Greetings and welcome Malcolm,
>hey folks - I've been a long time perl programmer and only recently
>tried my hand a python, so it's probable that these questions are
>non-sensical in this context but for the moment I'm trying to stay
>afloat
Although I wrote Python first, I spent years writing in both Python
and Perl, so I may be able to offer a tip or two.
OK, so on to your questions. I'll paste my examples from an
interactive Python3 shell. I hope you have already found that you
can simply run an interactive interpreter:
$ python3
Python 3.4.1 (default, May 23 2014, 17:48:28) [GCC] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
>I've been dabbling a bit with some lists and trying to work out how best
>to abitrarily sort and filter these. Perl has a number of operators that
>help with this in map(), grep() and sort() as follows:
>
> @raw = (2, 1, 4, 3);
> @grepped = grep { $_ >= 3 } @raw; # (4, 3)
> @mapped = map { $_ + 1 } @raw; # (3, 2, 5, 4)
> @sorted = sort { $a > $b } @raw; # (1, 2, 3, 4)
>
>in this case:
>
>grep() will return all list items for which the code block returns true
Use 'filter' in Python.
https://docs.python.org/3/library/functions.html#filter
Vaguely Perl-like invocation:
>>> list(filter(lambda x: x >= 3, [2, 1, 4, 3]))
[4, 3]
Old-school Pythonic way:
>>> def gt3(x):
... return x >= 3
>>> list(filter(gt3, [2, 1, 4, 3]))
[4, 3]
New-school Pythonic way:
>>> [x for x in [2, 1, 4, 3] if x >= 3]
[4, 3]
[But, also, please see my readability remarks below on the
immediately preceding example.]
>map() will return all list items as modified by the code block
Use 'map' in Python, too.
https://docs.python.org/3/library/functions.html#map
Vaguely Perl-like invocation:
>>> list(map(lambda x: x + 1, [2, 1, 4, 3]))
[3, 2, 5, 4]
Old-school Pythonic way:
>>> def plus1(x):
... return x + 1
>>> list(map(plus1, [2, 1, 4, 3]))
[3, 2, 5, 4]
New-school Pythonic way (which you already discovered):
>>> [x+1 for x in [2, 1, 4, 3]]
[3, 2, 5, 4]
>sort() will return a sorted list of items, using the code block to
>compare them (where $a and $b represent two items to be compared)
Use 'sorted' in Python.
https://docs.python.org/3/library/functions.html#sorted
Or, sort a list in place with the method .sort().
https://docs.python.org/3/library/stdtypes.html#list.sort
What's the difference, you ask? I'll generate a list of random
integers and then show you:
>>> import random
>>> raw = [random.randint(0, 100) for _ in range(10)]
>>> raw
[91, 79, 4, 89, 17, 83, 89, 50, 74, 71]
>>> new = sorted(raw)
>>> new
[4, 17, 50, 71, 74, 79, 83, 89, 89, 91]
>>> raw
[91, 79, 4, 89, 17, 83, 89, 50, 74, 71]
>>> raw.sort()
>>> raw
[4, 17, 50, 71, 74, 79, 83, 89, 89, 91]
You may have gotten accustomed to writing your own function (or a
block) for each call to Perl's sort. This technique is far less
common in Python.
The notion of the Schwartzian transform in Perl is referred to be a
more general name in Python-lands. It's called the
Decorate-Sort-Undecorate (DSU) technique. Much more often, though,
you will see a developer pass key= to the sort() method or the
sorted() function.
>so - I've been able to at least work out the map() case above with a
>list comprehension
>
> raw = [2, 1, 4, 3]
> mapped = [ x + 1 for x in raw] # [3, 2, 5, 4]
Yes! Another way to do it. You are using something called a list
comprehension. (You see I used it to generate random numbers.)
This is a wonderful idiomatic technique to use in Python.
I mentioned that I would remark about readability. When writing in
Python, readability matters an awful lot to the majority of Python
programmers. So, I refer again to my example:
[x for x in [2, 1, 4, 3] if x >= 3]
The 'if' condition is pretty far away from the action, and as such,
I generally, don't like to use an 'if' condition like this in a more
complex list comprehension. It takes me longer to figure out when I
read it later. So, I would make this into two lines, or use a
different approach entirely.
l = [2, 1, 4, 3]
[x for x in l if x >= 3]
>and I know that .sorted() would do what I want in this limited
>example, but I'm after the ability to put abitrary code in here to
>determine sort order or test an item for filtering (because the
>items they're testing may be complex structures rather than these
>simple integers, for example)
You definitely want to read this:
https://docs.python.org/3/howto/sorting.html
>these seem so useful things to want to do that I'd imagine they're
>probably a basic part of the language, but so far I've not seen
>anything that might cover them with the exeption of map() as above
>- I am slowly trawling my way through Learning Python (5ed) so I
>might yet get to something related, I don't know
Python's sort() is pretty darned good. Additionally, there's plenty
of flexibility with the key parameter. Here's an example. I'm
going to create a dead-simple object using a module you probably
haven't heard of before, but hopefully, this is enough like a dict()
[a Perl hash) or an object (did you use the grafted-on OO in Perl?)
that it'll make sense.
