139

Think about a function that I'm calling for its side effects, not return values (like printing to screen, updating GUI, printing to a file, etc.).

def fun_with_side_effects(x): ...side effects... return y

Now, is it Pythonic to use list comprehensions to call this func:

[fun_with_side_effects(x) for x in y if (...conditions...)]

Note that I don't save the list anywhere

Or should I call this func like this:

for x in y: if (...conditions...): fun_with_side_effects(x)

Which is better and why?

Improve this question
6
  • 10
    This is an easy choice. Readability counts - do it the second way. If you can't fit 2 extra lines on your screen get a bigger monitor :) Commented Apr 22, 2011 at 9:18
  • 1
    The list comprehension is unpythonic since it violates "explicit is better than implicit" -- you're hiding a loop in a different construct. Commented Apr 22, 2011 at 9:42
  • 3
    @larsmans: if only GvR had realised that when he introduced list comprehensions in the first place! Commented Apr 22, 2011 at 13:21
  • 2
    @larsmans, Steve Jessop, I think it's incorrect to conceive a list comprehension as a loop. It may well be implemented as a loop, but the point of constructs like this is to operate on aggregate data in a functional and (conceptually) parallel way. If there's a problem with the syntax, it's that for ... in is used in both cases -- leading to questions like this one! Commented Apr 23, 2011 at 15:39
  • 2
    @senderle: I think it depends on the side-effects, though. If the side-effects just alter one element at a time, independently, then I think it's totally reasonable to use functional-style constructs for that in an imperative language, because it's not the loop flow-control that's important, it's the application to every element. If the side-effects are such that order matters, possibly then the "comprehension" abstraction is starting to leak. Whether it's leaking enough to matter is another question, though - nobody's pretending Python does lazy evaluation. Commented Apr 24, 2011 at 13:32

7 Answers 7

Reset to default
119

It is very anti-Pythonic to do so, and any seasoned Pythonista will give you hell over it. The intermediate list is thrown away after it is created, and it could potentially be very, very large, and therefore expensive to create.

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

49

You shouldn't use a list comprehension, because as people have said that will build a large temporary list that you don't need. The following two methods are equivalent:

consume(side_effects(x) for x in xs) for x in xs: side_effects(x)

with the definition of consume from the itertools man page:

def consume(iterator, n=None): "Advance the iterator n-steps ahead. If n is none, consume entirely." # Use functions that consume iterators at C speed. if n is None: # feed the entire iterator into a zero-length deque collections.deque(iterator, maxlen=0) else: # advance to the empty slice starting at position n next(islice(iterator, n, n), None)

Of course, the latter is clearer and easier to understand.

Improve this answer

3 Comments

@Paul: I think it should be. And indeed you can, though map may not be as intuitive if one hasn't done functional programming before.
Not sure this is especially idiomatic. There's no advantage over using the explicit loop.
Solution is consume = collections.deque(maxlen=0).extend
30

List comprehensions are for creating lists. And unless you are actually creating a list, you should not use list comprehensions.

So I would got for the second option, just iterating over the list and then call the function when the conditions apply.

Improve this answer

1 Comment

I would go even further and state that side effects inside a list comprehension are unusual, unexpected, and therefore evil, even if you're using the resulting list when you're done.
13

Second is better.

Think of the person who would need to understand your code. You can get bad karma easily with the first :)

You could go middle between the two by using filter(). Consider the example:

y=[1,2,3,4,5,6] def func(x): print "call with %r"%x for x in filter(lambda x: x>3, y): func(x)
Improve this answer

2 Comments

You don't even need filter. Just put a generator expression in parens here: for el in (x for x in y if x > 3):. el and x can have the same name, but that might confuse people.
Note that those two xs are in different scopes, which is bad practice.
5

Depends on your goal.

If you are trying to do some operation on each object in a list, the second approach should be adopted.

If you are trying to generate a list from another list, you may use list comprehension.

Explicit is better than implicit. Simple is better than complex. (Python Zen)

Improve this answer

1 Comment

The best answer, in principle. Syntax obeys to rules, so that one can answer "right", "wrong", "this way", "that way". Style obeys to other considerations, such as common sense, elegance, conciseness, effectiveness, efficiency, etc. Those depend on one's purposes, and even on one's taste.
-1

You can do

for z in (fun_with_side_effects(x) for x in y if (...conditions...)): pass

but it's not very pretty.

Improve this answer

Comments

-1

Using a list comprehension for its side effects is ugly, non-Pythonic, inefficient, and I wouldn't do it. I would use a for loop instead, because a for loop signals a procedural style in which side-effects are important.

But, if you absolutely insist on using a list comprehension for its side effects, you should avoid the inefficiency by using a generator expression instead. If you absolutely insist on this style, do one of these two:

any(fun_with_side_effects(x) and False for x in y if (...conditions...))

or:

all(fun_with_side_effects(x) or True for x in y if (...conditions...))

These are generator expressions, and they do not generate a random list that gets tossed out. I think the all form is perhaps slightly more clear, though I think both of them are confusing and shouldn't be used.

I think this is ugly and I wouldn't actually do it in code. But if you insist on implementing your loops in this fashion, that's how I would do it.

I tend to feel that list comprehensions and their ilk should signal an attempt to use something at least faintly resembling a functional style. Putting things with side effects that break that assumption will cause people to have to read your code more carefully, and I think that's a bad thing.

Improve this answer

4 Comments

What if fun_with_side_effects returns True?
I think this cure is worse than the disease - itertools.consume is much cleaner.
@PaulMcG - itertools.consume no longer exists, probably because using comprehensions with side-effects is ugly.
It turns out I was mistaken, and it never existed as a method in the stdlib. It is a recipe in the itertools docs: docs.python.org/3/library/…

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.