[3.13] Align functools.reduce() docstring with PEP-257 (GH-126045) (#126113)

Yak-shave in preparation for Argument Clinic adaption in gh-125999.

(cherry picked from commit 9b14083)

Co-authored-by: Sergey B Kirpichev <[email protected]>

Author: miss-islington

Lib/functools.py

@@ -239,12 +239,14 @@ def reduce(function, sequence, initial=_initial_missing):
     """
     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
-    value.  For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates
-    ((((1+2)+3)+4)+5).  If 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.
+    Apply a function of two arguments cumulatively to the items of an iterable, from left to right.
+
+    This effectively reduces the iterable to a single value.  If 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.
+
+    For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])
+    calculates ((((1 + 2) + 3) + 4) + 5).
     """
 
     it = iter(sequence)

Modules/_functoolsmodule.c

@@ -780,12 +780,14 @@ functools_reduce(PyObject *self, PyObject *args)
 PyDoc_STRVAR(functools_reduce_doc,
 "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\
-value.  For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates\n\
-((((1+2)+3)+4)+5).  If initial is present, it is placed before the items\n\
-of the iterable in the calculation, and serves as a default when the\n\
-iterable is empty.");
+Apply a function of two arguments cumulatively to the items of an iterable, from left to right.\n\
+\n\
+This effectively reduces the iterable to a single value.  If initial is present,\n\
+it is placed before the items of the iterable in the calculation, and serves as\n\
+a default when the iterable is empty.\n\
+\n\
+For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])\n\
+calculates ((((1 + 2) + 3) + 4) + 5).");
 
 /* lru_cache object **********************************************************/