Now, I'll omit the interactive shell prompts (for easier pasting).
If I run this code to create a Pilot and store it in 'p' ...
from collections import namedtuple
Pilot = namedtuple('Pilot', ['surname', 'age', 'biplane'])
p = Pilot('Vernon', 32, 'Pander')
Then, p looks like this (if I were to print it):
Pilot(surname='Vernon', age=32, biplane='Pander')
So, now, I'll take a few of these Pilot()s and put them in a list
and show you how you can sort based on any of the attributes of the
Pilot:
aces = list()
aces.append(Pilot('Vernon', 32, 'Pander'))
aces.append(Pilot('Ehringhaus', 41, 'Curtiss'))
aces.append(Pilot('Wilkins', 28, 'Sopwith'))
aces.append(Pilot('Tessler', 37, 'de Havilland'))
Our flying aces are lined up! Which ones are wearing blue?
aces.sort(key=attrgetter('surname'))
Now the order would look like this:
[Pilot(surname='Ehringhaus', age=41, biplane='Curtiss'),
Pilot(surname='Tessler', age=37, biplane='de Havilland'),
Pilot(surname='Vernon', age=32, biplane='Pander'),
Pilot(surname='Wilkins', age=28, biplane='Sopwith')]
But, that's no good... I really wanted to order them by plane:
aces.sort(key=attrgetter('surname'))
And now:
[Pilot(surname='Ehringhaus', age=41, biplane='Curtiss'),
Pilot(surname='Vernon', age=32, biplane='Pander'),
Pilot(surname='Wilkins', age=28, biplane='Sopwith'),
Pilot(surname='Tessler', age=37, biplane='de Havilland')]
But, when choosing who gets to drive off the airfield first, it's always done
by age:
aces.sort(key=attrgetter('age'), reverse=True)
So, the story ends:
[Pilot(surname='Ehringhaus', age=41, biplane='Curtiss'),
Pilot(surname='Tessler', age=37, biplane='de Havilland'),
Pilot(surname='Vernon', age=32, biplane='Pander'),
Pilot(surname='Wilkins', age=28, biplane='Sopwith')]
I will make one other note about thinking in Perl vs. thinking in Python.
(This is something that I found strange and occasionally quite convenient in
Perl.)
Perl will flatten your lists when you pass them into functions. If you pass
two lists into a function.
&some_func(@a, @b)
In some_func, there's just a list of arguments, and all distinction between
the two lists is lost. That's why I saw (and wrote) much more often:
&some_func(\@a, \@b)
In Python, the list is not flattened (thank goodness). It's just like any
other variable. It may contain any type, basic, derived, class, function,
whatever. I do not know if other people have found this list behaviour
surprising when writing in both languages, but I do like the Python approach
better on the list handling.
The standard library contains a wider assortment of tools than the Perl
standard library, so when you are looking for a toolkit to handle something,
have a peek in the standard library list of modules; it may already be there.
https://docs.python.org/3/py-modindex.html
If not, check out PyPI (the Python equivalent of CPAN):
https://pypi.python.org/PyPI
Best of luck, Malcom, and welcome to the world of Python where everything is a
first class object.
-Martin
import pprint
from operator import attrgetter
from collections import namedtuple
Pilot = namedtuple('Pilot', ['surname', 'age', 'biplane'])
aces = list()
aces.append(Pilot('Vernon', 32, 'Pander'))
aces.append(Pilot('Ehringhaus', 41, 'Curtiss'))
aces.append(Pilot('Wilkins', 28, 'Sopwith'))
aces.append(Pilot('Tessler', 37, 'de Havilland'))
l = sorted(aces, key=attrgetter('surname'))
pprint.pprint(l)
l = sorted(aces, key=attrgetter('age'))
pprint.pprint(l)
l = sorted(aces, key=attrgetter('biplane'))
pprint.pprint(l)
--
Martin A. Brown
http://linux-ip.net/
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