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.. _metaclasses:
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:
Defining a metaclass
********************
.. code-block:: python
class M(type):
pass
class A(metaclass=M):
pass
In Python 2, the syntax for defining a metaclass is different:
.. code-block:: python
class A(object):
__metaclass__ = M
Mypy also supports using :py:func:`six.with_metaclass` and :py:func:`@six.add_metaclass <six.add_metaclass>`
to define metaclass in a portable way:
.. code-block:: python
import six
class A(six.with_metaclass(M)):
pass
@six.add_metaclass(M)
class C(object):
pass
.. _examples:
Metaclass usage example
***********************
Mypy supports the lookup of attributes in the metaclass:
.. code-block:: python
from typing import Type, TypeVar, ClassVar
T = TypeVar('T')
class M(type):
count: ClassVar[int] = 0
def make(cls: Type[T]) -> T:
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")
.. _limitations:
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:
.. code-block:: python
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 :py:class:`type` as potential metaclasses.
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