gh-137786: fix error msg for *args, **kwargs default value

Grammar/python.gram

@@ -1331,11 +1331,11 @@ invalid_default:
 invalid_star_etc:
     | a='*' (')' | ',' (')' | '**')) { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "named parameters must follow bare *") }
     | '*' ',' TYPE_COMMENT { RAISE_SYNTAX_ERROR("bare * has associated type comment") }
-    | '*' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter cannot have default value") }
+    | '*' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter's default value cannot be changed because it has the immutable default value ()") }
     | '*' (param_no_default | ',') param_maybe_default* a='*' (param_no_default | ',') {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "* may appear only once") }
 invalid_kwds:
-    | '**' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter cannot have default value") }
+    | '**' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter's default value cannot be changed because it has the immutable default value {}") }
     | '**' param ',' a=param { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameters cannot follow var-keyword parameter") }
     | '**' param ',' a[Token*]=('*'|'**'|'/') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameters cannot follow var-keyword parameter") }
 invalid_parameters_helper: # This is only there to avoid type errors
@@ -1359,11 +1359,11 @@ invalid_lambda_parameters_helper:
     | lambda_param_with_default+
 invalid_lambda_star_etc:
     | '*' (':' | ',' (':' | '**')) { RAISE_SYNTAX_ERROR("named parameters must follow bare *") }
-    | '*' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter cannot have default value") }
+    | '*' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter's default value cannot be changed because it has the immutable default value ()") }
     | '*' (lambda_param_no_default | ',') lambda_param_maybe_default* a='*' (lambda_param_no_default | ',') {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "* may appear only once") }
 invalid_lambda_kwds:
-    | '**' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter cannot have default value") }
+    | '**' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter's default value cannot be changed because it has the immutable default value {}") }
     | '**' lambda_param ',' a=lambda_param { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameters cannot follow var-keyword parameter") }
     | '**' lambda_param ',' a[Token*]=('*'|'**'|'/') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameters cannot follow var-keyword parameter") }
 invalid_double_type_comments:

Lib/test/test_syntax.py

@@ -461,22 +461,22 @@
 >>> def foo(a,*b=3,c):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-positional parameter cannot have default value
+SyntaxError: var-positional parameter's default value cannot be changed because it has the immutable default value ()
 
 >>> def foo(a,*b: int=,c):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-positional parameter cannot have default value
+SyntaxError: var-positional parameter's default value cannot be changed because it has the immutable default value ()
 
 >>> def foo(a,**b=3):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter cannot have default value
+SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
 
 >>> def foo(a,**b: int=3):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter cannot have default value
+SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
 
 >>> def foo(a,*a, b, **c, d):
 ...    pass
@@ -577,11 +577,11 @@
 
 >>> lambda a,*b=3,c: None
 Traceback (most recent call last):
-SyntaxError: var-positional parameter cannot have default value
+SyntaxError: var-positional parameter's default value cannot be changed because it has the immutable default value ()
 
 >>> lambda a,**b=3: None
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter cannot have default value
+SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
 
 >>> lambda a, *a, b, **c, d: None
 Traceback (most recent call last):

Misc/NEWS.d/next/Core_and_Builtins/2025-08-15-12-31-45.gh-issue-137786.HkKBqf.rst

@@ -0,0 +1,2 @@
+fix error msg for *args, **kwargs default value
+Yoonho Hann.