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

Start by reproducing the failure at exec(source, ChainMap()) with the class A example on CPython 3.12, then inspect the class-body name lookup path. Compare behavior with a real dict and review linked PR gh-121389; done means the outer name is available inside the class body without regressing existing exec behavior.