`DynamicClassAttribute` drops explicitly provided empty docstrings

Bug report

Bug description:

types.DynamicClassAttribute loses an explicitly provided empty docstring. Its constructor assigns self.__doc__ = doc or fget.__doc__, so when you pass doc='' and fget is None, the descriptor ends up with None.__doc__ (“The type of the None singleton.”) instead of retaining the empty string. This breaks parity with property, which preserves an empty doc, and cascades through setter()/deleter().

from types import DynamicClassAttribute

attr = DynamicClassAttribute(fget=None, fset=None, fdel=None, doc='')
print(repr(attr.__doc__))  # Expected '', but prints 'The type of the None singleton.'

None

Easy Fix:

 class DynamicClassAttribute:
     def __init__(self, fget=None, fset=None, fdel=None, doc=None):
         self.fget = fget
         self.fset = fset
         self.fdel = fdel
-        self.__doc__ = doc or fget.__doc__
-        self.overwrite_doc = doc is None
+        if doc is None and fget is not None:
+            doc = fget.__doc__
+        self.__doc__ = doc
+        self.overwrite_doc = doc is None

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-140975
• gh-141242

Start at the DynamicClassAttribute constructor in CPython's types implementation and inspect how an explicitly provided empty docstring is handled when fget is None. Confirm that the descriptor and its setter()/deleter() results preserve '', matching property behavior, and check the linked PRs for work already underway.