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Re: Nested class doesn't see class scope

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On Tue, Jul 5, 2016 at 5:40 AM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Tuesday 05 July 2016 14:41, Ian Kelly wrote:
>
>> 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.
>
>
> So, like nested functions in Python before "from __future__ import
> nested_scopes".

In Python 3.4+, the code for a class body does participate in
closures. The CPython compiler implements this using the
LOAD_CLASSDEREF instruction. However, classes don't create closures
and default to storing to the locals dict (as class attributes),
unless a name is declared global or nonlocal. Obviously writing to a
global or nonlocal won't create a class attribute. For example:

    def f():
        y = 0
        class C:
            global x
            nonlocal y
            x = 1
            y = 2
            z = 3
        return types.SimpleNamespace(**locals())

    >>> ns = f()
    >>> x
    1
    >>> ns.y
    2
    >>> ns.C.z
    3
    >>> sorted(vars(ns.C))
    ['__dict__', '__doc__', '__module__', '__weakref__', 'z']

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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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