gh-91210: Improve error message when non-default param follows default (GH-95933)

- Improve error message when parameter without a default follows one with a default
- Show same error message when positional-only params precede the default/non-default sequence

Grammar/python.gram

@@ -1162,14 +1162,14 @@ invalid_dict_comprehension:
     | '{' a='**' bitwise_or for_if_clauses '}' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "dict unpacking cannot be used in 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") }
     | a="/" ',' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "at least one argument must precede /") }
     | (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") }
+    | 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='/' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | param_maybe_default+ '/' a='*' {
@@ -1190,14 +1190,14 @@ 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") }
     | a="/" ',' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "at least one argument must precede /") }
     | (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") }
+    | 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='/' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | lambda_param_maybe_default+ '/' a='*' {

Lib/test/test_positional_only_arg.py

@@ -23,10 +23,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
             compile(codestr + "\n", "<test>", "single")
 
     def test_invalid_syntax_errors(self):
-        check_syntax_error(self, "def f(a, b = 5, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b=1, /, c, *, d=2): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "def f(a = 5, b, /): pass", "non-default argument follows default argument")
+        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(*args, /): pass")
         check_syntax_error(self, "def f(*args, a, /): pass")
         check_syntax_error(self, "def f(**kwargs, /): pass")
@@ -44,10 +45,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
         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", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b, /, c): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b=1, /, c, d=2): pass", "non-default argument follows default argument")
-        check_syntax_error(self, "async def f(a = 5, b, /): pass", "non-default argument follows default argument")
+        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(*args, /): pass")
         check_syntax_error(self, "async def f(*args, a, /): pass")
         check_syntax_error(self, "async def f(**kwargs, /): pass")
@@ -231,9 +233,11 @@ class PositionalOnlyTestCase(unittest.TestCase):
         self.assertEqual(x(1, 2), 3)
 
     def test_invalid_syntax_lambda(self):
-        check_syntax_error(self, "lambda a, b = 5, /, c: None", "non-default argument follows default argument")
-        check_syntax_error(self, "lambda a = 5, b, /, c: None", "non-default argument follows default argument")
-        check_syntax_error(self, "lambda a = 5, b, /: None", "non-default argument follows default argument")
+        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 *args, /: None")
         check_syntax_error(self, "lambda *args, a, /: None")
         check_syntax_error(self, "lambda **kwargs, /: None")

Lib/test/test_syntax.py

@@ -334,7 +334,12 @@ From ast_for_arguments():
 >>> def f(x, y=1, z):
 ...     pass
 Traceback (most recent call last):
-SyntaxError: non-default argument follows default argument
+SyntaxError: 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
 
 >>> def f(x, None):
 ...     pass
@@ -555,6 +560,14 @@ SyntaxError: expected default value expression
 Traceback (most recent call last):
 SyntaxError: expected default value expression
 
+>>> lambda a,d=3,c: None
+Traceback (most recent call last):
+SyntaxError: 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
+
 >>> import ast; ast.parse('''
 ... def f(
 ...     *, # type: int

Misc/NEWS.d/next/Core and Builtins/2022-08-12-18-13-49.gh-issue-91210.AWMSLj.rst

@@ -0,0 +1 @@
+Improve error message when a parameter without a default value follows one with a default value, and show the same message, even when the non-default/default sequence is preceded by positional-only parameters.