The doccumentation at https://docs.python.org/3.10/library/weakref.html cautions that the `WeakKeyDictionary` and `WeakValueDictionary` are susceptible to the problem of dictionary mutation during iteration.
These notes present the user with a problem that has no easy solution.
I dug into the implementation and found that fortunately, Antoine Pitrou already addressed this challenge (10 years ago!) by introducing an `_IterationGuard` context manager to the implementation, which delays mutation while an iteration is in progress.
I asked for confirmation and @pitrou agreed that these notes could be removed:
c1baa601e2 (commitcomment-39514438)
Reflecting changes to the code, removed weakref.ReferenceError from weakref.rst and exceptions.rst.
Issue submitter provided evidence that the `weakref.ReferenceError` alias for `ReferenceError` was removed from the code in 2007. Working with @gvanrossum at PyCascades CPython sprint we looked at the code and confirmed that `weakref.ReferenceError` was no longer in `weakref.py`.
Based on that analysis I removed references `weakref.ReferenceError` from the two documents where it was still being referenced: `weakref.rst` and `exceptions.rst`.
https://bugs.python.org/issue38374
Also add a missing word to gc entry, and delete a now-obsolete doc note
in the weakref __callback__ docs. (Opened an issue for rewriting
the section that compares finalizers and __del__ method.)
weakref callbacks.
This is 2e446e87ac5b except that collections/__init__.py has been
modified to import proxy from _weakref instead of weakref. This
eliminates an import cycle which seems to cause a problem on Unix but
not Windows.
