Patch #1678339: test case for bug in difflib.

Lib/test/outstanding_bugs.py

@@ -10,13 +10,44 @@ import unittest
 from test import test_support
 
 #
-# No test cases for outstanding bugs at the moment.
+# One test case for outstanding bugs at the moment:
 #
 
+class TestDifflibLongestMatch(unittest.TestCase):
+    # From Patch #1678339:
+    # The find_longest_match method in the difflib's SequenceMatcher has a bug.
+
+    # The bug is in turn caused by a problem with creating a b2j mapping which
+    # should contain a list of indices for each of the list elements in b.
+    # However, when the b2j mapping is being created (this is being done in
+    # __chain_b method in the SequenceMatcher) the mapping becomes broken. The
+    # cause of this is that for the frequently used elements the list of indices
+    # is removed and the element is being enlisted in the populardict mapping.
+
+    # The test case tries to match two strings like:
+    # abbbbbb.... and ...bbbbbbc
+
+    # The number of b is equal and the find_longest_match should have returned
+    # the proper amount. However, in case the number of "b"s is large enough, the
+    # method reports that the length of the longest common substring is 0. It
+    # simply can't find it.
+
+    # A bug was raised some time ago on this matter. It's ID is 1528074.
+
+    def test_find_longest_match(self):
+        import difflib
+        for i in (190, 200, 210):
+            text1 = "a" + "b"*i
+            text2 = "b"*i + "c"
+            m = difflib.SequenceMatcher(None, text1, text2)
+            (aptr, bptr, l) = m.find_longest_match(0, len(text1), 0, len(text2))
+            self.assertEquals(i, l)
+            self.assertEquals(aptr, 1)
+            self.assertEquals(bptr, 0)
+ 
 
 def test_main():
-    #test_support.run_unittest()
-    pass
+    test_support.run_unittest(TestDifflibLongestMatch)
 
 if __name__ == "__main__":
     test_main()