bpo-40334: Correctly identify invalid target in assignment errors (GH-20076)

Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Grammar/python.gram

@@ -640,8 +640,17 @@ invalid_assignment:
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "only single target (not tuple) can be annotated") }
     | a=expression ':' expression ['=' annotated_rhs] {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "illegal target for annotation") }
-    | a=expression ('=' | augassign) (yield_expr | star_expressions) {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot assign to %s", _PyPegen_get_expr_name(a)) }
+    | a=star_expressions '=' (yield_expr | star_expressions) {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            _PyPegen_get_invalid_target(a),
+            "cannot assign to %s", _PyPegen_get_expr_name(_PyPegen_get_invalid_target(a))) }
+    | a=star_expressions augassign (yield_expr | star_expressions) {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION( 
+            a, 
+            "'%s' is an illegal expression for augmented assignment",
+            _PyPegen_get_expr_name(a)
+        )}
+ 
 invalid_block:
     | NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
 invalid_comprehension:

Lib/test/test_dictcomps.py

@@ -77,7 +77,7 @@ class DictComprehensionTest(unittest.TestCase):
             compile("{x: y for y, x in ((1, 2), (3, 4))} = 5", "<test>",
                     "exec")
 
-        with self.assertRaisesRegex(SyntaxError, "cannot assign"):
+        with self.assertRaisesRegex(SyntaxError, "illegal expression"):
             compile("{x: y for y, x in ((1, 2), (3, 4))} += 5", "<test>",
                     "exec")
 

Lib/test/test_generators.py

@@ -1921,7 +1921,7 @@ SyntaxError: cannot assign to yield expression
 >>> def f(): (yield bar) += y
 Traceback (most recent call last):
   ...
-SyntaxError: cannot assign to yield expression
+SyntaxError: 'yield expression' is an illegal expression for augmented assignment
 
 
 Now check some throw() conditions:

Lib/test/test_genexps.py

@@ -158,7 +158,7 @@ Verify that syntax error's are raised for genexps used as lvalues
     >>> (y for y in (1,2)) += 10
     Traceback (most recent call last):
        ...
-    SyntaxError: cannot assign to generator expression
+    SyntaxError: 'generator expression' is an illegal expression for augmented assignment
 
 
 ########### Tests borrowed from or inspired by test_generators.py ############

Lib/test/test_peg_parser.py

@@ -625,7 +625,7 @@ FAIL_SPECIALIZED_MESSAGE_CASES = [
     ("(a, b): int", "only single target (not tuple) can be annotated"),
     ("[a, b]: int", "only single target (not list) can be annotated"),
     ("a(): int", "illegal target for annotation"),
-    ("1 += 1", "cannot assign to literal"),
+    ("1 += 1", "'literal' is an illegal expression for augmented assignment"),
     ("pass\n    pass", "unexpected indent"),
     ("def f():\npass", "expected an indented block"),
     ("def f(*): pass", "named arguments must follow bare *"),

Lib/test/test_syntax.py

@@ -100,30 +100,37 @@ expression inside that contain should still cause a syntax error.
 This test just checks a couple of cases rather than enumerating all of
 them.
 
-# All of the following also produce different error messages with pegen
-# >>> (a, "b", c) = (1, 2, 3)
-# Traceback (most recent call last):
-# SyntaxError: cannot assign to literal
+>>> (a, "b", c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to literal
 
-# >>> (a, True, c) = (1, 2, 3)
-# Traceback (most recent call last):
-# SyntaxError: cannot assign to True
+>>> (a, True, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
 
 >>> (a, __debug__, c) = (1, 2, 3)
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
 
-# >>> (a, *True, c) = (1, 2, 3)
-# Traceback (most recent call last):
-# SyntaxError: cannot assign to True
+>>> (a, *True, c) = (1, 2, 3)
+Traceback (most recent call last):
+SyntaxError: cannot assign to True
 
 >>> (a, *__debug__, c) = (1, 2, 3)
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
 
-# >>> [a, b, c + 1] = [1, 2, 3]
-# Traceback (most recent call last):
-# SyntaxError: cannot assign to operator
+>>> [a, b, c + 1] = [1, 2, 3]
+Traceback (most recent call last):
+SyntaxError: cannot assign to operator
+
+>>> [a, b[1], c + 1] = [1, 2, 3]
+Traceback (most recent call last):
+SyntaxError: cannot assign to operator
+
+>>> [a, b.c.d, c + 1] = [1, 2, 3]
+Traceback (most recent call last):
+SyntaxError: cannot assign to operator
 
