gh-129545 Improve Syntax Error Message for Default Parameter Order

Grammar/python.gram

@@ -1259,7 +1259,7 @@ invalid_parameters:
     | (slash_no_default | slash_with_default) param_maybe_default* a='/' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ may appear only once") }
     | slash_no_default? param_no_default* invalid_parameters_helper a=param_no_default {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameter without a default follows parameter with a default") }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "positional parameter without a default follows parameter with a default") }
     | param_no_default* a='(' param_no_default+ ','? b=')' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Function parameters cannot be parenthesized") }
     | (slash_no_default | slash_with_default)? param_maybe_default* '*' (',' | param_no_default) param_maybe_default* a='/' {
@@ -1287,7 +1287,7 @@ invalid_lambda_parameters:
     | (lambda_slash_no_default | lambda_slash_with_default) lambda_param_maybe_default* a='/' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ may appear only once") }
     | lambda_slash_no_default? lambda_param_no_default* invalid_lambda_parameters_helper a=lambda_param_no_default {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "parameter without a default follows parameter with a default") }
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "positional parameter without a default follows parameter with a default") }
     | lambda_param_no_default* a='(' ','.lambda_param+ ','? b=')' {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "Lambda expression parameters cannot be parenthesized") }
     | (lambda_slash_no_default | lambda_slash_with_default)? lambda_param_maybe_default* '*' (',' | lambda_param_no_default) lambda_param_maybe_default* a='/' {

Lib/test/test_positional_only_arg.py

@@ -24,11 +24,11 @@ def assertRaisesSyntaxError(self, codestr, regex="invalid syntax"):
             compile(codestr + "\n", "<test>", "single")
 
     def test_invalid_syntax_errors(self):
-        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "def f(a = 5, b, /): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "def f(a, /, b = 5, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a = 5, b, /): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "def f(a, /, b = 5, c): pass", "positional parameter without a default follows parameter with a default")
         check_syntax_error(self, "def f(*args, /): pass")
         check_syntax_error(self, "def f(*args, a, /): pass")
         check_syntax_error(self, "def f(**kwargs, /): pass")
@@ -46,11 +46,11 @@ def test_invalid_syntax_errors(self):
         check_syntax_error(self, "def f(a, *, c, /, d, e): pass")
 
     def test_invalid_syntax_errors_async(self):
-        check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "async def f(a = 5, b, /): pass", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "async def f(a, /, b = 5, c): pass", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a, b = 5, /, c): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a = 5, b, /): pass", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "async def f(a, /, b = 5, c): pass", "positional parameter without a default follows parameter with a default")
         check_syntax_error(self, "async def f(*args, /): pass")
         check_syntax_error(self, "async def f(*args, a, /): pass")
         check_syntax_error(self, "async def f(**kwargs, /): pass")
@@ -234,11 +234,11 @@ def test_lambdas(self):
         self.assertEqual(x(1, 2), 3)
 
     def test_invalid_syntax_lambda(self):
-        check_syntax_error(self, "lambda a, b = 5, /, c: None", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "lambda a = 5, b, /, c: None", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "lambda a = 5, b=1, /, c, *, d=2: None", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "lambda a = 5, b, /: None", "parameter without a default follows parameter with a default")
-        check_syntax_error(self, "lambda a, /, b = 5, c: None", "parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a, b = 5, /, c: None", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b, /, c: None", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b=1, /, c, *, d=2: None", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a = 5, b, /: None", "positional parameter without a default follows parameter with a default")
+        check_syntax_error(self, "lambda a, /, b = 5, c: None", "positional parameter without a default follows parameter with a default")
         check_syntax_error(self, "lambda *args, /: None")
         check_syntax_error(self, "lambda *args, a, /: None")
         check_syntax_error(self, "lambda **kwargs, /: None")

Lib/test/test_syntax.py

@@ -394,12 +394,12 @@
 >>> def f(x, y=1, z):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: parameter without a default follows parameter with a default
+SyntaxError: positional parameter without a default follows parameter with a default
 
 >>> def f(x, /, y=1, z):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: parameter without a default follows parameter with a default
+SyntaxError: positional parameter without a default follows parameter with a default
 
 >>> def f(x, None):
 ...     pass
@@ -622,11 +622,11 @@
 
 >>> lambda a,d=3,c: None
 Traceback (most recent call last):
-SyntaxError: parameter without a default follows parameter with a default
+SyntaxError: positional parameter without a default follows parameter with a default
 
 >>> lambda a,/,d=3,c: None
 Traceback (most recent call last):
-SyntaxError: parameter without a default follows parameter with a default
+SyntaxError: positional parameter without a default follows parameter with a default
 
 >>> import ast; ast.parse('''
 ... def f(

Misc/NEWS.d/next/Core_and_Builtins/2025-03-07-16-48-12.gh-issue-129545.t-9iTH.rst

@@ -0,0 +1,3 @@
+Change the text of the ``SyntaxError: parameter without a default follows parameter with a default``
+to ``SyntaxError: positional parameter without a default follows parameter with a default`` to improve
+error message clarity in certain cases.