gh-161004: Align Python implementation of functools.reduce with C implementation

Doc/library/functools.rst

@@ -476,7 +476,11 @@ The :mod:`!functools` module defines the following functions:
               value = next(it)
           else:
               value = initial
-          for element in it:
+          while(True):
+              try:
+                  element = next(it)
+              except StopIteration:
+                  break
               value = function(value, element)
           return value
 

Lib/functools.py

@@ -259,9 +259,13 @@ def reduce(function, sequence, /, initial=_initial_missing):
     else:
         value = initial
 
-    for element in it:
+    while(True):
+        try:
+            element = next(it)
+        except StopIteration:
+            # Stop when no elements are left
+            break
         value = function(value, element)
-
     return value
 
 try:

Lib/test/test_reduce.py

@@ -0,0 +1,28 @@
+import unittest
+import operator
+import sys
+import functools as c_functools
+
+class Test_reduce(unittest.TestCase):
+    def test_add_func(self):
+        del sys.modules['functools']
+        sys.modules['_functools'] = None  # block the C accelerator
+        import functools as py_functools
+
+        class Iterator:
+            def __init__(self):
+                self.items = [1, 2, 3, 4, 4]
+            def __next__(self):
+                if not self.items:
+                    raise StopIteration
+                return self.items.pop(0)
+        class Iterable:
+            def __iter__(self):
+                return Iterator()
+        self.assertEqual(
+            c_functools.reduce(operator.add, Iterable()),
+            py_functools.reduce(operator.add, Iterable())
+        )
+
+if __name__ == "__main__":
+    unittest.main()