their answers.

After studying your suggestions, I'll come back to you.

Thank you very much.

Vicente Soler
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Re: Speeding up network access: threading?

[Steve Holden]

Jens Müller wrote:
> Hi and sorry for double posting - had mailer problems,
> 
>> Terry said "queue". not "list". Use the Queue class (it's thread-safe)
>> in the "Queue" module (assuming you're using Python 2.x; in Python 3.x
>> it's called the "queue" module).
> 
> Yes yes, I know. I use a queue to realize the thread pool queue, that
> works all right.
> 
> But each worker thread calculates a result and needs to make it
> avaialable to the application in the main thread again. Therefore, it
> appends its result to a common list. This seems works as well, but I was
> thinking of possible conflict situations that maybe could happen when
> two threads append their results to that same result list at the same
> moment.
> 
If you don't need to take anything off the list ever, just create a
separate thread that reads items from an output Queue and appends them
to the list.

If you *do* take them off, then use a Queue.

regards
 Steve
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Re: Printing plain text with exact positioning on Windows

[Nobody]

On Tue, 05 Jan 2010 04:40:14 -0800, KvS wrote:

>> Did you mean borderless printing?
>> Every printer needs his margins, some more some less. Some printers have the
>> ability to do borderless printing but usualy they can do it only on special
>> or photo paper. So you can adjust the pdf as you wish, even with no margins,
>> and then try to find under printer options "borderless printing". That is
>> why I didn't understand :-)) it is a printer thing not pdf!
> 
> As much as possible "borderless", yes. Of course the printer will
> still apply some small margin, but that's ok. A margin of say <0.5 cm.
> is fine. So it's not a printer thing, I accept the (physical)
> limitations of the printer, but I want to avoid any extra margins due
> to software settings.

"Hardcopy" document formats such as PostScript and PDF use positions
relative to the edges of the page, not the margins.

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Re: parsing an Excel formula with the re module

[MRAB]

vsoler wrote:

On 5 ene, 19:35, MRAB  wrote:

vsoler wrote:

Hello,
I am acessing an Excel file by means of Win 32 COM technology.
For a given cell, I am able to read its formula. I want to make a map
of how cells reference one another, how different sheets reference one
another, how workbooks reference one another, etc.
Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?
I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?
For example"=Book1!A5+8" should be parsed into ["=","Book1", "!",
"A5","+","8"]
Can anybody help? Any suggestions?

Do you mean "how" or do you really mean "whether", ie, get a list of the
other cells that are referred to by a certain cell, for example,
"=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5"]?


I'd like to know how to do it, should it be possible.


Something like this should work:

references = re.findall(r"\b((?:\w+!)?[A-Za-z]+\d+)\b", formula)
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Re: twenty years ago Guido created Python

[Steve Holden]

Phlip wrote:
> On Dec 31 2009, 2:06 pm, Steve Howell  wrote:
> 
>> Python is a truly awesome programming language.  Not only is Guido a
>> genius language designer, but he is also a great project leader.  What
>> an accomplishment.  Congratulations to everybody who has contributed
>> to Python in the last two decades!
> 
> The more languages you learn before getting to Smalltalk, the more
> awesome Smalltalk will be for you.
> 
After implementing SmallTalk I more or less gave up OO programming for
ten years, resuming it only after I met Python. SmallTalk didn't seem
that awesome to me.

Though for its time it was an incredible system, its insistence on a
SmallTalk-only VM environment seemed a little solipsistic.

regards
 Steve
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Re: parsing an Excel formula with the re module

[vsoler]

On 5 ene, 20:05, Mensanator  wrote:
> On Jan 5, 12:35 pm, MRAB  wrote:
>
>
>
> > vsoler wrote:
> > > Hello,
>
> > > I am acessing an Excel file by means of Win 32 COM technology.
> > > For a given cell, I am able to read its formula. I want to make a map
> > > of how cells reference one another, how different sheets reference one
> > > another, how workbooks reference one another, etc.
>
> > > Hence, I need to parse Excel formulas. Can I do it by means only of re
> > > (regular expressions)?
>
> > > I know that for simple formulas such as "=3*A7+5" it is indeed
> > > possible. What about complex for formulas that include functions,
> > > sheet names and possibly other *.xls files?
>
> > > For example    "=Book1!A5+8" should be parsed into ["=","Book1", "!",
> > > "A5","+","8"]
>
> > > Can anybody help? Any suggestions?
>
> > Do you mean "how" or do you really mean "whether", ie, get a list of the
> > other cells that are referred to by a certain cell, for example,
> > "=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5]
>
> Ok, although "Book1" would be the default name of a workbook, with
> default
> worksheets labeled "Sheet1". "Sheet2", etc.
>
> If I had a worksheet named "Sheety" that wanted to reference a cell on
> "Sheetx"
> OF THE SAME WORKBOOK, it would be =Sheet2!A7. If the reference was to
> a completely
> different workbook (say Book1 with worksheets labeled "Sheet1",
> "Sheet2") then
> the cell might have =[Book1]Sheet1!A7.
>
> And don't forget the $'s! You may see =[Book1]Sheet1!$A$7.

Yes, Mensanator, but...  what re should I use? I'm looking for the re
statement. No doubt you can help!

Thank you.
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Fwd: I would like to install Python on my 64 bit Win 7

[aung paing Soe]

-- Forwarded message --
From: aung paing Soe 
Date: Tue, Jan 5, 2010 at 11:27 AM
Subject: I would like to install Python on my 64 bit Win 7
To: webmas...@python.org


Hello ,
  I would like to study about Python Programming . So I want to
install Python .
But my laptop is Window 7 64-bit home basic  .
So please give me a advice how to install Python in my 64 bit computer.
I really want to study python programming .
I am looking forward your reply.
Thank you very much

Yours,
Beginner
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Re: parsing an Excel formula with the re module

[Tim Chase]

vsoler wrote:

Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?

I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?


Where things start getting ugly is when you have nested function 
calls, such as


  =if(Sum(A1:A25)>42,Min(B1:B25), if(Sum(C1:C25)>3.14, 
(Min(C1:C25)+3)*18,Max(B1:B25)))


Regular expressions don't do well with nested parens (especially 
arbitrarily-nesting-depth such as are possible), so I'd suggest 
going for a full-blown parsing solution like pyparsing.


If you have fair control over what can be contained in the 
formulas and you know they won't contain nested parens/functions, 
you might be able to formulate some sort of "kinda, sorta, maybe 
parses some forms of formulas" regexp.


-tkc


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Re: parsing an Excel formula with the re module

[vsoler]

On 5 ene, 20:21, vsoler  wrote:
> On 5 ene, 20:05, Mensanator  wrote:
>
>
>
> > On Jan 5, 12:35 pm, MRAB  wrote:
>
> > > vsoler wrote:
> > > > Hello,
>
> > > > I am acessing an Excel file by means of Win 32 COM technology.
> > > > For a given cell, I am able to read its formula. I want to make a map
> > > > of how cells reference one another, how different sheets reference one
> > > > another, how workbooks reference one another, etc.
>
> > > > Hence, I need to parse Excel formulas. Can I do it by means only of re
> > > > (regular expressions)?
>
> > > > I know that for simple formulas such as "=3*A7+5" it is indeed
> > > > possible. What about complex for formulas that include functions,
> > > > sheet names and possibly other *.xls files?
>
> > > > For example    "=Book1!A5+8" should be parsed into ["=","Book1", "!",
> > > > "A5","+","8"]
>
> > > > Can anybody help? Any suggestions?
>
> > > Do you mean "how" or do you really mean "whether", ie, get a list of the
> > > other cells that are referred to by a certain cell, for example,
> > > "=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5]
>
> > Ok, although "Book1" would be the default name of a workbook, with
> > default
> > worksheets labeled "Sheet1". "Sheet2", etc.
>
> > If I had a worksheet named "Sheety" that wanted to reference a cell on
> > "Sheetx"
> > OF THE SAME WORKBOOK, it would be =Sheet2!A7. If the reference was to
> > a completely
> > different workbook (say Book1 with worksheets labeled "Sheet1",
> > "Sheet2") then
> > the cell might have =[Book1]Sheet1!A7.
>
> > And don't forget the $'s! You may see =[Book1]Sheet1!$A$7.
>
> Yes, Mensanator, but...  what re should I use? I'm looking for the re
> statement. No doubt you can help!
>
> Thank you.


Let me give you an example:

>>> import re
>>> re.split("([^0-9])", "123+456*/")
[’123’, ’+’, ’456’, ’*’, ’’, ’/’, ’’]

I find it excellent that one single statement is able to do a lexical
analysis of an expression!

If the expression contains variables, such as A12 or B9, I can try
another re expression. Which one should I use?

And if my expression contains parenthesis?   And the sin() function?

Vicente Soler


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please help shrink this each_with_index() implementation

[Phlip]

Hypo Nt:

def each_with_index(seq):
index = 0
result = []

for item in seq:
  result.append([item, index])
  index += 1

return result

My Pythonic sequencing skills are obviously feeble. Can anything think
of a way to write that in fewer lines?

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  http://c2.com/cgi/wiki?MoreliaViridis
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Re: Fwd: I would like to install Python on my 64 bit Win 7

[Dave McCormick]

aung paing Soe wrote:

  
  -- Forwarded message --
From: aung paing Soe 
Date: Tue, Jan 5, 2010 at 11:27 AM
Subject: I would like to install Python on my 64 bit Win 7
To: webmas...@python.org
  
  
Hello , 
  I would like to study about Python Programming . So I want to
install Python .
But my laptop is Window 7 64-bit home basic  . 
So please give me a advice how to install Python in my 64 bit computer.
I really want to study python programming .
I am looking forward your reply.
Thank you very much
  
Yours, 
Beginner 
  

I am using WIN7 64 bit also.  
Go to python.org and look for 
http://www.python.org/download/releases/2.5.4/
python-2.5.4.amd64.msi

Dave


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Re: Dynamic text color

[Dave McCormick]

John Posner wrote:


Dave, you're doing exactly the right thing: gradually expanding your 
program, to provide more functionality and to learn more about the 
available programming tools. It's also very good that you take care to 
close() the file after processing it. Now for the bad news ...

Seems like there is always bad news :)


1. Don't use "file" as a variable name -- it's a built-in object type. 
(Some people don't like the fact that Python allows you to redefine 
such "reserved words".)


2. It's probably not the best idea to use a single variable (you use 
"file") to do double-duty: to hold the name of a file, and to hold the 
open-file object returned by the open() function. It's perfectly 
legal, but it hides information that might be useful when you're 
debugging a program. This is better:


3. It might be better to use read() rather than readlines() to process 
the "red.txt" file. It depends on what that file is supposed to 
contain. For example, if you expect "red.txt" to contain exactly one 
line, which has one or more words, you can process the open-file 
object like this:

All noted and fixed.


It's certainly a mistake to use the expression "str(rList).split()". 
Using str() to convert the list "rList" into a string creates a mess 
that includes square-bracket characters. Did this actually work for you?
It sort of worked.  With one color file it seemed fine but after I 
posted I added another color file and things fell apart.
Now with the above fixes it works with three colors from three files.  
When the list are printed to the shell the list look like this:

redList ['red', 'dog', 'apple', '#']
blueList ['blue', 'ball', 'berry']
greenList ['green', 'grass', 'do']

But another problem is noticed.  It does not matter if the list is built 
in code or from a file.
If dog is entered, "do" will be green with the "g" being red.  
Back to the drawing board.


Here is the complete code"
##
from Tkinter import *
import re
RFfile = 'red.txt'
inpRF = open(RFfile,"r")
rList = inpRF.read()
inpRF.close()

BFfile = 'blue.txt'
inpBF = open(BFfile,"r")
bList = inpBF.read()
inpBF.close()

GFfile = 'green.txt'
inpGF = open(GFfile,"r")
gList = inpGF.read()
inpGF.close()

def get_complete_text(event):
   complete_text = Tbox.get("1.0", END)
   redList = str(rList).split()
   blueList = str(bList).split()
   greenList = str(gList).split()
   print "redList",redList
   print "blueList",blueList
   print "greenList",greenList
   Tbox.tag_remove("red", "1.0", END)
   Tbox.tag_remove("blue", "1.0", END)
   Tbox.tag_remove("green", "1.0", END)
RED
   redList_regexp = "|".join(redList)
   for matchobj in re.finditer(redList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("red", "1.0 + %d chars" % start,"1.0 + %d chars" % end)
   Tbox.tag_config("red", foreground="red")
BLUE###
   blueList_regexp = "|".join(blueList)
   for matchobj in re.finditer(blueList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("blue", "1.0 + %d chars" % start,"1.0 + %d chars" % 
end)

   Tbox.tag_config("blue", foreground="blue")
GREEN###
   greenList_regexp = "|".join(greenList)
   for matchobj in re.finditer(greenList_regexp, complete_text):
   start,end = matchobj.span()
   Tbox.tag_add("green", "1.0 + %d chars" % start,"1.0 + %d chars" 
% end)

   Tbox.tag_config("green", foreground="green")

root = Tk()
Tbox = Text(root, width=40, height=15, wrap=CHAR,
  font="Times 14 bold", bg="#dd")
Tbox.pack()
Tbox.bind("", get_complete_text)
Tbox.focus()
root.mainloop()

Thanks again,
Dave
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Re: twenty years ago Guido created Python

[Mensanator]

On Jan 5, 8:22 am, n00m  wrote:
> Stick your English into your ass

Most people would say "up your ass".
And use a period at the end of the sentence.

Got any more funny insults?
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Re: Fwd: I would like to install Python on my 64 bit Win 7

[Benjamin Kaplan]

On Tue, Jan 5, 2010 at 3:03 PM, Dave McCormick  wrote:
>
>
> aung paing Soe wrote:
>
> -- Forwarded message --
> From: aung paing Soe 
> Date: Tue, Jan 5, 2010 at 11:27 AM
> Subject: I would like to install Python on my 64 bit Win 7
> To: webmas...@python.org
>
>
> Hello ,
>   I would like to study about Python Programming . So I want to
> install Python .
> But my laptop is Window 7 64-bit home basic  .
> So please give me a advice how to install Python in my 64 bit computer.
> I really want to study python programming .
> I am looking forward your reply.
> Thank you very much
>
> Yours,
> Beginner
>
> I am using WIN7 64 bit also.
> Go to python.org and look for
> http://www.python.org/download/releases/2.5.4/
> python-2.5.4.amd64.msi
>
> Dave

Unless you need a package that requires Python 2.5, it's a good idea
to use Python 2.6 instead. 2.5 isn't getting any more bug fixes and
2.6 includes quite a few new features.
http://python.org/download/releases/2.6.4/
>
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>
>
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Re: please help shrink this each_with_index() implementation

[akean]

On Jan 6, 8:58 am, Phlip  wrote:
> Hypo Nt:
>
> def each_with_index(seq):
>     index = 0
>     result = []
>
>     for item in seq:
>       result.append([item, index])
>       index += 1
>
>     return result
>
> My Pythonic sequencing skills are obviously feeble. Can anything think
> of a way to write that in fewer lines?
>
> --
>   Phlip
>  http://c2.com/cgi/wiki?MoreliaViridis

 y = [[seq[i],i] for i in range(len(seq))]  gives the same result.
The index is accessible without assigning it to another variable.
--
Anita
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Re: please help shrink this each_with_index() implementation

[Carsten Haese]

Phlip wrote:
> Hypo Nt:
> 
> def each_with_index(seq):
> index = 0
> result = []
> 
> for item in seq:
>   result.append([item, index])
>   index += 1
> 
> return result
> 
> My Pythonic sequencing skills are obviously feeble. Can anything think
> of a way to write that in fewer lines?

Couldn't you just use the built-in enumerate() to replace the whole thing?

--
Carsten Haese
http://informixdb.sourceforge.net

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Re: please help shrink this each_with_index() implementation

[Marco Nawijn]

On Jan 5, 8:58 pm, Phlip  wrote:
> Hypo Nt:
>
> def each_with_index(seq):
>     index = 0
>     result = []
>
>     for item in seq:
>       result.append([item, index])
>       index += 1
>
>     return result
>
> My Pythonic sequencing skills are obviously feeble. Can anything think
> of a way to write that in fewer lines?
>
> --
>   Phlip
>  http://c2.com/cgi/wiki?MoreliaViridis

You could use the build-in function enumerate inside a list
comprehension.

>>> seq = range(5)
>>> [ (i,s) for i,s in enumerate(seq) ]
[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]

This will reduce the function to a one-liner.

Regards,

Marco
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Re: Dynamic text color

[John Posner]

On Tue, 05 Jan 2010 15:08:04 -0500, Dave McCormick   
wrote:




It's certainly a mistake to use the expression "str(rList).split()".  
Using str() to convert the list "rList" into a string creates a mess  
that includes square-bracket characters. Did this actually work for you?


It sort of worked.  With one color file it seemed fine but after I  
posted I added another color file and things fell apart.
Now with the above fixes it works with three colors from three files.   
When the list are printed to the shell the list look like this:

redList ['red', 'dog', 'apple', '#']
blueList ['blue', 'ball', 'berry']
greenList ['green', 'grass', 'do']

But another problem is noticed.  It does not matter if the list is built  
in code or from a file.
If dog is entered, "do" will be green with the "g" being red.  Back to  
the drawing board.


It sounds like the program is doing exactly what you TOLD it to do (which  
might not be what you WANT it to do):


 1. In an earlier pass on the text, color the string "dog" red.
 2. In a later pass, color the string "do" green.

You need to decide what you WANT to happen if one word to be colored is a  
substring of another word to be colored differently. Or maybe you want to  
outlaw such situations. After making that decision, you can start to think  
about how to write the appropriate code.


Best,
John
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Re: please help shrink this each_with_index() implementation

[Phlip]

> > My Pythonic sequencing skills are obviously feeble. Can anything think
> > of a way to write that in fewer lines?

Thanks, all!

> Couldn't you just use the built-in enumerate() to replace the whole thing?

Because that would involve, like, reading an entire Python book just
to locate that method?

GMAB I'm too busy writing high-end Django via TDD & BDD! C-:

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Re: please help shrink this each_with_index() implementation

[Antoine Pitrou]

>> Couldn't you just use the built-in enumerate() to replace the whole
>> thing?
> 
> Because that would involve, like, reading an entire Python book just to
> locate that method?

Actually, no. It just involves reading one of the most important pages in 
the documentation, the page which describes the built-in functions:

http://docs.python.org/library/functions.html

Don't forget that the Python documentation is rich and structured.
And good luck.


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Re: A null program - what is it doing?

[Gib Bogle]

r0g wrote:

Gib Bogle wrote:

No doubt a dumb question from a noob:

The following program (a cut down version of some test code) uses no
CPU, and does not terminate:

import sys
from PyQt4.QtCore import *

if __name__=="__main__":
app = QCoreApplication(sys.argv)
sys.exit(app.exec_())

What is the program doing?  What is the correct way to terminate the
execution?

Thanks in advance for educating me.




I've never used QT but other graphical toolkits I have used all start
their own "main loop" which is a loop that cycles round receiving,
queuing and dispatching "events". You probably need to call the
QCoreApplication's quit method to break out of this e.g.

app.exit() or something similar, have a look at some complete PyQt4
examples or google for PyQt4 mainloop.


Roger.


Thanks.  I've realized that QCoreApplication (or QApplication) manages the event 
loop, and normally when it is executed you are showing a widget of some kind, 
closing which ends the application.  Alt-F4 acts like Ctrl-C to terminate from 
the keyboard.

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Commands for a breakpoint in .pdbrc

[Pablo Torres N.]

Hi all,

I'd like to save the commands for a breakpoint in a .pdbrc, something
like:

b 81
commands 1
pp foo.attr1
pp foo.attr2
end
b 108
commands 2
pp bar.attr1
pp bar.attr2
end

This would automate setting the environment for the debugging session.
However, this does not work with 'python -m pdb script.py', because at
the line 'commands 1', the pdb prompt starts and asks me for the
commands for the first breakpoint, ignoring what I wrote in .pdbrc;
further, it raises a NameError after I type 'end' at the pdb prompt,
because of 'foo.attr1', 'foo.attr2' and even 'end'. The same happens
for the rest of the breakpoints, so I end up with them set but not
their commands.

What would be the correct way to do this? Is it even possible?


Thanks a lot,
Pablo Torres N.
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Re: Dynamic text color

[Dave McCormick]

John Posner wrote:
On Tue, 05 Jan 2010 15:08:04 -0500, Dave McCormick 
 wrote:


It sounds like the program is doing exactly what you TOLD it to do 
(which might not be what you WANT it to do):


 1. In an earlier pass on the text, color the string "dog" red.
 2. In a later pass, color the string "do" green.

You need to decide what you WANT to happen if one word to be colored 
is a substring of another word to be colored differently. Or maybe you 
want to outlaw such situations. After making that decision, you can 
start to think about how to write the appropriate code.


