gh-123562: Improve SyntaxError message for case ... as a.b (#123563)

Author: sobolevn

Grammar/python.gram

@@ -1375,7 +1375,9 @@ invalid_case_block:
         RAISE_INDENTATION_ERROR("expected an indented block after 'case' statement on line %d", a->lineno) }
 invalid_as_pattern:
     | or_pattern 'as' a="_" { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot use '_' as a target") }
-    | or_pattern 'as' !NAME a=expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "invalid pattern target") }
+    | or_pattern 'as' a=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
+            a, "cannot use %s as pattern target", _PyPegen_get_expr_name(a)) }
 invalid_class_pattern:
     | name_or_attr '(' a=invalid_class_argument_pattern  { RAISE_SYNTAX_ERROR_KNOWN_RANGE(
         PyPegen_first_item(a, pattern_ty),

Lib/test/test_patma.py

@@ -3015,6 +3015,13 @@ def test_multiple_assignments_to_name_in_pattern_5(self):
                 pass
         """)
 
+    def test_multiple_assignments_to_name_in_pattern_6(self):
+        self.assert_syntax_error("""
+        match ...:
+            case a as a + 1:  # NAME and expression with no ()
+                pass
+        """)
+
     def test_multiple_starred_names_in_sequence_pattern_0(self):
         self.assert_syntax_error("""
         match ...:

Lib/test/test_syntax.py

@@ -1932,7 +1932,31 @@
     ...   case 42 as 1+2+4:
     ...     ...
     Traceback (most recent call last):
-    SyntaxError: invalid pattern target
+    SyntaxError: cannot use expression as pattern target
+
+    >>> match ...:
+    ...   case 42 as a.b:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use attribute as pattern target
+
+    >>> match ...:
+    ...   case 42 as (a, b):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use tuple as pattern target
+
+    >>> match ...:
+    ...   case 42 as (a + 1):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use expression as pattern target
+
+    >>> match ...:
+    ...   case (32 as x) | (42 as a()):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use function call as pattern target
 
     >>> match ...:
     ...   case Foo(z=1, y=2, x):
@@ -2817,6 +2841,22 @@ def test_except_stmt_invalid_as_expr(self):
             end_offset=22 + len("obj.attr"),
         )
 
+    def test_match_stmt_invalid_as_expr(self):
+        self._check_error(
+            textwrap.dedent(
+                """
+                match 1:
+                    case x as obj.attr:
+                        ...
+                """
+            ),
+            errtext="cannot use attribute as pattern target",
+            lineno=3,
+            end_lineno=3,
+            offset=15,
+            end_offset=15 + len("obj.attr"),
+        )
+
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

Misc/NEWS.d/next/Core_and_Builtins/2024-09-01-12-08-39.gh-issue-123562.aJPKVu.rst

@@ -0,0 +1,2 @@
+Improve :exc:`SyntaxError` message for using ``case ... as ...`` with not a
+name.