handle dunder names in @declared_attr separately

Fixed Mypy crash which would occur when using Mypy plugin against code
which made use of :class:`_orm.declared_attr` methods for non-mapped names
like ``__mapper_args__``, ``__table_args__``, or other dunder names, as the
plugin would try to interpret these as mapped attributes which would then
be later mis-handled. As part of this change, the decorated function is
still converted by the plugin into a generic assignment statement (e.g.
``__mapper_args__: Any``) so that the argument signature can continue to be
annotated in the same way one would for any other ``@classmethod`` without
Mypy complaining about the wrong argument type for a method that isn't
explicitly ``@classmethod``.

Fixes: #7321
Change-Id: I55656e867876677c5c55143449db371344be8600

2 files changed, 19 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/mypy/decl_class.py b/lib/sqlalchemy/ext/mypy/decl_class.py
index b85ec0f69..0d7462d5b 100644
--- a/lib/sqlalchemy/ext/mypy/decl_class.py
+++ b/lib/sqlalchemy/ext/mypy/decl_class.py
@@ -241,7 +241,20 @@ def _scan_declarative_decorator_stmt(
 
     left_hand_explicit_type: Optional[ProperType] = None
 
-    if isinstance(stmt.func.type, CallableType):
+    if util.name_is_dunder(stmt.name):
+        # for dunder names like __table_args__, __tablename__,
+        # __mapper_args__ etc., rewrite these as simple assignment
+        # statements; otherwise mypy doesn't like if the decorated
+        # function has an annotation like ``cls: Type[Foo]`` because
+        # it isn't @classmethod
+        any_ = AnyType(TypeOfAny.special_form)
+        left_node = NameExpr(stmt.var.name)
+        left_node.node = stmt.var
+        new_stmt = AssignmentStmt([left_node], TempNode(any_))
+        new_stmt.type = left_node.node.type
+        cls.defs.body[dec_index] = new_stmt
+        return
+    elif isinstance(stmt.func.type, CallableType):
         func_type = stmt.func.type.ret_type
         if isinstance(func_type, UnboundType):
             type_id = names.type_id_for_unbound_type(func_type, cls, api)
diff --git a/lib/sqlalchemy/ext/mypy/util.py b/lib/sqlalchemy/ext/mypy/util.py
index a3825f175..4d55cb728 100644
--- a/lib/sqlalchemy/ext/mypy/util.py
+++ b/lib/sqlalchemy/ext/mypy/util.py
@@ -1,3 +1,4 @@
+import re
 from typing import Any
 from typing import Iterable
 from typing import Iterator
@@ -82,6 +83,10 @@ class SQLAlchemyAttribute:
         return cls(typ=typ, info=info, **data)
 
 
+def name_is_dunder(name):
+    return bool(re.match(r"^__.+?__$", name))
+
+
 def _set_info_metadata(info: TypeInfo, key: str, data: Any) -> None:
     info.metadata.setdefault("sqlalchemy", {})[key] = data