 >>> a if 1 else b = 1
 Traceback (most recent call last):
@@ -131,15 +138,15 @@ SyntaxError: cannot assign to conditional expression
 
 >>> a, b += 1, 2
 Traceback (most recent call last):
-SyntaxError: invalid syntax
+SyntaxError: 'tuple' is an illegal expression for augmented assignment
 
 >>> (a, b) += 1, 2
 Traceback (most recent call last):
-SyntaxError: cannot assign to tuple
+SyntaxError: 'tuple' is an illegal expression for augmented assignment
 
 >>> [a, b] += 1, 2
 Traceback (most recent call last):
-SyntaxError: cannot assign to list
+SyntaxError: 'list' is an illegal expression for augmented assignment
 
 From compiler_complex_args():
 
@@ -346,16 +353,16 @@ More set_context():
 
 >>> (x for x in x) += 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to generator expression
+SyntaxError: 'generator expression' is an illegal expression for augmented assignment
 >>> None += 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to None
+SyntaxError: 'None' is an illegal expression for augmented assignment
 >>> __debug__ += 1
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
 >>> f() += 1
 Traceback (most recent call last):
-SyntaxError: cannot assign to function call
+SyntaxError: 'function call' is an illegal expression for augmented assignment
 
 
 Test continue in finally in weird combinations.
@@ -688,6 +695,7 @@ class SyntaxTestCase(unittest.TestCase):
     def test_assign_call(self):
         self._check_error("f() = 1", "assign")
 
+    @unittest.skipIf(support.use_old_parser(), "The old parser cannot generate these error messages")
     def test_assign_del(self):
         self._check_error("del (,)", "invalid syntax")
         self._check_error("del 1", "delete literal")

Parser/pegen/parse.c

@@ -10747,7 +10747,8 @@ invalid_named_expression_rule(Parser *p)
 //     | tuple ':'
 //     | star_named_expression ',' star_named_expressions* ':'
 //     | expression ':' expression ['=' annotated_rhs]
-//     | expression ('=' | augassign) (yield_expr | star_expressions)
+//     | star_expressions '=' (yield_expr | star_expressions)
+//     | star_expressions augassign (yield_expr | star_expressions)
 static void *
 invalid_assignment_rule(Parser *p)
 {
@@ -10841,19 +10842,40 @@ invalid_assignment_rule(Parser *p)
         }
         p->mark = _mark;
     }
-    { // expression ('=' | augassign) (yield_expr | star_expressions)
+    { // star_expressions '=' (yield_expr | star_expressions)
+        Token * _literal;
         void *_tmp_128_var;
-        void *_tmp_129_var;
         expr_ty a;
         if (
-            (a = expression_rule(p))  // expression
+            (a = star_expressions_rule(p))  // star_expressions
             &&
-            (_tmp_128_var = _tmp_128_rule(p))  // '=' | augassign
+            (_literal = _PyPegen_expect_token(p, 22))  // token='='
+            &&
+            (_tmp_128_var = _tmp_128_rule(p))  // yield_expr | star_expressions
+        )
+        {
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( _PyPegen_get_invalid_target ( a ) , "cannot assign to %s" , _PyPegen_get_expr_name ( _PyPegen_get_invalid_target ( a ) ) );
+            if (_res == NULL && PyErr_Occurred()) {
+                p->error_indicator = 1;
+                return NULL;
+            }
+            goto done;
+        }
+        p->mark = _mark;
+    }
+    { // star_expressions augassign (yield_expr | star_expressions)
+        void *_tmp_129_var;
+        expr_ty a;
+        AugOperator* augassign_var;
+        if (
+            (a = star_expressions_rule(p))  // star_expressions
+            &&
+            (augassign_var = augassign_rule(p))  // augassign
             &&
             (_tmp_129_var = _tmp_129_rule(p))  // yield_expr | star_expressions
         )
         {
-            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "cannot assign to %s" , _PyPegen_get_expr_name ( a ) );
+            _res = RAISE_SYNTAX_ERROR_KNOWN_LOCATION ( a , "'%s' is an illegal expression for augmented assignment" , _PyPegen_get_expr_name ( a ) );
             if (_res == NULL && PyErr_Occurred()) {
                 p->error_indicator = 1;
                 return NULL;
@@ -16675,7 +16697,7 @@ _tmp_127_rule(Parser *p)
     return _res;
 }
 
