Docs: how to work around metaclass limitation

Author: robsdedude

docs/source/error_code_list.rst

@@ -231,6 +231,9 @@ must be a subclass of ``type``. Further, the class hierarchy must yield
 a consistent metaclass. For more details, see the
 `Python documentation <https://docs.python.org/3.13/reference/datamodel.html#determining-the-appropriate-metaclass>`_
 
+Note that mypy's metaclass checking is limited and may produce false-positives.
+See also :ref:`limitations`.
+
 Example with an error:
 
 .. code-block:: python

docs/source/metaclasses.rst

@@ -86,3 +86,34 @@ so it's better not to combine metaclasses and class hierarchies:
   such as ``class A(metaclass=f()): ...``
 * Mypy does not and cannot understand arbitrary metaclass code.
 * Mypy only recognizes subclasses of :py:class:`type` as potential metaclasses.
+
+For some builtin types, mypy assumes that their metaclass is :py:class:`abc.ABCMeta`
+even if it's :py:class:`type`. In those cases, you can either
+
+* use :py:class:`abc.ABCMetaclass` instead of :py:class:`type` as the
+  superclass of your metaclass if that works in your use case,
+* mute the error with ``# type: ignore[metaclass]``, or
+* compute the metaclass' superclass dynamically, which mypy doesn't understand
+  so it will also need to be muted.
+
+.. code-block:: python
+
+    import abc
+
+    assert type(tuple) is type  # metaclass of tuple is type
+
+    # the problem:
+    class M0(type): pass
+    class A0(tuple, metaclass=M1): pass  # Mypy Error: metaclass conflict
+
+    # option 1: use ABCMeta instead of type
+    class M1(abc.ABCMeta): pass
+    class A1(tuple, metaclass=M1): pass
+
+    # option 2: mute the error
+    class M2(type): pass
+    class A2(tuple, metaclass=M2): pass  # type: ignore[metaclass]
+
+    # option 3: compute the metaclass dynamically
+    class M3(type(tuple)): pass  # type: ignore[metaclass]
+    class A3(tuple, metaclass=M3): pass

mypy/checker.py

@@ -2809,7 +2809,7 @@ class C(B, A[int]): ...  # this is unsafe because...
             self.msg.base_class_definitions_incompatible(name, base1, base2, ctx)
 
     def check_metaclass_compatibility(self, typ: TypeInfo) -> None:
-        """Ensure that metaclasses of all parent types are compatible."""
+        """Ensures that metaclasses of all parent types are compatible."""
         if (
             typ.is_metaclass()
             or typ.is_protocol

mypy/errorcodes.py

@@ -261,11 +261,7 @@ def __hash__(self) -> int:
     "General",
     default_enabled=False,
 )
-METACLASS: Final[ErrorCode] = ErrorCode(
-    "metaclass",
-    "Ensure that metaclass is valid",
-    "General",
-)
+METACLASS: Final[ErrorCode] = ErrorCode("metaclass", "Ensure that metaclass is valid", "General")
 
 # Syntax errors are often blocking.
 SYNTAX: Final[ErrorCode] = ErrorCode("syntax", "Report syntax errors", "General")

mypy/semanal.py

@@ -2648,7 +2648,11 @@ def get_declared_metaclass(
             elif isinstance(metaclass_expr, MemberExpr):
                 metaclass_name = get_member_expr_fullname(metaclass_expr)
             if metaclass_name is None:
-                self.fail(f'Dynamic metaclass not supported for "{name}"', metaclass_expr, code=codes.METACLASS)
+                self.fail(
+                    f'Dynamic metaclass not supported for "{name}"',
+                    metaclass_expr,
+                    code=codes.METACLASS,
+                )
                 return None, False, True
             sym = self.lookup_qualified(metaclass_name, metaclass_expr)
             if sym is None:
@@ -2677,7 +2681,9 @@ def get_declared_metaclass(
                 metaclass_info = sym.node
 
             if not isinstance(metaclass_info, TypeInfo) or metaclass_info.tuple_type is not None:
-                self.fail(f'Invalid metaclass "{metaclass_name}"', metaclass_expr, code=codes.METACLASS)
+                self.fail(
+                    f'Invalid metaclass "{metaclass_name}"', metaclass_expr, code=codes.METACLASS
+                )
                 return None, False, False
             if not metaclass_info.is_metaclass():
                 self.fail(