Untokenize: An logically incorrect assert tested user input validity.

Replace it with correct logic that raises ValueError for bad input.
Issues #8478 and #12691 reported the incorrect logic.
Add an Untokenize test case and an initial test method.

1 files changed, 3 insertions, 1 deletions

diff --git a/Lib/tokenize.py b/Lib/tokenize.py
index 294bf9a068..c156450d04 100644
--- a/Lib/tokenize.py
+++ b/Lib/tokenize.py
@@ -229,7 +229,9 @@ class Untokenizer:
 
     def add_whitespace(self, start):
         row, col = start
-        assert row <= self.prev_row
+        if row < self.prev_row or row == self.prev_row and col < self.prev_col:
+            raise ValueError("start ({},{}) precedes previous end ({},{})"
+                             .format(row, col, self.prev_row, self.prev_col))
         col_offset = col - self.prev_col
         if col_offset:
             self.tokens.append(" " * col_offset)