PEP 584: Add info regarding Mapping/MutableMapping (#1331)

Also add a note about other stdlib mappings having these added.

pep-0584.rst

@@ -21,6 +21,11 @@ Abstract
 This PEP proposes adding merge (``|``) and update (``|=``) operators
 to the built-in ``dict`` class.
 
+.. note::
+   After this PEP was accepted, the decision was made to also
+   implement the new operators for `several other standard library
+   mappings <https://bugs.python.org/issue36144>`_.
+
 
 ==========
 Motivation
@@ -421,6 +426,30 @@ seen wins", however, has the advantage of consistency with other dict
 operations (and the proposed union operators).
 
 
+----------------------------------------------
+What About ``Mapping`` And ``MutableMapping``?
+----------------------------------------------
+
+``collections.abc.Mapping`` and ``collections.abc.MutableMapping``
+should define ``|`` and ``|=``, so subclasses could just inherit the
+new operators instead of having to define them.
+
+
+''''''''
+Response
+''''''''
+
+There are two primary reasons why adding the new operators to these
+classes would be problematic:
+
+- Currently, neither defines a ``copy`` method, which would be
+  necessary for ``|`` to create a new instance.
+
+- Adding ``|=`` to ``MutableMapping`` (or a ``copy`` method to
+  ``Mapping``) would create compatibility issues for virtual
+  subclasses.
+
+
 ==============
 Rejected Ideas
 ==============