Why does list comprehension in my Python Interactive shell append a list of Nones?

Question

I'm testing some Django functionality in my interactive shell

Here's my attempt to probe these objects, note the list of Nones at the end

>>> [print(foo) for foo in CharacterSkillLink.objects.all() if foo.speciality]
Streetwise (Street Countdown) Roran
[None]

And with a more orthodox list comprehension:

>>> [print(foo) for foo in range(1,10)]
1
2
3
4
5
6
7
8
9
[None, None, None, None, None, None, None, None, None]

Nine Nones, all in a row.

Why am I getting that?

[Is it Pythonic to use list comprehensions for just side effects?](http://stackoverflow.com/questions/5753597/is-it-pythonic-to-use-list-comprehensions-for-just-side-effects) — Ashwini Chaudhary, Feb 11 '15 at 09:36
Since everybody already said why it's behaving like that, I'll hint how to print it: `print('\n'.join([str(foo) for foo in range(1,10)]))` — Igor Hatarist, Feb 11 '15 at 09:36
@AshwiniChaudhary yup, unpacking a range would be more appropriate given py3k :) I just showed how to do it with a list comprehension — Igor Hatarist, Feb 11 '15 at 09:39
@AshwiniChaudhary thanks for those nuggets of information, in the python world, I'd barely call myself a milk snake. — AncientSwordRage, Feb 11 '15 at 09:40
@IgorHatarist but I'm guessing I couldn't unpack a range with an `if` statement in there, right? — AncientSwordRage, Feb 11 '15 at 09:42
@Pureferret Right. You can use [`filter()`](https://docs.python.org/3/library/functions.html#filter) for that, but it'll grow quickly and will look pretty ugly. Ashwini has provided a great relevant question about side effects :) — Igor Hatarist, Feb 11 '15 at 09:44

score 6 · Accepted Answer · answered Feb 11 '15 at 09:35

6

Because print returns a value, namely None. What it prints and what it returns are two different things.

answered Feb 11 '15 at 09:35

André Laszlo

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thiruvenkadam · Answer 2 · 2015-02-11T09:53:13.127

1

This is because, you use Python 3.x, in which print function returns None after printing to the console and thus you are getting this output. Whereas, if you have used Python 2.x, you will correctly get a SyntaxError for the print function.

A better example will be this(in python 2.x as your example wont work in python 2.x)

>>> b = []
>>> [b.append(i) for i in range(10)]
...[None, None, None, None, None, None, None, None, None, None]
>>> print b
...[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If you wish to print things AND add it to the list, it should be like this:

[(print(foo) or foo) for foo in CharacterSkillLink.objects.all() if foo.speciality]

However, in my view, refrain from using such things as things might get ugly after some period of time.

edited Feb 11 '15 at 09:53

answered Feb 11 '15 at 09:43

thiruvenkadam

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It's not a statement anymore, it is a function now.(Python 3) – Ashwini Chaudhary Feb 11 '15 at 09:50
My bad :-) will change the answer to reflect that :-D – thiruvenkadam Feb 11 '15 at 09:50

Why does list comprehension in my Python Interactive shell append a list of Nones?

2 Answers2