Issue 21635: Fix caching in difflib.SequenceMatcher.get_matching_blocks().

2 files changed, 11 insertions, 2 deletions

diff --git a/Lib/difflib.py b/Lib/difflib.py
index 38dfef4..7eb42a9 100644
--- a/Lib/difflib.py
+++ b/Lib/difflib.py
@@ -511,8 +511,8 @@ class SequenceMatcher:
             non_adjacent.append((i1, j1, k1))
 
         non_adjacent.append( (la, lb, 0) )
-        self.matching_blocks = non_adjacent
-        return map(Match._make, self.matching_blocks)
+        self.matching_blocks = list(map(Match._make, non_adjacent))
+        return self.matching_blocks
 
     def get_opcodes(self):
         """Return list of 5-tuples describing how to turn a into b.
diff --git a/Lib/test/test_difflib.py b/Lib/test/test_difflib.py
index 325449a..0ba8f0e 100644
--- a/Lib/test/test_difflib.py
+++ b/Lib/test/test_difflib.py
@@ -76,6 +76,15 @@ class TestSFbugs(unittest.TestCase):
         diff_gen = difflib.unified_diff([], [])
         self.assertRaises(StopIteration, next, diff_gen)
 
+    def test_matching_blocks_cache(self):
+        # Issue #21635
+        s = difflib.SequenceMatcher(None, "abxcd", "abcd")
+        first = s.get_matching_blocks()
+        second = s.get_matching_blocks()
+        self.assertEqual(second[0].size, 2)
+        self.assertEqual(second[1].size, 2)
+        self.assertEqual(second[2].size, 0)
+
     def test_added_tab_hint(self):
         # Check fix for bug #1488943
         diff = list(difflib.Differ().compare(["\tI am a buggy"],["\t\tI am a bug"]))