Issue #21827: Fixed textwrap.dedent() for the case when largest common

whitespace is a substring of smallest leading whitespace.
Based on patch by Robert Li.

Lib/test/test_textwrap.py

@@ -748,6 +748,11 @@ def foo():
         expect = "hello there\n  how are you?"
         self.assertEqual(expect, dedent(text))
 
+        # test margin is smaller than smallest indent
+        text = "  \thello there\n   \thow are you?\n \tI'm fine, thanks"
+        expect = " \thello there\n  \thow are you?\n\tI'm fine, thanks"
+        self.assertEqual(expect, dedent(text))
+
 
 # Test textwrap.indent
 class IndentTestCase(unittest.TestCase):

Lib/textwrap.py

@@ -444,11 +444,15 @@ def dedent(text):
         elif margin.startswith(indent):
             margin = indent
 
-        # Current line and previous winner have no common whitespace:
-        # there is no margin.
+        # Find the largest common whitespace between current line and previous
+        # winner.
         else:
-            margin = ""
+            for i, (x, y) in enumerate(zip(margin, indent)):
+                if x != y:
+                    margin = margin[:i]
                     break
+            else:
+                margin = margin[:len(indent)]
 
     # sanity check (testing/debugging only)
     if 0 and margin:

Misc/ACKS

@@ -847,6 +847,7 @@ Mark Levitt
 Ivan Levkivskyi
 William Lewis
 Akira Li
+Robert Li
 Xuanji Li
 Robert van Liere
 Ross Light

Misc/NEWS

@@ -45,6 +45,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #21827: Fixed textwrap.dedent() for the case when largest common
+  whitespace is a substring of smallest leading whitespace.
+  Based on patch by Robert Li.
+
 - Issue #25447: The lru_cache() wrapper objects now can be copied and pickled
   (by returning the original object unchanged).