bpo-45716: Improve the error message when using True/False/None as keywords in a call (GH-29413)

Author: pablogsal

Grammar/python.gram

@@ -1062,6 +1062,8 @@ invalid_arguments:
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, asdl_seq_GET(b, b->size-1)->target, "Generator expression must be parenthesized") }
     | a=args ',' args { _PyPegen_arguments_parsing_error(p, a) }
 invalid_kwarg:
+    | a[Token*]=('True'|'False'|'None') b='=' {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "cannot assign to %s", PyBytes_AS_STRING(a->bytes)) }
     | a=NAME b='=' expression for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Maybe you meant '==' or ':=' instead of '='?")}
     | !(NAME '=') a=expression b='=' {

Lib/test/test_syntax.py

@@ -524,9 +524,15 @@
 >>> f((x)=2)
 Traceback (most recent call last):
 SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
->>> f(True=2)
+>>> f(True=1)
 Traceback (most recent call last):
-SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
+SyntaxError: cannot assign to True
+>>> f(False=1)
+Traceback (most recent call last):
+SyntaxError: cannot assign to False
+>>> f(None=1)
+Traceback (most recent call last):
+SyntaxError: cannot assign to None
 >>> f(__debug__=1)
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__

Misc/NEWS.d/next/Core and Builtins/2021-11-04-20-19-07.bpo-45716.5C0pA1.rst

@@ -0,0 +1,2 @@
+Improve the :exc:`SyntaxError` message when using ``True``, ``None`` or
+``False`` as keywords in a function call. Patch by Pablo Galindo.