From 233b9a683f98fd6dc6277489b802885961d4b739 Mon Sep 17 00:00:00 2001
From: Uno <uno@daangn.com>
Date: Fri, 15 Aug 2025 12:34:51 +0900
Subject: [PATCH 1/2] fix: error msg for *args, **kwargs default value

---
 Grammar/python.gram                                  |  8 ++++----
 Lib/test/test_syntax.py                              | 12 ++++++------
 .../2025-08-15-12-31-45.gh-issue-137786.HkKBqf.rst   |  2 ++
 Parser/parser.c                                      |  8 ++++----
 4 files changed, 16 insertions(+), 14 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-08-15-12-31-45.gh-issue-137786.HkKBqf.rst

diff --git a/Grammar/python.gram b/Grammar/python.gram
index ff54e42111005a..3da69fe152448b 100644
--- a/Grammar/python.gram
+++ b/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:
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index c52d24219410c2..8046dc58425106 100644
--- a/Lib/test/test_syntax.py
+++ b/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):
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-08-15-12-31-45.gh-issue-137786.HkKBqf.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-08-15-12-31-45.gh-issue-137786.HkKBqf.rst
new file mode 100644
index 00000000000000..01e02c18e64a06
--- /dev/null
+++ b/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.
diff --git a/Parser/parser.c b/Parser/parser.c
index de5cdc9b6f7f34..e5db365345e8bd 100644
--- a/Parser/parser.c
+++ b/Parser/parser.c
@@ -22710,7 +22710,7 @@ invalid_star_etc_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_star_etc[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'*' param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter cannot have default value" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter's default value cannot be changed because it has the immutable default value ()" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -22795,7 +22795,7 @@ invalid_kwds_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_kwds[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'**' param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter cannot have default value" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter's default value cannot be changed because it has the immutable default value {}" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -23289,7 +23289,7 @@ invalid_lambda_star_etc_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_lambda_star_etc[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'*' lambda_param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter cannot have default value" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter's default value cannot be changed because it has the immutable default value ()" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -23377,7 +23377,7 @@ invalid_lambda_kwds_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_lambda_kwds[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'**' lambda_param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter cannot have default value" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter's default value cannot be changed because it has the immutable default value {}" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;

From 98cbe835da54e301889ce3132a647af6363793b0 Mon Sep 17 00:00:00 2001
From: Uno <uno@daangn.com>
Date: Fri, 15 Aug 2025 15:21:07 +0900
Subject: [PATCH 2/2] fix: add explanation without modifying the original
 message

---
 Grammar/python.gram     |  8 ++++----
 Lib/test/test_syntax.py | 12 ++++++------
 Parser/parser.c         |  8 ++++----
 3 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/Grammar/python.gram b/Grammar/python.gram
index 3da69fe152448b..12c06cb5b8b5fb 100644
--- a/Grammar/python.gram
+++ b/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's default value cannot be changed because it has the immutable default value ()") }
+    | '*' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter cannot have default value, pass values explicitly") }
     | '*' (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's default value cannot be changed because it has the immutable default value {}") }
+    | '**' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter cannot have default value, pass values explicitly") }
     | '**' 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's default value cannot be changed because it has the immutable default value ()") }
+    | '*' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-positional parameter cannot have default value, pass values explicitly") }
     | '*' (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's default value cannot be changed because it has the immutable default value {}") }
+    | '**' lambda_param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter cannot have default value, pass values explicitly") }
     | '**' 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:
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 8046dc58425106..bc2070d5bfbb67 100644
--- a/Lib/test/test_syntax.py
+++ b/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's default value cannot be changed because it has the immutable default value ()
+SyntaxError: var-positional parameter cannot have default value, pass values explicitly
 
 >>> def foo(a,*b: int=,c):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-positional parameter's default value cannot be changed because it has the immutable default value ()
+SyntaxError: var-positional parameter cannot have default value, pass values explicitly
 
 >>> def foo(a,**b=3):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
+SyntaxError: var-keyword parameter cannot have default value, pass values explicitly
 
 >>> def foo(a,**b: int=3):
 ...    pass
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
+SyntaxError: var-keyword parameter cannot have default value, pass values explicitly
 
 >>> 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's default value cannot be changed because it has the immutable default value ()
+SyntaxError: var-positional parameter cannot have default value, pass values explicitly
 
 >>> lambda a,**b=3: None
 Traceback (most recent call last):
-SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}
+SyntaxError: var-keyword parameter cannot have default value, pass values explicitly
 
 >>> lambda a, *a, b, **c, d: None
 Traceback (most recent call last):
diff --git a/Parser/parser.c b/Parser/parser.c
index e5db365345e8bd..82656345e9b332 100644
--- a/Parser/parser.c
+++ b/Parser/parser.c
@@ -22710,7 +22710,7 @@ invalid_star_etc_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_star_etc[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'*' param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter's default value cannot be changed because it has the immutable default value ()" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter cannot have default value, pass values explicitly" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -22795,7 +22795,7 @@ invalid_kwds_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_kwds[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'**' param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter's default value cannot be changed because it has the immutable default value {}" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter cannot have default value, pass values explicitly" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -23289,7 +23289,7 @@ invalid_lambda_star_etc_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_lambda_star_etc[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'*' lambda_param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter's default value cannot be changed because it has the immutable default value ()" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-positional parameter cannot have default value, pass values explicitly" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;
@@ -23377,7 +23377,7 @@ invalid_lambda_kwds_rule(Parser *p)
         )
         {
             D(fprintf(stderr, "%*c+ invalid_lambda_kwds[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "'**' lambda_param '='"));
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter's default value cannot be changed because it has the immutable default value {}" );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "var-keyword parameter cannot have default value, pass values explicitly" );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 p->level--;