gh-121676: Raise a ``DeprecationWarning`` if the Python implementation of ``functools.reduce`` is called with `function` or `sequence` as a keyword args (#121677)

Python implementation of `functools` allows calling `reduce`
with `function` or `sequence` as keyword args. This doesn't
match behavior of our C accelerator and our documentation
for `functools.reduce` states that `function`and `sequence`
are positional-only arguments.

Now calling a Python implementation of `functools.reduce`
with `function` or `sequence` as keyword args would raise
a `DeprecationWarning` and is planned to be prohibited in
Python 3.16.


Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
index 4a0252c..3222486 100644
--- a/Lib/test/test_functools.py
+++ b/Lib/test/test_functools.py
@@ -1045,6 +1045,12 @@ 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: