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Re: Nested class doesn't see class scope

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On Mon, Jul 4, 2016 at 10:41 PM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Mon, Jul 4, 2016 at 9:20 PM, Steven D'Aprano <steve@pearwood.info> wrote:
>> I got this in Python 3.6:
>>
>>
>> py> class A:
>> ...     var = 999
>> ...     print(var)  # succeeds
>> ...     class B:
>> ...         x = var
>> ...
>> 999
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>>   File "<stdin>", line 3, in A
>>   File "<stdin>", line 4, in B
>> NameError: name 'var' is not defined
>>
>>
>> I expected that `var` would be available during the construction of B, just
>> as it was available inside A, but not to methods inside B. Obviously my
>> expectations are invalid. Can anyone explain the actual behaviour?
>
> Class definitions don't create closures like functions do. When Python
> executes a class definition, the metaclass creates a dict, and then
> the interpreter execs the class body using that dict as the locals.
> The body of class A has one locals dict, and the body of class B has a
> completely separate locals dict. The only way to share variables
> between them (prior to the class objects actually being constructed)
> is via globals.

Or I suppose one could write a metaclass that does something fancy
when creating the dicts.

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Thread

Nested class doesn't see class scope Steven D'Aprano <steve@pearwood.info> - 2016-07-05 13:20 +1000
  Re: Nested class doesn't see class scope Rustom Mody <rustompmody@gmail.com> - 2016-07-04 20:37 -0700
  Re: Nested class doesn't see class scope Paul Rubin <no.email@nospam.invalid> - 2016-07-04 20:42 -0700
    Re: Nested class doesn't see class scope Ian Kelly <ian.g.kelly@gmail.com> - 2016-07-04 22:37 -0600
  Re: Nested class doesn't see class scope Ian Kelly <ian.g.kelly@gmail.com> - 2016-07-04 22:41 -0600
    Re: Nested class doesn't see class scope Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-07-05 15:40 +1000
      Re: Nested class doesn't see class scope eryk sun <eryksun@gmail.com> - 2016-07-05 12:45 +0000
  Re: Nested class doesn't see class scope Ian Kelly <ian.g.kelly@gmail.com> - 2016-07-04 22:43 -0600

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