bpo-40612: Fix SyntaxError edge cases in traceback module

Author: gvanrossum

Lib/test/test_traceback.py

@@ -58,13 +58,13 @@ def test_caret(self):
                                         SyntaxError)
         self.assertIn("^", err[2]) # third line has caret
         self.assertEqual(err[2].count('\n'), 1)   # and no additional newline
-        self.assertEqual(err[1].find("+"), err[2].find("^"))  # in the right place
+        self.assertEqual(err[1].find("+") + 1, err[2].find("^"))  # in the right place
 
         err = self.get_exception_format(self.syntax_error_with_caret_non_ascii,
                                         SyntaxError)
         self.assertIn("^", err[2]) # third line has caret
         self.assertEqual(err[2].count('\n'), 1)   # and no additional newline
-        self.assertEqual(err[1].find("+"), err[2].find("^"))  # in the right place
+        self.assertEqual(err[1].find("+") + 1, err[2].find("^"))  # in the right place
 
     def test_nocaret(self):
         exc = SyntaxError("error", ("x.py", 23, None, "bad syntax"))
@@ -78,7 +78,7 @@ def test_bad_indentation(self):
         self.assertEqual(len(err), 4)
         self.assertEqual(err[1].strip(), "print(2)")
         self.assertIn("^", err[2])
-        self.assertEqual(err[1].find(")"), err[2].find("^"))
+        self.assertEqual(err[1].find(")") + 1, err[2].find("^"))
 
         err = self.get_exception_format(self.syntax_error_bad_indentation2,
                                         IndentationError)
@@ -656,7 +656,7 @@ def outer_raise():
         self.assertIn('inner_raise() # Marker', blocks[2])
         self.check_zero_div(blocks[2])
 
-    @support.skip_if_new_parser("Pegen is arguably better here, so no need to fix this")
+    @unittest.skipIf(support.use_old_parser(), "Pegen is arguably better here, so no need to fix this")
     def test_syntax_error_offset_at_eol(self):
         # See #10186.
         def e():
@@ -666,7 +666,7 @@ def e():
         def e():
             exec("x = 5 | 4 |")
         msg = self.get_report(e).splitlines()
-        self.assertEqual(msg[-2], '              ^')
+        self.assertEqual(msg[-2], '               ^')
 
     def test_message_none(self):
         # A message that looks like "None" should not be treated specially

Lib/traceback.py

@@ -569,9 +569,12 @@ def format_exception_only(self):
 
         if not issubclass(self.exc_type, SyntaxError):
             yield _format_final_exc_line(stype, self._str)
-            return
+        else:
+            yield from self._format_syntax_error(stype)
 
-        # It was a syntax error; show exactly where the problem was found.
+    def _format_syntax_error(self, stype):
+        """Format SyntaxError exceptions (internal helper)."""
+        # Show exactly where the problem was found.
         filename = self.filename or "<string>"
         lineno = str(self.lineno) or '?'
         yield '  File "{}", line {}\n'.format(filename, lineno)
@@ -580,12 +583,13 @@ def format_exception_only(self):
         offset = self.offset
         if badline is not None:
             yield '    {}\n'.format(badline.strip())
-            if offset is not None:
+            if offset is not None and offset >= 1:
                 caretspace = badline.rstrip('\n')
-                offset = min(len(caretspace), offset) - 1
+                # Convert to 0-based offset, and clip to text length
+                offset = min(len(caretspace), offset - 1)
                 caretspace = caretspace[:offset].lstrip()
                 # non-space whitespace (likes tabs) must be kept for alignment
-                caretspace = ((c.isspace() and c or ' ') for c in caretspace)
+                caretspace = ((c if c.isspace() else ' ') for c in caretspace)
                 yield '    {}^\n'.format(''.join(caretspace))
         msg = self.msg or "<no detail available>"
         yield "{}: {}\n".format(stype, msg)