-// _tmp_128: '=' | augassign
+// _tmp_128: yield_expr | star_expressions
 static void *
 _tmp_128_rule(Parser *p)
 {
@@ -16684,24 +16706,24 @@ _tmp_128_rule(Parser *p)
     }
     void * _res = NULL;
     int _mark = p->mark;
-    { // '='
-        Token * _literal;
+    { // yield_expr
+        expr_ty yield_expr_var;
         if (
-            (_literal = _PyPegen_expect_token(p, 22))  // token='='
+            (yield_expr_var = yield_expr_rule(p))  // yield_expr
         )
         {
-            _res = _literal;
+            _res = yield_expr_var;
             goto done;
         }
         p->mark = _mark;
     }
-    { // augassign
-        AugOperator* augassign_var;
+    { // star_expressions
+        expr_ty star_expressions_var;
         if (
-            (augassign_var = augassign_rule(p))  // augassign
+            (star_expressions_var = star_expressions_rule(p))  // star_expressions
         )
         {
-            _res = augassign_var;
+            _res = star_expressions_var;
             goto done;
         }
         p->mark = _mark;

Parser/pegen/pegen.c

@@ -2054,3 +2054,49 @@ _PyPegen_make_module(Parser *p, asdl_seq *a) {
     }
     return Module(a, type_ignores, p->arena);
 }
+
+// Error reporting helpers
+
+expr_ty
+_PyPegen_get_invalid_target(expr_ty e)
+{
+    if (e == NULL) {
+        return NULL;
+    }
+
+#define VISIT_CONTAINER(CONTAINER, TYPE) do { \
+        Py_ssize_t len = asdl_seq_LEN(CONTAINER->v.TYPE.elts);\
+        for (Py_ssize_t i = 0; i < len; i++) {\
+            expr_ty other = asdl_seq_GET(CONTAINER->v.TYPE.elts, i);\
+            expr_ty child = _PyPegen_get_invalid_target(other);\
+            if (child != NULL) {\
+                return child;\
+            }\
+        }\
+    } while (0)
+
+    // We only need to visit List and Tuple nodes recursively as those
+    // are the only ones that can contain valid names in targets when
+    // they are parsed as expressions. Any other kind of expression
+    // that is a container (like Sets or Dicts) is directly invalid and
+    // we don't need to visit it recursively.
+
+    switch (e->kind) {
+        case List_kind: {
+            VISIT_CONTAINER(e, List);
+            return NULL;
+        }
+        case Tuple_kind: {
+            VISIT_CONTAINER(e, Tuple);
+            return NULL;
+        }
+        case Starred_kind:
+            return _PyPegen_get_invalid_target(e->v.Starred.value);
+        case Name_kind:
+        case Subscript_kind:
+        case Attribute_kind:
+            return NULL;
+        default:
+            return e;
+    }
+}

Parser/pegen/pegen.h

@@ -260,6 +260,10 @@ void *_PyPegen_arguments_parsing_error(Parser *, expr_ty);
 int _PyPegen_check_barry_as_flufl(Parser *);
 mod_ty _PyPegen_make_module(Parser *, asdl_seq *);
 
+// Error reporting helpers
+
+expr_ty _PyPegen_get_invalid_target(expr_ty e);
+
 void *_PyPegen_parse(Parser *);
 
 #endif

Python/ast.c

@@ -3164,10 +3164,7 @@ ast_for_expr_stmt(struct compiling *c, const node *n)
         expr1 = ast_for_testlist(c, ch);
         if (!expr1)
             return NULL;
-        if(!set_context(c, expr1, Store, ch))
-            return NULL;
-        /* set_context checks that most expressions are not the left side.
-          Augmented assignments can only have a name, a subscript, or an
+        /* Augmented assignments can only have a name, a subscript, or an
           attribute on the left, though, so we have to explicitly check for
           those. */
         switch (expr1->kind) {
@@ -3176,7 +3173,13 @@ ast_for_expr_stmt(struct compiling *c, const node *n)
             case Subscript_kind:
                 break;
             default:
-                ast_error(c, ch, "illegal expression for augmented assignment");
+                ast_error(c, ch, "'%s' is an illegal expression for augmented assignment",
+                          get_expr_name(expr1));
+                return NULL;
+        }
+
+        /* set_context checks that most expressions are not the left side. */
+        if(!set_context(c, expr1, Store, ch)) {
             return NULL;
         }