Best,
John
Darn thing doing what I told it to do...  Guess that means I did 
something right :)
But it is not what I am wanting. 
I first thought to make it look for a space but that would not work when 
a single character like "#" is to be colored if there is a "string" of 
them.  Or if all of the characters between quotes are to be colored. 


I always did like puzzles!

Dave
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Python multiprocessing: Permission denied

[t0ster]

Hi guys, I'm getting an error when trying to execute python program
that uses multiprocessing package:

  File "/usr/local/lib/python2.6/multiprocessing/__init__.py", line
178, in RLock
return RLock()
  File "/usr/local/lib/python2.6/multiprocessing/synchronize.py", line
142, in __init__
SemLock.__init__(self, RECURSIVE_MUTEX, 1, 1)
  File "/usr/local/lib/python2.6/multiprocessing/synchronize.py", line
49, in __init__
sl = self._semlock = _multiprocessing.SemLock(kind, value,
maxvalue)
OSError: [Errno 13] Permission denied

It looks like the user don't have permission to access shared memory.
When executing with root privileges it works fine.

Is there any solution to run it as normal user(not root)?

Python version 2.6.2 , OS is Linux 2.6.18 (CentOS release 5.4) and
it's VPS machine.
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Re: it gets worse (was: How do you configure IDLE on a Mac...)

[Ned Deily]

In article 
<6672dad2-26ba-458b-8075-21bac6506...@e37g2000yqn.googlegroups.com>,
 Mensanator  wrote:
[...]
> So, for all practical purposes, the macports install is broken also.
> 
> IDLE simply does not work in an X11 window (you think someone would
> have noticed that). The missing preferences is just the beginning.
> Apparently NONE of the menu item shortcuts work.
> 
> For example, the Cut, Copy, Paste shortcuts are given as Command-X,
> Command-C and Command-V. But that doesn't work in an X11 window,
> apperently only in an Aqua Tk (parent application appears as IDLE).
> 
> Of course, I can do Control-X, Control-C and Control-V to do Cut,
> Copy and Paste. Don't know if this works for all shortcuts, but
> I suppose I could just pick them from the menu (and I can bang
> my head against the wall while I'm at it).
> 
> What do you think, suppose I copy the gmpy built with the macports
> install over to the directory where the python.org version is? Would
> it
> import? If that'll work, I can switch back to using the python.org
> install and use it's version of IDLE. I certainly won't be needing
> distutils once I have a working version of gmpy.

Let's go back to your original problem, which, if I understand 
correctly, was trying to get going with Python 3 and gmpy on OS X 10.6.   
(Sorry I was away over the holidays and didn't get a chance to respond 
to your original postings at the time.)  I believe the problems you 
originally encountered with installing gmpy were all due to a couple of 
problems with building C extension modules on 10.6 when using the 
current 3.1.1 OS X python.org.  Unfortunately, 3.1.1 was released before 
10.6 was so there are a couple of important fixes that haven't yet been 
released for 3.1 (but are in the 2.6.4 installer which was released 
after 10.6 came out).  Fortunately, though, there are simple workarounds 
for the problems.  Keep in mind, though, that, at the moment, the 
python.org installers for OS X are 32-bit only; that will change in the 
future but if you do need a 64-bit Python 3 you'll need to stick to 
other solutions like MacPorts for the time being.

First, make sure the gmp library you've installed has 32-bit support.  
If you installed it using MacPorts, check with the file command:

$ file /opt/local/lib/libgmp.dylib
/opt/local/lib/libgmp.dylib: Mach-O universal binary with 2 architectures
/opt/local/lib/libgmp.dylib (for architecture i386):  Mach-O dynamically 
linked shared library i386
/opt/local/lib/libgmp.dylib (for architecture x86_64):   Mach-O 64-bit 
dynamically linked shared library x86_64

If it doesn't have an i386 variant, reinstall the gmp library from 
MacPorts:

$ sudo port selfupdate# make sure MacPorts is up-to-date
$ sudo port clean gmp
$ sudo port install gmp +universal # install 32-/64-bit variants

Second, you need to install the MacOSX10.4u SDK because the current 
python.org pythons are built with it.  That SDK is included in the Snow 
Leopard Xcode installer package but it is not installed by default.  
There should be an Xcode.mpkg somewhere, perhaps on your hard disk if 
your system came with Snow Leopard factory-installed or perhaps on a 
restore DVD.  If not, it's on the retail Snow Leopard DVD and can be 
downloaded from the Apple Developer site.  After launching the Xcode 
installer, just select and install the "Mac OS 10.4 Support" package 
from the Custom Install menu.

Third, you need to tell Distutils to use the older gcc-4.0 instead of 
the gcc-4.2 which is now the default on 10.6.

$ cd /path/to/gmpy-1.11rc1
$ export CC=/usr/bin/gcc-4.0
$ /usr/local/bin/python3.1 setup.py install
...
$ /usr/local/bin/python3.1 test3/gmpy_test.py
Unit tests for gmpy 1.11
on Python 3.1.1 (r311:74543, Aug 24 2009, 18:44:04) 
[GCC 4.0.1 (Apple Inc. build 5493)]
Testing gmpy 1.11 (GMP 4.3.1), default caching (100, 128)
...
1500 tests in 42 items.
1500 passed and 0 failed.

-- 
 Ned Deily,
 n...@acm.org

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Re: parsing an Excel formula with the re module

[MRAB]

Mensanator wrote:

On Jan 5, 12:35 pm, MRAB  wrote:

vsoler wrote:

Hello,
I am acessing an Excel file by means of Win 32 COM technology.
For a given cell, I am able to read its formula. I want to make a map
of how cells reference one another, how different sheets reference one
another, how workbooks reference one another, etc.
Hence, I need to parse Excel formulas. Can I do it by means only of re
(regular expressions)?
I know that for simple formulas such as "=3*A7+5" it is indeed
possible. What about complex for formulas that include functions,
sheet names and possibly other *.xls files?
For example"=Book1!A5+8" should be parsed into ["=","Book1", "!",
"A5","+","8"]
Can anybody help? Any suggestions?

Do you mean "how" or do you really mean "whether", ie, get a list of the
other cells that are referred to by a certain cell, for example,
"=3*A7+5" should give ["A7"] and "=Book1!A5+8" should give ["Book1!A5]


Ok, although "Book1" would be the default name of a workbook, with
default
worksheets labeled "Sheet1". "Sheet2", etc.

If I had a worksheet named "Sheety" that wanted to reference a cell on
"Sheetx"
OF THE SAME WORKBOOK, it would be =Sheet2!A7. If the reference was to
a completely
different workbook (say Book1 with worksheets labeled "Sheet1",
"Sheet2") then
the cell might have =[Book1]Sheet1!A7.

And don't forget the $'s! You may see =[Book1]Sheet1!$A$7.


I forgot about the dollars! In that case, the regex is:

references = re.findall(r"\b((?:\w+!)?\$?[A-Za-z]+\$?\d+)\b", formula)
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Re: Exception as the primary error handling mechanism?

[Ben Finney]

r0g  writes:

> Of course I'm now guilty of pedantry too :/ I might have let it slip
> had you not started your reply with the word "No", that just p* me
> off.

Well, if being told “no” is going to piss you off, I think you're in for
a rough time.

> Having said that I find the mental image of you slamming your fist on
> the table and shouting it out loud whenever you read something you
> disagree with on usenet quite amusing!

In return, I find myself quite amused that you would get such an image.
To reduce the stress you describe above, you might want to read what is
written, rather than inventing fanciful emotion where it wasn't in the
message to begin with.

-- 
 \  “One time I went to a drive-in in a cab. The movie cost me |
  `\  ninety-five dollars.” —Steven Wright |
_o__)  |
Ben Finney
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Re: please help shrink this each_with_index() implementation

[Phlip]

On Jan 5, 1:10 pm, Antoine Pitrou  wrote:

> http://docs.python.org/library/functions.html
>
> Don't forget that the Python documentation is rich and structured.
> And good luck.

Does it say how to convert a string containing either an integer
representation, or something alphabetic, into an integer, or a zero,
in like 1 method call? (No except: ?)

Nothing personal, but I'm finding the super-hard stuff very facile &
tractable, and the easy stuff absurdly hard around here...
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Re: Python multiprocessing: Permission denied

[Steven D'Aprano]

On Tue, 05 Jan 2010 13:52:18 -0800, t0ster wrote:

> It looks like the user don't have permission to access shared memory.
> When executing with root privileges it works fine.
> 
> Is there any solution to run it as normal user(not root)?

Then give the user permission to access shared memory.

Why do you expect that Python would be able to over-ride the operating 
system's security? This problem is no different from saying "It looks 
like the user doesn't have permission to access this file".


-- 
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Re: Exception as the primary error handling mechanism?

[Steven D'Aprano]

On Wed, 06 Jan 2010 09:39:08 +1100, Ben Finney wrote:

> r0g  writes:
> 
>> Of course I'm now guilty of pedantry too :/ I might have let it slip
>> had you not started your reply with the word "No", that just p* me
>> off.
> 
> Well, if being told “no” is going to piss you off, I think you're in for
> a rough time.

Oh, you're in trouble now! If you think he gets upset at being told no, 
you should see how upset he gets at being told he's in for a rough time!!!

*wink*



-- 
Steven
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Re: please help shrink this each_with_index() implementation

[Steven D'Aprano]

On Tue, 05 Jan 2010 14:40:49 -0800, Phlip wrote:

> On Jan 5, 1:10 pm, Antoine Pitrou  wrote:
> 
>> http://docs.python.org/library/functions.html
>>
>> Don't forget that the Python documentation is rich and structured. And
>> good luck.
> 
> Does it say how to convert a string containing either an integer
> representation, or something alphabetic, into an integer, or a zero, in
> like 1 method call? (No except: ?)

If you mean something like this:

>>> int('153')
153

then yes it does. But if you mean something like this:


>>> some_mysterious_function('one hundred and fifty-three')
153

then no, it doesn't.



> Nothing personal, but I'm finding the super-hard stuff very facile &
> tractable, and the easy stuff absurdly hard around here...


Then perhaps you should work through the tutorial to learn the basics.



