gh-92810: Add What's New entry

Author: dolfinus

Doc/whatsnew/3.15.rst

@@ -413,6 +413,29 @@ arguments (:pep:`791`).
 Improved modules
 ================
 
+abc
+---
+
+* Reduce memory usage of :func:`issubclass` checks for :class:`abc.ABCMeta` subclasses.
+
+  Unlike regular classes, there subclass checks can lead to upstream calls like this:
+
+  ``issubclass(Some, Child)`` -> ``issubclass(Some, Parent)`` -> ``issubclass(Some, Top)``
+  (stops here)
+
+  :class:`abc.ABCMeta` subclasses may also trigger downstream checks:
+
+  ``issubclass(Some, Child)`` -> ``issubclass(Some, Parent)`` -> ``issubclass(Some, Top)``
+  (nothing found) -> ``issubclass(Some, Child.__subclasses__)`` -> ``issubclass(Some, ...)``
+
+  This could lead to memory bloat in large ABC class tree (thousands of subclasses).
+
+  Most of classes don't override ``__subclasses`` method, so this check is redundant.
+  Now :meth:`!abc.ABCMeta.__new__` checks if method is present, and if not, skips
+  recursive downstream checks in :func:`issubclass`.
+
+  (Contributed by Maxim Martynov in :gh:`92810`.)
+
 argparse
 --------
 

Misc/NEWS.d/next/Library/2025-11-07-01-36-39.gh-issue-92810.H765mk.rst

@@ -0,0 +1,2 @@
+Reduce memory usage by :meth:`~type.__subclasscheck__` for
+:class:`abc.ABCMeta` with large class trees.