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

首先，在 CPython 3.12 上使用类 A 示例重现 exec(source, ChainMap()) 处的失败，然后检查类体名称查找路径。将其行为与真正的 dict 进行比较，并审查链接的 PR gh-121389；完成的标准是外部名称可在类体内使用，同时不回归现有的 exec 行为。