Gotchas and limitations of metaclass support

Note that metaclasses pose some requirements on the inheritance structure,so it’s better not to combine metaclasses and class hierarchies:

class M1(type): pass
class M2(type): pass
 
class A1(metaclass=M1): pass
class A2(metaclass=M2): pass
 
class B1(A1, metaclass=M2): pass  # Mypy Error: Inconsistent metaclass structure for 'B1'
# At runtime the above definition raises an exception
# TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases
 
# Same runtime error as in B1, but mypy does not catch it yet
class B12(A1, A2): pass

• Mypy does not understand dynamically-computed metaclasses,such as class A(metaclass=f()): …
• Mypy does not and cannot understand arbitrary metaclass code.
• Mypy only recognizes subclasses of type as potential metaclasses.