-- 
Steven
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Re: please help shrink this each_with_index() implementation

[Phlip]

> > Does it say how to convert a string containing either an integer
> > representation, or something alphabetic, into an integer, or a zero, in
> > like 1 method call? (No except: ?)
>
> If you mean something like this:
>
> >>> int('153')
>
> 153

The point: int('') or int('something') both throw an error. In
general, this is hand-holding, but in specific I don't think the "rich
and structured" documentation will cover how to beat a 0 out of it in
less than 3 lines. So I will persist in my idiotic questions here!

> Then perhaps you should work through the tutorial to learn the basics.

They will tell me how to use except: (which is a good example why a
program should not use exceptions for its normal control flow if at
all possible).

Please, please, please save your newbie admonitions for those who
qualify!
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subprocess.Popen does not close pipe in an error case

[Steven K. Wong]

Below, I have a Python script that launches 2 child programs, prog1
and prog2, with prog1's stdout connected to prog2's stdin via a pipe.
(It's like executing "prog1 | prog2" in the shell.)

If both child programs exit with 0, then the script runs to
completion. But if prog2 exits with non-0, prog1 does not exit and the
script hangs (i.e. prog1.poll() always returns None) -- unless I
uncomment the 2 lines marked by XXX to close prog1.stdout.

I was expecting that I don't have to explicitly close prog1.stdout,
whether prog2 succeeds or fails. Is the current behavior a bug in the
subprocess module or is it expected? Or am I doing something wrong?

Thanks.

import subprocess
import time

# prog1: a program that writes lots of data to the pipe
cmd = ['zcat', '--force', 'a_large_file']
prog1 = subprocess.Popen(cmd, bufsize=-1, stdout=subprocess.PIPE)

# prog2: a program that fails without reading much data from the pipe
cmd = ['python', '-c', 'import time; time.sleep(10); asdf']
prog2 = subprocess.Popen(cmd, bufsize=-1, stdin=prog1.stdout,
stdout=open('popen.out', 'w'))
print 'waiting for a while'

retCodeProg2 = prog2.wait()
print 'prog2 returns', retCodeProg2
# XXX
# if retCodeProg2 != 0:
# prog1.stdout.close()
while prog1.poll() is None:
print 'sleep a bit'
time.sleep(1)
retCodeProg1 = prog1.poll()
print 'prog1 returns', retCodeProg1
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Re: Exception as the primary error handling mechanism?

[Phlip]

Peng Yu wrote:

> Otherwise, could some python expert explain to me why exception is
> widely used for error handling in python? Is it because the efficiency
> is not the primary goal of python?

It's not about efficiency, it's about making assumptions for the
programmer about what kind of rigor they need.

Why can't int('nonnumeric') return None?

Why can't a Django Record.objects.get(pk=-1) return a None? That's
what it's for.

(A related question - why can't I just go 'if record = method():  use
(record)'. Why extra lines just to trap and assign the variable before
using it?)

There are workarounds that sometimes benefit the code. In the case of
collections, like recordsets, you might be better off using for ... all
():

Then your controlled block efficiently does not happen if it saw no
records. "Efficiently" in terms of programmer complexity - the number
and meaning of lines that a programmer must comprehend.

And why can't Record.objects.get(pk='nonnumeric') return None?
Because, of course, deep inside it calls int(). I can't simplify the
calling code, and rely on garbage-in-None-out, because Python decided
which simplifications I should avoid with self-righteous indignation.

The Samurai Principle (return victorious, or not at all) is very
useful, sometimes. But other times it just prematurely depletes your
supply of Samurai...

--
  Phlip
  http://zeekland.zeroplayer.com/
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Re: subprocess.Popen does not close pipe in an error case

[Steven K. Wong]

BTW, I'm using Python 2.6.2 on Linux.
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Re: please help shrink this each_with_index() implementation

[Mackrackit]

On Jan 5, 2010, at 4:30 PM, Phlip  wrote:





The point: int('') or int('something') both throw an error. In
general, this is hand-holding, but in specific I don't think the "rich
and structured" documentation will cover how to beat a 0 out of it in
less than 3 lines. So I will persist in my idiotic questions here!



What does an extra two or three lines of code matter?

Dave-- 
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Re: Exception as the primary error handling mechanism?

[r0g]

Steven D'Aprano wrote:
> On Wed, 06 Jan 2010 09:39:08 +1100, Ben Finney wrote:
> 
>> r0g  writes:
>>
>>> Of course I'm now guilty of pedantry too :/ I might have let it slip
>>> had you not started your reply with the word "No", that just p* me
>>> off.
>> Well, if being told “no” is going to piss you off, I think you're in for
>> a rough time.
> 
> Oh, you're in trouble now! If you think he gets upset at being told no, 
> you should see how upset he gets at being told he's in for a rough time!!!
> 
> *wink*
> 
> 
> 


NO! It's a rude way to start a sentence don't you think? Just because
you're correcting someone doesn't mean you have to be combative and try
and make them feel small. Unless they've adopted a hostile or wilfully
ignorant tone there's no reason to be so brusqe with people. You can be
both nice AND terse you  know.

I can't imagine why I expect good manners on usenet though, AFAICT it's
never been like that (well not since I got on it anyway).

Roger.


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Re: Exception as the primary error handling mechanism?

