gh-133850: Improved SyntaxError message for missing commas

Grammar/python.gram

@@ -1316,6 +1316,8 @@ invalid_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | param_maybe_default+ '/' a='*' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
+    | a=NAME b=NAME {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected comma between parameters") }
 invalid_default:
     | a='=' &(')'|',') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected default value expression") }
 invalid_star_etc:
@@ -1344,6 +1346,8 @@ invalid_lambda_parameters:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "/ must be ahead of *") }
     | lambda_param_maybe_default+ '/' a='*' {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "expected comma between / and *") }
+    | a=NAME b=NAME {
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected comma between parameters") }
 invalid_lambda_parameters_helper:
     | a=lambda_slash_with_default { _PyPegen_singleton_seq(p, a) }
     | lambda_param_with_default+

Lib/test/test_syntax.py

@@ -521,6 +521,16 @@
 Traceback (most recent call last):
 SyntaxError: expected comma between / and *
 
+>>> def foo(a b):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
+>>> def foo(a d=1):
+...    pass
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
 >>> def foo(a=1,d=,c):
 ...    pass
 Traceback (most recent call last):
@@ -568,6 +578,10 @@
 Traceback (most recent call last):
 SyntaxError: expected comma between / and *
 
+>>> lambda a b: None
+Traceback (most recent call last):
+SyntaxError: expected comma between parameters
+
 >>> lambda a,*b=3,c: None
 Traceback (most recent call last):
 SyntaxError: var-positional argument cannot have default value

Misc/NEWS.d/next/Core_and_Builtins/2025-05-10-20-34-00.gh-issue-133850.asdsd.rst

@@ -0,0 +1 @@
+Improved error message for missing comma between parameters in function/lambda definition.