gh-145241: specialize SyntaxError for single trailing-comma with item

Grammar/python.gram

@@ -1447,6 +1447,8 @@ invalid_import_from_targets:
         RAISE_SYNTAX_ERROR_STARTING_FROM(token, "Expected one or more names after 'import'") }
 
 invalid_with_stmt:
+    | ['async'] 'with' expression ['as' star_target] ',' ':' {
+        RAISE_SYNTAX_ERROR("single 'with' item has a trailing comma") }
     | ['async'] 'with' ','.(expression ['as' star_target])+ NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
     | ['async'] 'with' '(' ','.(expressions ['as' star_target])+ ','? ')' NEWLINE { RAISE_SYNTAX_ERROR("expected ':'") }
 invalid_with_stmt_indent:

Lib/test/test_syntax.py

@@ -2061,6 +2061,14 @@
 Traceback (most recent call last):
 SyntaxError: trailing comma not allowed without surrounding parentheses
 
+>>> with item,: pass
+Traceback (most recent call last):
+SyntaxError: single 'with' item has a trailing comma
+
+>>> with item as x,: pass
+Traceback (most recent call last):
+SyntaxError: single 'with' item has a trailing comma
+
 >>> import a from b
 Traceback (most recent call last):
 SyntaxError: Did you mean to use 'from ... import ...' instead?

Misc/NEWS.d/next/Core_and_Builtins/2026-02-26-18-00-00.gh-issue-145241.hL2k9Q.rst

@@ -0,0 +1,3 @@
+Specialized the parser error for a single unparenthesized ``with`` item followed
+by a trailing comma (for example, ``with item,:``), raising a clearer
+:exc:`SyntaxError` message. Patch by Pablo Galindo.