Surprising class definition scope behavior

Asked Jul 31 '17 at 16:36

Active Jul 31 '17 at 16:44

Viewed 39 times

Say I have some function:

def f(x):
    return x

...and some class definition using it, like so:

class Cgood():
    x = None
    y = f('foo')

This works fine:

>>> Cgood().y
'foo'

However, if I try to do something like this:

class Cbad():
    x = None
    y = [f(x) for _ in 'abcdefg']

We get an error:

NameError: name 'x' is not defined

Why is x not available in the scope of the list comprehension? Is there some other way to access x? The only way I see is ditching the list comp and resorting to a normal for loop.

EDIT: I am on Python 3.6.2

edited Jul 31 '17 at 16:44

jasonharper

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asked Jul 31 '17 at 16:36

Rick supports Monica

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List comprehension or `for` loop both have no own scope, so Cbad should be working like Cgood do. – Alex Pertsev Jul 31 '17 at 16:40
@A.Haaji I agree. Sadly, it doesn't. – Rick supports Monica Jul 31 '17 at 16:41
1

i just tried to run your example and i got `Out[3]: [None, None, None, None, None, None, None]` for `In [3]: Cbad.y` – Alex Pertsev Jul 31 '17 at 16:42
Isn't it like there must be an operation of the iterator variable on the left side of the for-in loop and not some other variable? – Aakash Verma Jul 31 '17 at 16:44
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Also check the explanation under: https://docs.python.org/3.6/reference/executionmodel.html#resolution-of-names – Ashwini Chaudhary Jul 31 '17 at 16:46
Good lord I swear I searched before posting this! Thanks for the link @AshwiniChaudhary. – Rick supports Monica Jul 31 '17 at 16:47
In Python 3, list comprehensions are now implemented with a function (which has its own scope) that is called to produce the list value. This makes them work the same as generator expressions, – Kallz Jul 31 '17 at 16:47

Surprising class definition scope behavior

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