gh-147957: pop items from UserDict in LIFO order

Lib/collections/__init__.py

@@ -1253,6 +1253,13 @@ def copy(self):
         c.update(self)
         return c
 
+
+    # This method has a default implementation in MutableMapping, but dict's
+    # equivalent is last-in, first-out instead of first-in, first-out.
+    def popitem(self):
+        return self.data.popitem()
+
+
     @classmethod
     def fromkeys(cls, iterable, value=None):
         d = cls()

Misc/NEWS.d/next/Library/2026-04-01-07-10-49.gh-issue-147957.QXf5Xx.rst

@@ -0,0 +1 @@
+Causes the ``popitem`` method for ``collections.UserDict`` to pop in last-in, first-out order rather than first-in, first-out. The former has been the default for ``dict`` instances since Python 3.7, so this change will make subclasses of ``UserDict`` behave more like ``dict``.