Clarify target parsing contracts and AST expectations

Author: pablogsal

Grammar/python.gram

@@ -786,7 +786,8 @@ comparison[expr_ty]:
 
 compare_op_bitwise_or_pair[CmpopExprPair*]:
     | '==' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, Eq, a) }
-    | (tok='!=' { _PyPegen_check_barry_as_flufl(p, tok) ? NULL : tok}) a=bitwise_or {_PyPegen_cmpop_expr_pair(p, NotEq, a) }
+    | (tok='!=' { _PyPegen_check_barry_as_flufl(p, tok) ? NULL : tok }) a=bitwise_or {
+        _PyPegen_cmpop_expr_pair(p, NotEq, a) }
     | '<=' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, LtE, a) }
     | '<' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, Lt, a) }
     | '>=' a=bitwise_or { _PyPegen_cmpop_expr_pair(p, GtE, a) }
@@ -1110,8 +1111,7 @@ star_targets[expr_ty]:
         _PyAST_Tuple(CHECK(asdl_expr_seq*, _PyPegen_seq_insert_in_front(p, a, b)), Store, EXTRA) }
 
 star_target[expr_ty] (memo):
-    | '*' a=(!'*' star_target) {
-        _PyAST_Starred(a, Store, EXTRA) }
+    | '*' a=(!'*' star_target) { _PyAST_Starred(a, Store, EXTRA) }
     | target
 
 target[expr_ty] (memo):

Lib/test/test_ast/test_ast.py

@@ -712,7 +712,14 @@ def test_primary_target_context(self):
                             self.assertIsInstance(child.ctx, ast.Load)
 
     def test_container_target_context(self):
-        for expression in ('(a, [b, c])', '[a, (b, c)]', '((a))', '()', '[]'):
+        cases = (
+            ('(a, [b, c])', '(a, [b, c])'),
+            ('[a, (b, c)]', '[a, (b, c)]'),
+            ('((a))', 'a'),
+            ('()', '()'),
+            ('[]', '[]'),
+        )
+        for expression, expected_segment in cases:
             for context in (ast.Load, ast.Store, ast.Del):
                 with self.subTest(expression=expression, context=context):
                     if context is ast.Load:
@@ -727,8 +734,7 @@ def test_container_target_context(self):
                         if hasattr(child, 'ctx'):
                             self.assertIsInstance(child.ctx, context)
                     self.assertEqual(ast.get_source_segment(source, node),
-                                     ast.get_source_segment(expression,
-                                         ast.parse(expression, mode='eval').body))
+                                     expected_segment)
 
         node = ast.parse('[a, *[b, *c]] = value').body[0].targets[0]
         for child in ast.walk(node):

Parser/action_helpers.c

@@ -346,7 +346,8 @@ _PyPegen_set_expr_context(Parser *p, expr_ty expr, expr_context_ty ctx)
     return new;
 }
 
-/* Check the target form before copying it into Store or Del context. */
+/* Invalid targets return NULL without raising, allowing parsing to backtrack.
+   Copy valid targets to preserve the memoized expression's context. */
 expr_ty
 _PyPegen_make_target(Parser *p, expr_ty expr, TARGETS_TYPE targets_type)
 {
@@ -1249,7 +1250,8 @@ _PyPegen_get_invalid_target(expr_ty e, TARGETS_TYPE targets_type)
         return NULL;
     }
 
-    if (targets_type == SINGLE_TARGETS || targets_type == ATTRIBUTE_OR_SUBSCRIPT_TARGETS) {
+    if (targets_type == SINGLE_TARGETS ||
+        targets_type == ATTRIBUTE_OR_SUBSCRIPT_TARGETS) {
         if (targets_type == SINGLE_TARGETS && e->kind == Name_kind) {
             return NULL;
         }