bpo-41681: Fix for f-string/str.format error description when using 2 , in format specifier

Lib/test/test_format.py

@@ -495,6 +495,10 @@ def test_g_format_has_no_trailing_zeros(self):
         self.assertEqual(format(12300050.0, ".6g"), "1.23e+07")
         self.assertEqual(format(12300050.0, "#.6g"), "1.23000e+07")
 
+    def test_with_two_commas_in_format_specifier(self):
+        error_msg = "Cannot specify ',' with ','."
+        with self.assertRaisesRegex(ValueError, error_msg):
+            '{:,,}'.format(1)
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_fstring.py

@@ -1198,6 +1198,10 @@ def test_invalid_syntax_error_message(self):
         with self.assertRaisesRegex(SyntaxError, "f-string: invalid syntax"):
             compile("f'{a $ b}'", "?", "exec")
 
+    def test_with_two_commas_in_format_specifier(self):
+        error_msg = "Cannot specify ',' with ','."
+        with self.assertRaisesRegex(ValueError, error_msg):
+            f'{1:,,}'
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core and Builtins/2020-08-31-17-49-02.bpo-41681.3-VJiH.rst

@@ -0,0 +1,2 @@
+Fixes the wrong error description in the error raised by using 2 `,` in
+format string in f-string and :meth:`str.format`.

Python/formatter_unicode.c

@@ -252,8 +252,10 @@ parse_internal_render_format_spec(PyObject *format_spec,
         ++pos;
     }
     if (end-pos && READ_spec(pos) == ',') {
-        invalid_comma_and_underscore();
-        return 0;
+        if (format->thousands_separators == LT_UNDERSCORE_LOCALE) {
+            invalid_comma_and_underscore();
+            return 0;
+        }
     }
 
     /* Parse field precision */