bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381)

Lib/difflib.py

@@ -1634,6 +1634,7 @@ def _mdiff(fromlines, tolines, context=None, linejunk=None,
                 lines_to_write -= 1
             # Now yield the context lines after the change
             lines_to_write = context-1
+            try:
                 while(lines_to_write):
                     from_line, to_line, found_diff = next(line_pair_iterator)
                     # If another change within the context, extend the context
@@ -1642,6 +1643,9 @@ def _mdiff(fromlines, tolines, context=None, linejunk=None,
                     else:
                         lines_to_write -= 1
                     yield from_line, to_line, found_diff
+            except StopIteration:
+                # Catch exception from next() and return normally
+                return
 
 
 _file_template = """

Lib/test/test_difflib.py

@@ -93,6 +93,14 @@ class TestSFbugs(unittest.TestCase):
         self.assertEqual("+ \t\tI am a bug", diff[2])
         self.assertEqual("? +\n", diff[3])
 
+    def test_mdiff_catch_stop_iteration(self):
+        # Issue #33224
+        self.assertEqual(
+            list(difflib._mdiff(["2"], ["3"], 1)),
+            [((1, '\x00-2\x01'), (1, '\x00+3\x01'), True)],
+        )
+
+
 patch914575_from1 = """
    1. Beautiful is beTTer than ugly.
    2. Explicit is better than implicit.

Misc/NEWS.d/next/Library/2018-04-04-23-41-30.bpo-33224.pyR0jB.rst

@@ -0,0 +1,2 @@
+Update difflib.mdiff() for PEP 479.  Convert an uncaught StopIteration in a
+generator into a return-statement.