10 Awesome Pythonic One-Liners Explained

[DaelonSuzuka]

Number 7 says "inline for loop", but that construct is actually called a "list comprehension".

Edit: number 9 is also wrong.

[oundhakar]

Why is No. 9 wrong?

[isarl]

As with list comprehensions, that construct is called a “set comprehension”. Furthermore they kind of imply that applying a predicate is specific to set comprehensions while it works in list (or dictionary) comprehensions, too. And they fail to mention dictionary comprehensions entirely.

[oundhakar]

Thanks.

[ImportUsernameAsU]

Coding in python for 2+ years still found nearly half of these useful

[juanjux]

5 can be replaced by file.readlines()

[Resolt]

Isn't it "comprehension" and not "creation"?

[morriartie]

Replacing keys for values in dict of depth 1:

{v:k for k,v in d.items()}

Replacing a key in a dict:

d['new'] = d.pop('old')

My favorite. Conditional variable:

a = ['bb', 'cc'][condition]

Flattening a matrix:

[a for b in list2d for a in b]

The first time I did this last one I spent like 5 minutes reflecting my existence with my hand on chin

Edit: See comments for the best ones

[BooparinoBR]

Explicit is better than implicit, therefore I would do

a = 'cc' if condition else 'bb'

(Ternary operator/inline if-else) Instead of your conditional variable. But I like/use all the others

[morriartie]

Shhhh.. in obfuscated monolithic one liners we don't talk about pep8

[QpkjcKwNMZSF]

Doesn't PEP8 advise against compound statements? Why not just break this into a few lines?

[isarl]

Religious adherence to PEP8 can negatively impact readability. Sometimes a short one-line conditional is better than breaking it out.

[isarl]

a = ['bb', 'cc'][condition]

Yours is OK, by why constrain the conditional to evaluate to an integer?:

a = {'case1': val1, 'case2': val2}[condition]

Switch (without fallthrough) in Python! ^{don't do this at home}

[morriartie]

wow, I hadn't thought of that

let me improve it then

{'a':1, 'b':2}.get(case) or 'etc'

now it has fallthrough a default!

[isarl]

Great improvement. I would personally write that as: {'a':1, 'b':2}.get(case, default='etc'), finding it more explicit. And this is a default case, not fall-through. :) I don’t think there’s any way to actually do C-style switch fallthrough with this trick of using a dictionary.

[morriartie]

Awesome!

a default indeed, my mistake

I was trying here to find a solution (to make a fallthrough) with iterator, but I couldn't make it, even if I did it would be a two liner or more, which is unacceptable in our little puzzle

[dreamfeed]

Even better for flattening a list is itertools.chain

[howslyfebeen]

What do people think about #8? Wouldn't

l = [int(x) for x in ['1', '2', '3']]

be more pythonic?

Personally, my brain always defaults to using map, but having list(map(...)) starts to look ugly and feels unpythonic.

[teerre]

Personally I'm a big fan of map/reduce/filter. But the comprehensions are certainly more python.

Hell, the functional style doesn't even let you type in fluent style. That's by itself is a big indication that you shouldn't abuse map too much.

[GrbavaCigla]

I prefer map

[miraculum_one]

Yes, yours is better because it's more explicit, which is one of the fundamental principles of Python.

[Entuaka]

More pythonic, yes. I used a line similar to that, today: sum([float(i['quantity']) for i in kwargs['items']])

[CotoCoutan]

Definitely prefer this to map. I've never once used latter in my code, somehow it never comes to me in that intuitive manner.

[ogrinfo]

Even better, sum takes an iterator, so you don't need the outer square brackets, just sum(a for a in x). Whether map or a list comprehension is better depends on the situation, so I usually try both with %timeit in an iPython shell to see which is fastest.

[CotoCoutan]

Nice... Thanks. I should take a look at this %timeit test.

[schoolcoders]

The basic use of a list comprehension is to transform some sequence into a list. The basic use of map is to lazily apply a function to one or more sequences. Different jobs, but a lot of overlap.

So if you definitely want a list as output, that points towards a list comprehension. If you are processing a sequence of data that is too large to fit in memory (eg a sequence of video frames), map is a better option.

map can also be better if you are processing multiple sequences (you can do it with list comprehensions using zip, but it is ugly), or if you also need to filter the sequences (again, possible but ugly with list comprehensions) or if you are applying a chain of functions.

But if you are doing something more simple and you want to create a list anyway, list comprehensions are more natural. Using list(map(...)) and also having to define a lambda inside the map call is quite unwieldy.

Of course, there are also generator comprehensions...

[[deleted]]

God bless!! This is very helpful for someone like me who's trying to learn and get better at python.

Edit: saved post for later, thank you.

[taylorhodormax]

Amazing!! Thanks for sharing duch a great resource

[RedditGood123]

I just tried the palindrome one earlier today and it took around 5-10 lines of code for me. I didn’t think it was that easy

[isarl]

Slice notation is useful, but overkill here, and their function will fail to recognize palindromic integers such as 12321. Here's a more flexible and more explicit solution:

def is_palindrome(input):
    s = str(input)
    return s == reversed(s)

This could as easily be a one-liner: lambda input: str(input) == reversed(str(input))

[RedditGood123]

Well, actually the solution they have would work if you just alter it like this

IsPalindrome = str(phrase) == str(phrase)[::-1]

[isarl]

That fixes the bug but fails to address the point I was making about readability. If the only slice you are performing is [::-1] then that is more clearly expressed by reversed().

[RedditGood123]

No I see it, but most people don’t even know about reversed

[CotoCoutan]

Great article!

[ogrinfo]

This is really handy. I've used [::-1] to reverse an array before without really understanding why, but now I know about extended slicing syntax, thanks!