bpo-40334: produce specialized errors for invalid del targets (GH-19911)

Author: hauntsaninja

Grammar/python.gram

@@ -583,15 +583,19 @@ ann_assign_subscript_attribute_target[expr_ty]:
     | a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Store, EXTRA) }
 
 del_targets[asdl_seq*]: a=','.del_target+ [','] { a }
+# The lookaheads to del_target_end ensure that we don't match expressions where a prefix of the
+# expression matches our rule, thereby letting these cases fall through to invalid_del_target.
 del_target[expr_ty] (memo):
-    | a=t_primary '.' b=NAME !t_lookahead { _Py_Attribute(a, b->v.Name.id, Del, EXTRA) }
-    | a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Del, EXTRA) }
+    | a=t_primary '.' b=NAME &del_target_end { _Py_Attribute(a, b->v.Name.id, Del, EXTRA) }
+    | a=t_primary '[' b=slices ']' &del_target_end { _Py_Subscript(a, b, Del, EXTRA) }
     | del_t_atom
 del_t_atom[expr_ty]:
-    | a=NAME { _PyPegen_set_expr_context(p, a, Del) }
+    | a=NAME &del_target_end { _PyPegen_set_expr_context(p, a, Del) }
     | '(' a=del_target ')' { _PyPegen_set_expr_context(p, a, Del) }
     | '(' a=[del_targets] ')' { _Py_Tuple(a, Del, EXTRA) }
     | '[' a=[del_targets] ']' { _Py_List(a, Del, EXTRA) }
+    | invalid_del_target
+del_target_end: ')' | ']' | ',' | ';' | NEWLINE
 
 targets[asdl_seq*]: a=','.target+ [','] { a }
 target[expr_ty] (memo):
@@ -649,3 +653,6 @@ invalid_lambda_star_etc:
 invalid_double_type_comments:
     | TYPE_COMMENT NEWLINE TYPE_COMMENT NEWLINE INDENT {
         RAISE_SYNTAX_ERROR("Cannot have two type comments on def") }
+invalid_del_target:
+    | a=star_expression &del_target_end {
+        RAISE_SYNTAX_ERROR("cannot delete %s", _PyPegen_get_expr_name(a)) }

Lib/test/test_grammar.py

@@ -801,6 +801,23 @@ def test_del_stmt(self):
         del abc
         del x, y, (z, xyz)
 
+        x, y, z = "xyz"
+        del x
+        del y,
+        del (z)
+        del ()
+
+        a, b, c, d, e, f, g = "abcdefg"
+        del a, (b, c), (d, (e, f))
+
+        a, b, c, d, e, f, g = "abcdefg"
+        del a, [b, c], (d, [e, f])
+
+        abcd = list("abcd")
+        del abcd[1:2]
+
+        compile("del a, (b[0].c, (d.e, f.g[1:2])), [h.i.j], ()", "<testcase>", "exec")
+
     def test_pass_stmt(self):
         # 'pass'
         pass

Lib/test/test_syntax.py

@@ -63,10 +63,9 @@
 Traceback (most recent call last):
 SyntaxError: cannot assign to function call
 
-# Pegen does not support this yet
-# >>> del f()
-# Traceback (most recent call last):
-# SyntaxError: cannot delete function call
+>>> del f()
+Traceback (most recent call last):
+SyntaxError: cannot delete function call
 
 >>> a + 1 = 2
 Traceback (most recent call last):
@@ -665,7 +664,7 @@ def _check_error(self, code, errtext,
                 self.fail("SyntaxError is not a %s" % subclass.__name__)
             mo = re.search(errtext, str(err))
             if mo is None:
-                self.fail("SyntaxError did not contain '%r'" % (errtext,))
+                self.fail("SyntaxError did not contain %r" % (errtext,))
             self.assertEqual(err.filename, filename)
             if lineno is not None:
                 self.assertEqual(err.lineno, lineno)
@@ -677,10 +676,36 @@ def _check_error(self, code, errtext,
     def test_assign_call(self):
         self._check_error("f() = 1", "assign")
 
-    @support.skip_if_new_parser("Pegen does not produce a specialized error "
-                                "message yet")
     def test_assign_del(self):
-        self._check_error("del f()", "delete")
+        self._check_error("del (,)", "invalid syntax")
+        self._check_error("del 1", "delete literal")
+        self._check_error("del (1, 2)", "delete literal")
+        self._check_error("del None", "delete None")
+        self._check_error("del *x", "delete starred")
+        self._check_error("del (*x)", "delete starred")
+        self._check_error("del (*x,)", "delete starred")
+        self._check_error("del [*x,]", "delete starred")
+        self._check_error("del f()", "delete function call")
+        self._check_error("del f(a, b)", "delete function call")
+        self._check_error("del o.f()", "delete function call")
+        self._check_error("del a[0]()", "delete function call")
+        self._check_error("del x, f()", "delete function call")
+        self._check_error("del f(), x", "delete function call")
+        self._check_error("del [a, b, ((c), (d,), e.f())]", "delete function call")
+        self._check_error("del (a if True else b)", "delete conditional")
+        self._check_error("del +a", "delete operator")
+        self._check_error("del a, +b", "delete operator")
+        self._check_error("del a + b", "delete operator")
+        self._check_error("del (a + b, c)", "delete operator")
+        self._check_error("del (c[0], a + b)", "delete operator")
+        self._check_error("del a.b.c + 2", "delete operator")
+        self._check_error("del a.b.c[0] + 2", "delete operator")
+        self._check_error("del (a, b, (c, d.e.f + 2))", "delete operator")
+        self._check_error("del [a, b, (c, d.e.f[0] + 2)]", "delete operator")
+        self._check_error("del (a := 5)", "delete named expression")
+        # We don't have a special message for this, but make sure we don't
+        # report "cannot delete name"
+        self._check_error("del a += b", "invalid syntax")
 
     def test_global_param_err_first(self):
         source = """if 1: