_find_width_index and _handle_long_word change

Lib/test/test_textwrap.py

@@ -1118,5 +1118,18 @@ def test_shorten_placeholder(self):
                            text_len=self.text_len)
 
 
+class ZeroWidthTestCase(BaseTestCase):
+    def text_len(self, text):
+        return sum(
+            0 if c == 'Q' else 1
+            for c in text
+        )
+
+    def test_zero_width_text_len(self):
+
+        text = "0QQ1234QQ56789"
+        self.check_wrap(text, 6, ["0QQ1234QQ5", "6789"], text_len=self.text_len)
+
+
 if __name__ == '__main__':
     unittest.main()

Lib/textwrap.py

@@ -199,6 +199,23 @@ def _fix_sentence_endings(self, chunks):
             else:
                 i += 1
 
+    def _find_width_index(self, text, width):
+        """_find_width_index(text : string, width: int)
+
+        Find at which index the text has the required width.
+        """
+        # In most cases text_len will just use the number of characters, so this heuristic prevents calculating width
+        # for each character
+        if self.text_len(text[:width]) == width:
+            # For character widths greater than one, width can be more than the number of characters
+            return min(width, len(text))
+        cur_text = ''
+        for i, c in enumerate(text):
+            cur_text += c
+            cur_width = self.text_len(cur_text)
+            if cur_width >= width:
+                return i+1
+
     def _handle_long_word(self, reversed_chunks, cur_line, cur_len, width):
         """_handle_long_word(chunks : [string],
                              cur_line : [string],
@@ -217,12 +234,12 @@ def _handle_long_word(self, reversed_chunks, cur_line, cur_len, width):
         # If we're allowed to break long words, then do so: put as much
         # of the next chunk onto the current line as will fit.
         if self.break_long_words:
-            end = space_left
             chunk = reversed_chunks[-1]
+            end = self._find_width_index(chunk, space_left)
             if self.break_on_hyphens and self.text_len(chunk) > space_left:
                 # break after last hyphen, but only if there are
                 # non-hyphens before it
-                hyphen = chunk.rfind('-', 0, space_left)
+                hyphen = chunk.rfind('-', 0, end)
                 if hyphen > 0 and any(c != '-' for c in chunk[:hyphen]):
                     end = hyphen + 1
             cur_line.append(chunk[:end])