Allow non-final __match_args__ and overriding (#12415)

Allow subclasses to override `__match_args__` freely, and don't require
`__match_args__` to be final. 

This matches runtime behavior. For example, if `B` subclasses `A`, 
`case A(...)` also matches instances of `B`, using the `__match_args__` 
from `A`.

The issue was brough up by @AlexWaygood in 
#12411 (comment).

mypy/checker.py

@@ -1439,12 +1439,7 @@ def check_setattr_method(self, typ: Type, context: Context) -> None:
             self.msg.invalid_signature_for_special_method(typ, context, '__setattr__')
 
     def check_match_args(self, var: Var, typ: Type, context: Context) -> None:
-        """Check that __match_args__ is final and contains literal strings"""
-
-        if not var.is_final:
-            self.note("__match_args__ must be final for checking of match statements to work",
-                      context, code=codes.LITERAL_REQ)
-
+        """Check that __match_args__ contains literal strings"""
         typ = get_proper_type(typ)
         if not isinstance(typ, TupleType) or \
                 not all([is_string_literal(item) for item in typ.items]):
@@ -2276,11 +2271,16 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
 
             # Defer PartialType's super type checking.
             if (isinstance(lvalue, RefExpr) and
-                    not (isinstance(lvalue_type, PartialType) and lvalue_type.type is None)):
+                    not (isinstance(lvalue_type, PartialType) and
+                         lvalue_type.type is None) and
+                    not (isinstance(lvalue, NameExpr) and lvalue.name == '__match_args__')):
                 if self.check_compatibility_all_supers(lvalue, lvalue_type, rvalue):
                     # We hit an error on this line; don't check for any others
                     return
 
+            if isinstance(lvalue, MemberExpr) and lvalue.name == '__match_args__':
+                self.fail(message_registry.CANNOT_MODIFY_MATCH_ARGS, lvalue)
+
             if lvalue_type:
                 if isinstance(lvalue_type, PartialType) and lvalue_type.type is None:
                     # Try to infer a proper type for a variable with a partial None type.
@@ -2377,7 +2377,8 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
 
             if inferred:
                 rvalue_type = self.expr_checker.accept(rvalue)
-                if not inferred.is_final:
+                if not (inferred.is_final or (isinstance(lvalue, NameExpr) and
+                                              lvalue.name == '__match_args__')):
                     rvalue_type = remove_instance_last_known_values(rvalue_type)
                 self.infer_variable_type(inferred, lvalue, rvalue_type, rvalue)
             self.check_assignment_to_slots(lvalue)

mypy/message_registry.py

@@ -247,3 +247,4 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
 CLASS_PATTERN_DUPLICATE_KEYWORD_PATTERN: Final = 'Duplicate keyword pattern "{}"'
 CLASS_PATTERN_UNKNOWN_KEYWORD: Final = 'Class "{}" has no attribute "{}"'
 MULTIPLE_ASSIGNMENTS_IN_PATTERN: Final = 'Multiple assignments to name "{}" in pattern'
+CANNOT_MODIFY_MATCH_ARGS: Final = 'Cannot assign to "__match_args__"'

test-data/unit/check-python310.test

@@ -881,16 +881,16 @@ reveal_type(z)  # N: Revealed type is "builtins.int*"
 
 [case testMatchNonFinalMatchArgs]
 class A:
-    __match_args__ = ("a", "b")  # N: __match_args__ must be final for checking of match statements to work
+    __match_args__ = ("a", "b")
     a: str
     b: int
 
 m: object
 
 match m:
     case A(i, j):
-        reveal_type(i)  # N: Revealed type is "Any"
-        reveal_type(j)  # N: Revealed type is "Any"
+        reveal_type(i)  # N: Revealed type is "builtins.str"
+        reveal_type(j)  # N: Revealed type is "builtins.int"
 [builtins fixtures/tuple.pyi]
 
 [case testMatchAnyTupleMatchArgs]
@@ -1528,3 +1528,37 @@ class A:
 class B:
     def __enter__(self) -> B: ...
     def __exit__(self, x, y, z) -> None: ...
+
+[case testOverrideMatchArgs]
+class AST:
+   __match_args__ = ()
+
+class stmt(AST): ...
+
+class AnnAssign(stmt):
+   __match_args__ = ('target', 'annotation', 'value', 'simple')
+   target: str
+   annotation: int
+   value: str
+   simple: int
+
+reveal_type(AST.__match_args__)  # N: Revealed type is "Tuple[]"
+reveal_type(stmt.__match_args__)  # N: Revealed type is "Tuple[]"
+reveal_type(AnnAssign.__match_args__)  # N: Revealed type is "Tuple[Literal['target']?, Literal['annotation']?, Literal['value']?, Literal['simple']?]"
+
+AnnAssign.__match_args__ = ('a', 'b', 'c', 'd')  # E: Cannot assign to "__match_args__"
+__match_args__ = 0
+
+def f(x: AST) -> None:
+    match x:
+        case AST():
+            reveal_type(x)  # N: Revealed type is "__main__.AST"
+    match x:
+        case stmt():
+            reveal_type(x)  # N: Revealed type is "__main__.stmt"
+    match x:
+        case AnnAssign(a, b, c, d):
+            reveal_type(a)  # N: Revealed type is "builtins.str"
+            reveal_type(b)  # N: Revealed type is "builtins.int"
+            reveal_type(c)  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]