bpo-43914: Highlight invalid ranges in SyntaxErrors (#25525)

To improve the user experience understanding what part of the error messages associated with SyntaxErrors is wrong, we can highlight the whole error range and not only place the caret at the first character. In this way:

>>> foo(x, z for z in range(10), t, w)
  File "<stdin>", line 1
    foo(x, z for z in range(10), t, w)
           ^
SyntaxError: Generator expression must be parenthesized

becomes

>>> foo(x, z for z in range(10), t, w)
  File "<stdin>", line 1
    foo(x, z for z in range(10), t, w)
           ^^^^^^^^^^^^^^^^^^^^
SyntaxError: Generator expression must be parenthesized

1 files changed, 5 insertions, 5 deletions

diff --git a/Lib/test/test_cmd_line_script.py b/Lib/test/test_cmd_line_script.py
index 2e0b5f72c22..af29c171d42 100644
--- a/Lib/test/test_cmd_line_script.py
+++ b/Lib/test/test_cmd_line_script.py
@@ -601,7 +601,7 @@ class CmdLineTest(unittest.TestCase):
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), 'ascii').read()
             # Confirm that the caret is located under the '=' sign
-            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
+            self.assertIn("\n    ^^^^^\n", text)
 
     def test_syntaxerror_indented_caret_position(self):
         script = textwrap.dedent("""\
@@ -612,8 +612,8 @@ class CmdLineTest(unittest.TestCase):
             script_name = _make_test_script(script_dir, 'script', script)
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), 'ascii').read()
-            # Confirm that the caret is located under the first 1 character
-            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
+            # Confirm that the caret starts under the first 1 character
+            self.assertIn("\n    1 + 1 = 2\n    ^^^^^\n", text)
 
             # Try the same with a form feed at the start of the indented line
             script = (
@@ -624,7 +624,7 @@ class CmdLineTest(unittest.TestCase):
             exitcode, stdout, stderr = assert_python_failure(script_name)
             text = io.TextIOWrapper(io.BytesIO(stderr), "ascii").read()
             self.assertNotIn("\f", text)
-            self.assertIn("\n    1 + 1 = 2\n          ^\n", text)
+            self.assertIn("\n    1 + 1 = 2\n    ^^^^^\n", text)
 
     def test_syntaxerror_multi_line_fstring(self):
         script = 'foo = f"""{}\nfoo"""\n'
@@ -650,7 +650,7 @@ class CmdLineTest(unittest.TestCase):
             self.assertEqual(
                 stderr.splitlines()[-3:],
                 [   b'    foo = """\\q"""',
-                    b'          ^',
+                    b'          ^^^^^^^^',
                     b'SyntaxError: invalid escape sequence \\q'
                 ],
             )