Johannes Bauer <[email protected]> writes:
> I stumbled about something that I cannot quite explain while doing some
> stupid naming of variables in my code, in particular using "collections"
> as an identifier. However, what results is strange. I've created a
> minimal example. Consider this:
>
> import collections
>
> class Test(object):
> def __init__(self):
> z = {
> "y": collections.defaultdict(list),
This mention of collections refers to ...
> }
> for (_, collections) in z.items():
... this local variable.
> pass
>
> Test()
The same thing would happen in a plain function:
import collections
def f():
z = { "y": collections.defaultdict(list) }
for (_, collections) in z.items():
pass
f()
> In my opinion, this should run. However, this is what happens on Python
> 3.6.3 (default, Oct 3 2017, 21:45:48) [GCC 7.2.0] on linux):
>
> Traceback (most recent call last):
> File "x.py", line 11, in <module>
> Test()
> File "x.py", line 6, in __init__
> "y": collections.defaultdict(list),
> UnboundLocalError: local variable 'collections' referenced before assignment
>
> Interestingly, when I remove the class:
The significant change is removing the function that creates a local scope.
> import collections
This introduces "collections" as a global ...
> z = {
> "y": collections.defaultdict(list),
> }
> for (_, collections) in z.items():
... and this uses that global ...
> pass
>
> It works as expected (doesn't throw).
... except now collections is bound to a list (from the for) and no
longer refers to the module.
--
Ben.
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