From 01b731fc2b04744a11e32f93aba8bfb9ddb3dd29 Mon Sep 17 00:00:00 2001
From: Raymond Hettinger <rhettinger@users.noreply.github.com>
Date: Thu, 5 Apr 2018 11:19:57 -0700
Subject: bpo-33224: PEP 479 fix for difflib.mdiff() (GH-6381)

---
 Lib/difflib.py                                       | 20 ++++++++++++--------
 Lib/test/test_difflib.py                             |  8 ++++++++
 .../Library/2018-04-04-23-41-30.bpo-33224.pyR0jB.rst |  2 ++
 3 files changed, 22 insertions(+), 8 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2018-04-04-23-41-30.bpo-33224.pyR0jB.rst

diff --git a/Lib/difflib.py b/Lib/difflib.py
index 043a169..887c3c2 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -1634,14 +1634,18 @@ 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
-            while(lines_to_write):
-                from_line, to_line, found_diff = next(line_pair_iterator)
-                # If another change within the context, extend the context
-                if found_diff:
-                    lines_to_write = context-1
-                else:
-                    lines_to_write -= 1
-                yield from_line, to_line, found_diff
+            try:
+                while(lines_to_write):
+                    from_line, to_line, found_diff = next(line_pair_iterator)
+                    # If another change within the context, extend the context
+                    if found_diff:
+                        lines_to_write = context-1
+                    else:
+                        lines_to_write -= 1
+                    yield from_line, to_line, found_diff
+            except StopIteration:
+                # Catch exception from next() and return normally
+                return
 
 
 _file_template = """
diff --git a/Lib/test/test_difflib.py b/Lib/test/test_difflib.py
index aaefe6d..745ccbd 100644
--- a/Lib/test/test_difflib.py
+++ b/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.
diff --git a/Misc/NEWS.d/next/Library/2018-04-04-23-41-30.bpo-33224.pyR0jB.rst b/Misc/NEWS.d/next/Library/2018-04-04-23-41-30.bpo-33224.pyR0jB.rst
new file mode 100644
index 0000000..87ff100
--- /dev/null
+++ b/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.
-- 
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