gh-131490: Clarify defaultdict keyword argument behavior in docs

Doc/library/collections.rst

@@ -727,6 +727,15 @@ stack manipulations such as ``dup``, ``drop``, ``swap``, ``over``, ``pick``,
     as if they were passed to the :class:`dict` constructor, including keyword
     arguments.
 
+    Note that this means ``defaultdict(default_factory=list)`` (for example),
+    will create a dict with a key of ``'default_factory'`` and a value of
+    :class:`list`. Instead, call ``defaultdict(list)``.
+
+        >>> defaultdict(default_factory=list)
+        defaultdict(None, {'default_factory': <class 'list'>})
+        >>> defaultdict(list)
+        defaultdict(<class 'list'>, {})
+        >>>
 
     :class:`defaultdict` objects support the following method in addition to the
     standard :class:`dict` operations:

Modules/_collectionsmodule.c

@@ -2476,6 +2476,10 @@ a new value when a key is not present, in __getitem__ only.\n\
 A defaultdict compares equal to a dict with the same items.\n\
 All remaining arguments are treated the same as if they were\n\
 passed to the dict constructor, including keyword arguments.\n\
+\n\
+Note that this means defaultdict(default_factory=list) (for\n\
+example), will create a dict with a key of 'default_factory'\n\
+and a value of <class 'list'>. Instead, call defaultdict(list).\n\
 ");
 
 /* See comment in xxsubtype.c */