Add backcompat and ref implementation sections (#5)

python · JelleZijlstra · Mar 14, 2023 · Nov 7, 2022 · Jan 12, 2023 · Jan 12, 2023
commit a1bfab3851f44b812a8d2a33eee71c02a456a600
@@ -241,6 +241,27 @@ Discussion:
     f(b)  # Runtime error: int + str
 
 
+Backwards Compatibility
+=======================
+
+This PEP changes the rules for how ``TypedDict`` behaves (allowing subclasses to
+inherit from ``TypedMapping`` protocols in a way that changes the resulting
+overloads), so code that inspects ``TypedDict`` types will have to change. This
+is expected to mainly affect type-checkers.
+
+The ``TypedMapping`` type will be added to the ``typing_extensions`` module,
+enabling its use in older versions of Python. Users wanting to inherit from
+both ``TypedDict`` and ``TypedMapping`` will need to use
+``typing_extensions.TypedDict`` in these versions, as the ``typing`` declaration
+of ``TypedDict`` does an ``is`` check on the metaclass of all base classes.
+
+
+Reference Implementation
+========================
+
+No reference implementation exists yet.
+
+
 Rejected Alternatives
 =====================