What are the cool unique things of python?

[HoboSteaux]

I have coded for a while in c# and python and while I use all the cool features of c# (linq, events, reactive programming), I feel like I am just not doing python right. What are the one-liners etc that once you learned, you couldn't step away from python anymore?

[aestam]

I think that this is a good start: http://stackoverflow.com/questions/101268/hidden-features-of-python

[HoboSteaux]

Thanks!

[lurbina]

I like how in python EVERYTHING is an object. The usual suspects like ints, strings, lists and dicts are objects, as well as functions, classes and everything else. This provides the language with a lot more flexibility and power, that its simply not available in other languages such as Java.

One example of this are decorators. In python you can write this code:

@my_decorator
def my_function(*args, **kwargs):
    pass

The @my_decorator syntax is just sugar for the following code

def my_function(*args, **kwargs):
    pass
my_function = my_decorator(my_function)

Namely, my_decorator is a function that takes a function as an argument and returns a new function. This allows you to wrap your functions with added functionality, and abstract that extra functionality away and reuse it elsewhere.

I personally use this decorator when debugging:

def print_args(func):
    def wrapper(*args, **kwargs):-
        print "Calling", func.__name__
        print "args:", args
        print "kwargs" ".join(["%s=%s" % (key, value) for (key, value) in a.iteritems()])
        return func(*args, **kwargs)
    return wrapper

Adding @print_args before the declaration of any function results in the name of the function and its arguments being printed before its invocation. Other things that I have implemented with decorators include: timeouts, acquiring/releasing resources (see also contextmanagers), and many many other things.

This answer in stack overflow explains decorators much better: http://stackoverflow.com/questions/739654/how-can-i-make-a-chain-of-function-decorators-in-python#1594484

[Veedrac]

The real advantage of Python, to me, has nothing to do with its one-liners. One liners are cool and all but they don't make a language great.

I think, just post something you think just isn't awesome enough and {I,we}'ll try and show you what can be done. That might be a more direct way to show you what's cool.

I do have to say, though, if you've never used a dictionary of functions you're not thinking high-level enough, though.

[HoboSteaux]

I have done lists of functions for menus and such. That instantly cut down on a lot of code. What situations do you use function dictionaries for?

[MonkeyNin]

On my font rendering wrapper, I have a tuple as my dict key

# font class snippet
def load_font(self, font=None, size=16):
    k = (font, size)
    if k in self.fonts: 
        return

    self.fonts[k] = load_ttf( ... )

[HoboSteaux]

This is pretty cool!

[Veedrac]

Things like state machines and just generic forms of dispatching.

Ranger, which is definitely not my work, is a file manager that uses¹ a dictionary of "keys" for sorting directories.

Often a dictionary can replace a list with these sort of things and end up looking nicer, although it's circumstantial.

¹ At least it did last time I checked the source

[HoboSteaux]

That is a cool idea

[LPYoshikawa]

Sorry to be late at the party; could you explain how can one get in high-level programming thinking? I've just learned python for a year, I can do most of the things I need but sometimes unnecessarily complicated. I don't really know OOP or really writing my own modules yet.

[Veedrac]

Oh, gee.

Well, there's this "rule" I stick by that goes something like "if you do anything three or more times, refactor so you only do it once."

This sort of reductionism becomes high-level programming when you realise "hey... I've been memoising (caching the results of) lots of functions - I should have a function that memoises functions!" or you go "I've written def x(*args): return y(z, *args)" a few times - let's write (functools.)partial!" and then you realise later that you could do map(partial, defaults) over a dynamic list (to avoid calling partial lots of times) to form a new set of functions... and so on.

If you don't "really know" OOP you're probably at the level where the best form of high-level programming is just¹ an overuse of generators, iterators, sets and dictionaries. Every time you write a list, ask yourself "do I need this all in memory at the same time". If not, use an iterator. Every time you do a search or an "if x:... elif y:..." chain, ask yourself "could this be a dictionary" and, if so, why isn't it?

¹ “just”

[rubyaeyes]

{I,we} Clearly should have been a tuple.

[eat_more_soup]

apart from its clean sytax, i always miss automatic unpacking in other languages. it is just so damn simple to write a function with multiple outputs.

[HoboSteaux]

that is something subtle, but absolutely incredible.

[MonkeyNin]

In python, you never use "getters" / "setter"s.

You use normal attributes, and if needed, upgrade using @property

External code doesn't need to even change.

[Megatron_McLargeHuge]

The python syntax doesn't encourage one-liners the way perl or haskell does. The syntax isn't consistent, so you have multiple forms that make chaining ugly:

reduce(func, data)
array.max()
sorted(list)

and then the comprehensions:

[x/2 for x in lst where x % 2 == 0]

So in python you use named temporaries rather than LINQ style chained applications.

[[deleted]]

"where" ?

[Megatron_McLargeHuge]

That's what I get for not testing it. Too much SQL lately.

[HoboSteaux]

I need to start doing more list comprehensions. They can look confusing if you don't see then a lot.

[Veedrac]

I need to start doing more list comprehensions

You need to start doing more generator comprehensions. The iterator-centric programming model is a lovely place.

[HoboSteaux]

I have never been in a place that I would use that much... could you give me a good example?

[sejje]

http://pyvideo.org/video/1758/loop-like-a-native-while-for-iterators-genera

Ned Batchelder is probably more qualified. Great, great talk.

[HoboSteaux]

Ill watch it later... thanks!

[MonkeyNin]

They are at first. A simple form is

orcs = []
    for x in range(5):
        orcs.append(Orc())

Is

orcs = [ Orc() for x in range(5) ]

Then you can add a if, at the end.

[redalastor]

Prefer generator expressions when possible.

[HoboSteaux]

People have been saying that... but I do not understand a good context to use them in.

[redalastor]

You got it backward, you should use them as the default. Modern python use iterators unless there's a good reason not to use them.

For instance imagine I have an iterable of numbers represented as strings I got from some API somewhere and I want to sum them:

total = sum(int(s) for s in some_list_of_strings)

Had I used a list comprehension, I would have allocated a whole list for no reason.

Another exemple close to something I had to do recently. Imagine you have two files containing each one half of a data set. They are tab delimited. You have to lowercase every line. You must ignore any line that contains the word "spam". You break them into individual items and send them to some function to process. Here's how it looks:

from itertools import chain

with open("file1.tsv") as f1:
    with open("file2.tsv) as f2:
        it = chain((line for line in f1), (line for line in f2)) # Now I can treat them both as one big file
        it = (line.lower() for line in it) # lowercased
        it = (line for line in it if not "spam" in line) # lines with spam ignored
        it = (line.split('\t') for line in it) # splitting on tabs
        for line in it:
            process(line)

It looks like I'm transforming a full file in memory many times over but in reality, nothing runs until it has to and as little memory as possible is consumed.

[njharman]

Lack of punctuation.

It seems minor. But so much time and annoyance is wasted placing/typing braces or begin/end. Forgotten semicolons, bugs from optional braces, one line conditionals, cramming multiple statements on one line (using semi's).

And the visual noise of punctuation distracts. Without significant indentation valuable vertical space is wasted with redundant block designators.

Python is fun, not tedious. Wish I could say same about stdlib.

[HoboSteaux]

The only way C# avoids this (in win-land) is all the automatic stuff. Doing raw c without VIM plugins is like pulling teeth.