missing global names inside a class body (calling `exec` with a `ChainMap`)

Bug report

Bug description:

I need to implement layered context running python code, so I choose ChainMap. But Python needs every globals passed to exec to be a dict, so I mixins it into ChainMap.

This patch runs well as far, but in a class it can't get the value from the outside.

I think this may because in the class context CPython uses some function like PyDict_Get instead of __getitem__, so the values didn't get copied into the class's globals context.

This bug caused this code fails:

from collections import ChainMap

class ChainMap(ChainMap, dict):  # globals must be a real dict
    pass

source = """
a = 1

class A:
    print(a)
"""

exec(source, ChainMap())  # this line raises NameError

CPython versions tested on:

3.12

Operating systems tested on:

Linux, Windows

Linked PRs

• gh-121389

Comece reproduzindo a falha em exec(source, ChainMap()) com o exemplo da classe A no CPython 3.12 e, em seguida, inspecione o caminho de resolução de nomes do corpo da classe. Compare o comportamento com um dict real e analise o PR vinculado gh-121389; o trabalho estará concluído quando o nome externo estiver disponível dentro do corpo da classe sem regredir o comportamento existente de exec.