bpo-45450: Improve syntax error for parenthesized arguments (GH-28906)

Grammar/python.gram

@@ -1140,12 +1140,16 @@ invalid_dict_comprehension:
 invalid_parameters:
     | param_no_default* invalid_parameters_helper a=param_no_default {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "non-default argument follows default argument") }
+    | param_no_default* a='(' param_no_default+ ','? b=')' {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Function parameters cannot be parenthesized") }
 invalid_parameters_helper: # This is only there to avoid type errors
     | a=slash_with_default { _PyPegen_singleton_seq(p, a) }
     | param_with_default+
 invalid_lambda_parameters:
     | lambda_param_no_default* invalid_lambda_parameters_helper a=lambda_param_no_default {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "non-default argument follows default argument") }
+    | lambda_param_no_default* a='(' ','.lambda_param+ ','? b=')' {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Lambda expression parameters cannot be parenthesized") }
 invalid_lambda_parameters_helper:
     | a=lambda_slash_with_default { _PyPegen_singleton_seq(p, a) }
     | lambda_param_with_default+

Lib/test/test_syntax.py

@@ -909,6 +909,44 @@ Missing parens after function definition
    Traceback (most recent call last):
    SyntaxError: expected '('
 
+Parenthesized arguments in function definitions
+
+   >>> def f(x, (y, z), w):
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: Function parameters cannot be parenthesized
+
+   >>> def f((x, y, z, w)):
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: Function parameters cannot be parenthesized
+
+   >>> def f(x, (y, z, w)):
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: Function parameters cannot be parenthesized
+
+   >>> def f((x, y, z), w):
+   ...    pass
+   Traceback (most recent call last):
+   SyntaxError: Function parameters cannot be parenthesized
+
+   >>> lambda x, (y, z), w: None
+   Traceback (most recent call last):
+   SyntaxError: Lambda expression parameters cannot be parenthesized
+
+   >>> lambda (x, y, z, w): None
+   Traceback (most recent call last):
+   SyntaxError: Lambda expression parameters cannot be parenthesized
+
+   >>> lambda x, (y, z, w): None
+   Traceback (most recent call last):
+   SyntaxError: Lambda expression parameters cannot be parenthesized
+
+   >>> lambda (x, y, z), w: None
+   Traceback (most recent call last):
+   SyntaxError: Lambda expression parameters cannot be parenthesized
+
 Custom error messages for try blocks that are not followed by except/finally
 
    >>> try:

Misc/NEWS.d/next/Core and Builtins/2021-10-12-18-22-44.bpo-45450.d9a-bX.rst

@@ -0,0 +1,2 @@
+Improve the syntax error message for parenthesized arguments. Patch by Pablo
+Galindo.