[Chris Rebert]

On Tue, Jan 5, 2010 at 3:51 PM, Phlip  wrote:
> Peng Yu wrote:
>> Otherwise, could some python expert explain to me why exception is
>> widely used for error handling in python? Is it because the efficiency
>> is not the primary goal of python?
>
> It's not about efficiency, it's about making assumptions for the
> programmer about what kind of rigor they need.
>
> Why can't int('nonnumeric') return None?

Errors should never pass silently.
Unless explicitly silenced.
-- The Zen of Python (http://www.python.org/dev/peps/pep-0020/)

Better to throw an exception and ensure the case is specifically dealt
with one way or another than to silently return an error flag result
which may only delay the error until later in the program, making it
harder to debug. Is it that much of a burden to write and use the
small function that does what you want?

def int_or_None(string):
try:
return int(string)
except ValueError:
return None

Heck, you can even write it inline and dispense with the function if you want:

try: foo = int(bar)
except ValueError: foo = None

Quibbling over a mere one more line of code (or writing one short
function) seems a bit petty.

> (A related question - why can't I just go 'if record = method():  use
> (record)'. Why extra lines just to trap and assign the variable before
> using it?)

I believe that's disallowed so as to prevent the subtle bugs seen in C
code which result from when someone makes a typo and omits the second
"=" in their `if foo == bar():` test.

Cheers,
Chris
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Re: Exception as the primary error handling mechanism?

[r0g]

Lie Ryan wrote:
> On 1/6/2010 1:48 AM, r0g wrote:
>> Steven D'Aprano wrote:
>>> On Tue, 05 Jan 2010 13:06:20 +, r0g wrote:
 If
 that's the case how can you expect it to validate anything at all in
 production?
>>>
>>> The asserts still operate so long as you don't use the -O switch.
>>>

> checking, since the function relies on the caller obeying the
> contract[2] and never calling it with an invalid input.
> 
> DbC uses assertions[1] spuriously, unlike the traditional approach which
> is much more conservative when using assertions.
> 
> [1] or explicit language support which is just syntax sugar for assertions
> [2] of course, on a debug release, the contract validation code will
> still be enforced to catch logic/consistency bugs that causes the violation


Thanks for the responses Steven/Dave/Lie, that's some really insightful
stuff :)

Roger.
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Re: Exception as the primary error handling mechanism?

[Phlip]

On Jan 5, 5:01 pm, Chris Rebert  wrote:

> > Why can't int('nonnumeric') return None?
>
> Errors should never pass silently.

You are saying I, as the programmer, cannot decide what is an error
and what is a pass-thru. The decision is made for me. (Yes yes I can
write int_or_None(), etc...)

Here's a super easy example:

  { 42: 'forty two' }.get(41, None)

Because I can supply a default, I can decide what is an error and what
is .

Now the equivalent in a language that does not enjoy this false "Zen":

  { 42: 'forty two' }[41]  # returns None
  { 42: 'forty two' }.fetch(41, None)  # ibid
  { 42: 'forty two' }.fetch(41)  # raises an exception

The quicky validation is available if I _request_ it.

> Quibbling over a mere one more line of code (or writing one short
> function) seems a bit petty.

Because that "Zen of Python" is an empty sophistry that forces me to
add a "mere one more line of code" over and over again...

> > (A related question - why can't I just go 'if record = method():  use
> > (record)'. Why extra lines just to trap and assign the variable before
> > using it?)
>
> I believe that's disallowed so as to prevent the subtle bugs seen in C
> code which result from when someone makes a typo and omits the second
> "=" in their `if foo == bar():` test.

Don't prevent me from using a technique just because others had
trouble with it.

And if bar() == foo is the superior technique anyway, because the ==
happens in chronological and lexical order after the bar() call.
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Re: unittest inconsistent

[Phlip]

On Jan 5, 4:14 pm, Matt Haggard  wrote:
> Can anyone tell me why this test fails?
>
> http://pastebin.com/f20039b17
>
> This is a minimal example of a much more complex thing I'm trying to
> do.  I'm trying to hijack a function and inspect the args passed to it
> by another function.
>
> The reason the 'Tester' object has no attribute 'arg1' is because
> "self" still refers to the object made for testA.

I hope someone else can spot the low-level reason...

...but why aren't you using http://pypi.python.org/pypi/mock/ ? Look
up its patch_object facility...
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Re: twenty years ago Guido created Python

[Daniel Fetchinson]

>>> Python is a truly awesome programming language.  Not only is Guido a
>>> genius language designer, but he is also a great project leader.  What
>>> an accomplishment.  Congratulations to everybody who has contributed
>>> to Python in the last two decades!
>>
>> The more languages you learn before getting to Smalltalk, the more
>> awesome Smalltalk will be for you.
>>
> After implementing SmallTalk I more or less gave up OO programming for
> ten years,

But why? What was so frightening about the OO model of SmallTalk?

Cheers,
Daniel

> resuming it only after I met Python. SmallTalk didn't seem
> that awesome to me.
>
> Though for its time it was an incredible system, its insistence on a
> SmallTalk-only VM environment seemed a little solipsistic.


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Re: Exception as the primary error handling mechanism?

[Phlip]

> Errors should never pass silently.
> Unless explicitly silenced.
> -- The Zen of Python (http://www.python.org/dev/peps/pep-0020/)

"The person who says it cannot be done should never interrupt the
person doing it"
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