pythongh-74690: Optimise isinstance() checks for nominal subclasses…

… of runtime-checkable protocols
AlexWaygood · Mar 30, 2023 · 694960d · 694960d
1 parent ecc5441
commit 694960d
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Showing 2 changed files with 6 additions and 1 deletion.
diff --git a/Lib/typing.py b/Lib/typing.py
@@ -1998,6 +1998,8 @@ class _ProtocolMeta(ABCMeta):
     # This metaclass is really unfortunate and exists only because of
     # the lack of __instancehook__.
     def __instancecheck__(cls, instance):
+        if super().__instancecheck__(instance):
+            return True
         # We need this method for situations where attributes are
         # assigned in __init__.
         if (
@@ -2019,7 +2021,7 @@ def __instancecheck__(cls, instance):
                      getattr(instance, attr) is not None)
                     for attr in _get_protocol_attrs(cls)):
                 return True
-        return super().__instancecheck__(instance)
+        return False
 
 
 class Protocol(Generic, metaclass=_ProtocolMeta):

diff --git a/Misc/NEWS.d/next/Library/2023-03-30-23-31-41.gh-issue-74690.DeUPLy.rst b/Misc/NEWS.d/next/Library/2023-03-30-23-31-41.gh-issue-74690.DeUPLy.rst
@@ -0,0 +1,3 @@
+Speed up :func:`isinstance` checks against :func:`runtime-checkable
+protocols <typing.runtime_checkable>` by 20x for nominal subclasses of the
+protocol. Patch by Alex Waygood.