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metaclasses.rst

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Metaclasses

A :ref:`metaclass <python:metaclasses>` is a class that describes the construction and behavior of other classes, similarly to how classes describe the construction and behavior of objects. The default metaclass is :py:class:`type`, but it's possible to use other metaclasses. Metaclasses allows one to create "a different kind of class", such as :py:class:`~enum.Enum`s, :py:class:`~typing.NamedTuple`s and singletons.

Mypy has some special understanding of :py:class:`~abc.ABCMeta` and EnumMeta.

Defining a metaclass

class M(type):
    pass

class A(metaclass=M):
    pass

Metaclass usage example

Mypy supports the lookup of attributes in the metaclass:

from typing import ClassVar, TypeVar

S = TypeVar("S")

class M(type):
    count: ClassVar[int] = 0

    def make(cls: type[S]) -> S:
        M.count += 1
        return cls()

class A(metaclass=M):
    pass

a: A = A.make()  # make() is looked up at M; the result is an object of type A
print(A.count)

class B(A):
    pass

b: B = B.make()  # metaclasses are inherited
print(B.count + " objects were created")  # Error: Unsupported operand types for + ("int" and "str")

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: metaclass conflict
# 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

class B12(A1, A2): pass  # Mypy Error: metaclass conflict

# This can be solved via a common metaclass subtype:
class CorrectMeta(M1, M2): pass
class B2(A1, A2, metaclass=CorrectMeta): pass  # OK, runtime is also OK
  • 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 :py:class:`type` as potential metaclasses.
  • Self is not allowed as annotation in metaclasses as per PEP 673.