gh-121676: Raise a DeprecationWarning if the Python implementation of functools.reduce is called with a keyword args

Doc/deprecations/pending-removal-in-3.16.rst

@@ -40,3 +40,8 @@ Pending Removal in Python 3.16
 
   * The undocumented and unused :attr:`!TarFile.tarfile` attribute
     has been deprecated since Python 3.13.
+
+* :mod:`functools`:
+
+  * Calling the Python implementation of :func:`functools.reduce` with *function*
+    or *sequence* as keyword arguments has been deprecated since Python 3.14.

Lib/functools.py

@@ -265,11 +265,6 @@ def reduce(function, sequence, initial=_initial_missing):
 
     return value
 
-try:
-    from _functools import reduce
-except ImportError:
-    pass
-
 
 ################################################################################
 ### partial() argument application
@@ -1122,3 +1117,26 @@ def __get__(self, instance, owner=None):
         return val
 
     __class_getitem__ = classmethod(GenericAlias)
+
+def _warn_kwargs(func):
+    @wraps(func)
+    def wrapper(*args, **kwargs):
+        if 'function' in kwargs or 'sequence' in kwargs:
+            import os
+            import warnings
+            warnings.warn(
+                'Calling functools.reduce with keyword arguments '
+                '"function" or "sequence" '
+                'is deprecated in Python 3.14 and will be '
+                'forbidden in Python 3.16.',
+                DeprecationWarning,
+                skip_file_prefixes=(os.path.dirname(__file__),))
+        return func(*args, **kwargs)
+    return wrapper
+
+reduce = _warn_kwargs(reduce)
+
+try:
+    from _functools import reduce
+except ImportError:
+    pass

Lib/test/test_functools.py

@@ -1015,6 +1015,12 @@ class TestReduceC(TestReduce, unittest.TestCase):
 class TestReducePy(TestReduce, unittest.TestCase):
     reduce = staticmethod(py_functools.reduce)
 
+    def test_reduce_with_kwargs(self):
+        with self.assertWarns(DeprecationWarning):
+            self.reduce(function=lambda x, y: x + y, sequence=[1, 2, 3, 4, 5], initial=1)
+        with self.assertWarns(DeprecationWarning):
+            self.reduce(lambda x, y: x + y, sequence=[1, 2, 3, 4, 5], initial=1)
+
 
 class TestCmpToKey:
 

Misc/NEWS.d/next/Library/2024-07-13-13-25-31.gh-issue-121676.KDLS11.rst

@@ -0,0 +1,3 @@
+Deprecate calling the Python implementation of :meth:`functools.reduce`
+with a ``function`` or ``sequence`` as a :term:`keyword argument`.
+This will be forbidden in Python 3.16 in order to match the C implementation.