gh-102757: fix function signature mismatch for functools.reduce between code and documentation

Doc/library/functools.rst

@@ -403,25 +403,27 @@ The :mod:`functools` module defines the following functions:
    .. versionadded:: 3.4
 
 
-.. function:: reduce(function, iterable[, initializer])
+.. function:: reduce(function, iterable[, initial], /)
 
    Apply *function* of two arguments cumulatively to the items of *iterable*, from
    left to right, so as to reduce the iterable to a single value.  For example,
    ``reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])`` calculates ``((((1+2)+3)+4)+5)``.
    The left argument, *x*, is the accumulated value and the right argument, *y*, is
-   the update value from the *iterable*.  If the optional *initializer* is present,
+   the update value from the *iterable*.  If the optional *initial* is present,
    it is placed before the items of the iterable in the calculation, and serves as
-   a default when the iterable is empty.  If *initializer* is not given and
+   a default when the iterable is empty.  If *initial* is not given and
    *iterable* contains only one item, the first item is returned.
 
    Roughly equivalent to::
 
-      def reduce(function, iterable, initializer=None):
+      initial_missing = object()
+
+      def reduce(function, iterable, initial=initial_missing, /):
           it = iter(iterable)
-          if initializer is None:
+          if initial is initial_missing:
               value = next(it)
           else:
-              value = initializer
+              value = initial
           for element in it:
               value = function(value, element)
           return value

Lib/functools.py

@@ -236,7 +236,7 @@ def __ge__(self, other):
 
 def reduce(function, sequence, initial=_initial_missing):
     """
-    reduce(function, iterable[, initial]) -> value
+    reduce(function, iterable[, initial], /) -> value
 
     Apply a function of two arguments cumulatively to the items of a sequence
     or iterable, from left to right, so as to reduce the iterable to a single

Misc/NEWS.d/next/Documentation/2023-03-16-15-39-26.gh-issue-102759.ehpHw6.rst

@@ -0,0 +1,2 @@
+Align function signature for ``functools.reduce`` in documentation and docstring
+with the C implementation.

Modules/_functoolsmodule.c

@@ -725,7 +725,7 @@ functools_reduce(PyObject *self, PyObject *args)
 }
 
 PyDoc_STRVAR(functools_reduce_doc,
-"reduce(function, iterable[, initial]) -> value\n\
+"reduce(function, iterable[, initial], /) -> value\n\
 \n\
 Apply a function of two arguments cumulatively to the items of a sequence\n\
 or iterable, from left to right, so as to reduce the iterable to a single\n\