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bpo-40895: Update weakref documentation to remove old warnings (GH-20687) 

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)
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Daniel Fortunov 2020-06-10 21:26:49 +01:00 committed by GitHub
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@ -163,14 +163,6 @@ Extension types can easily be made to support weak references; see
application without adding attributes to those objects. This can be especially
useful with objects that override attribute accesses.
.. note::
Caution: Because a :class:`WeakKeyDictionary` is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a :class:`WeakKeyDictionary` because actions
performed by the program during iteration may cause items in the
dictionary to vanish "by magic" (as a side effect of garbage collection).
.. versionchanged:: 3.9
Added support for ``|`` and ``|=`` operators, specified in :pep:`584`.
@ -192,14 +184,6 @@ than needed.
Mapping class that references values weakly. Entries in the dictionary will be
discarded when no strong reference to the value exists any more.
.. note::
Caution: Because a :class:`WeakValueDictionary` is built on top of a Python
dictionary, it must not change size when iterating over it. This can be
difficult to ensure for a :class:`WeakValueDictionary` because actions performed
by the program during iteration may cause items in the dictionary to vanish "by
magic" (as a side effect of garbage collection).
.. versionchanged:: 3.9
Added support for ``|`` and ``|=`` operators, as specified in :pep:`584`.