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

1 files changed, 14 insertions, 1 deletions

diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index ae1066924b..69a86231e2 100644
--- a/Lib/test/test_syntax.py
+++ b/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