# `abc` — Abstract Base Classes

Source code: Lib/abc.py

This module provides the infrastructure for defining abstract base classes (ABCs) in Python, as outlined in PEP 3119; see the PEP for why this was added to Python. (See also PEP 3141 and the `numbers` module regarding a type hierarchy for numbers based on ABCs.)

The `collections` module has some concrete classes that derive from ABCs; these can, of course, be further derived. In addition, the `collections.abc` submodule has some ABCs that can be used to test whether a class or instance provides a particular interface, for example, if it is hashable or if it is a mapping.

This module provides the metaclass `ABCMeta` for defining ABCs and a helper class `ABC` to alternatively define ABCs through inheritance:

class abc.ABC

  A helper class that has `ABCMeta` as its metaclass. With this class, an abstract base class can be created by simply deriving from `ABC` avoiding sometimes confusing metaclass usage, for example:

      from abc import ABC

      class MyABC(ABC):
          pass

  Note that the type of `ABC` is still `ABCMeta`, therefore inheriting from `ABC` requires the usual precautions regarding metaclass usage, as multiple inheritance may lead to metaclass conflicts. One may also define an abstract base class by passing the metaclass keyword and using `ABCMeta` directly, for example:

      from abc import ABCMeta

      class MyABC(metaclass=ABCMeta):
          pass

  Added in version 3.4.

class abc.ABCMeta

  Pośrednie uogólnienie dla definiowania abstrakcyjnych uogólnień podstawowych (w skr. z ang. - ABC - Abstract Base Class).

  Używaj tego uogólnienia pośredniego aby utworzyć abstrakcyjne uogólnienie podstawowe (w skr. z ang. ABC). ABC może być wykorzystane do tworzenia następnych uogólnień bezpośrednio, i wtedy działa jako uogólnienie domieszki. Możesz także zarejestrować niezwiązane uogólnienia konkretne (nawet wbudowane uogólnienia) i niezwiązane abstrakcyjne uogólnienia podstawowe jako „sztuczne podrzędne uogólnienia” – te uogólnienia i pochodne po nich będą uznawane za podrzędne uogólnienia rejestrowanego uogólnienia podstawowego abstrakcyjnego przez wbudowane zadanie `issubclass()`, ale rejestrowanie abstrakcyjnego uogólnienia bazowego nie pokaże się w ich sposobie rozwiązywania sposobów postępowania (w skr. MRO - z ang. - Method Resolution Order) ani też wypełnienia sposobów postępowania określone przez rejestrowanie abstrakcyjnych bazowych uogólnień nie będą wywoływalne (nawet nie przez użycie `super()`). [1]

  Classes created with a metaclass of `ABCMeta` have the following method:

  register(subclass)

    Zarejestruj podrzędne uogólnienie jako „wirtualne podrzędne uogólnienie” tego uogólnienia abstrakcyjnego podstawowego. Na przykład:

        from abc import ABC

        class MyABC(ABC):
            pass

        MyABC.register(tuple)

        assert issubclass(tuple, MyABC)
        assert isinstance((), MyABC)

    Zmienione w wersji 3.3: Returns the registered subclass, to allow usage as a class decorator.

    Zmienione w wersji 3.4: To detect calls to `register()`, you can use the `get_cache_token()` function.

  Możesz także obejść ten sposób postępowania w abstrakcyjnym bazowym uogólnieniu:

  __subclasshook__(subclass)

    (Musi być określona jako sposób postępowania uogólnienia.)

    Check whether subclass is considered a subclass of this ABC. This means that you can customize the behavior of `issubclass()` further without the need to call `register()` on every class you want to consider a subclass of the ABC. (This class method is called from the `__subclasscheck__()` method of the ABC.)

    This method should return `True`, `False` or `NotImplemented`. If it returns `True`, the subclass is considered a subclass of this ABC. If it returns `False`, the subclass is not considered a subclass of this ABC, even if it would normally be one. If it returns `NotImplemented`, the subclass check is continued with the usual mechanism.

  Dla demonstracji tych pomysłów, popatrz na tę przykładową definicję abstrakcyjnego uogólnienia podstawowego:

      class Foo:
          def __getitem__(self, index):
              ...
          def __len__(self):
              ...
          def get_iterator(self):
              return iter(self)

      class MyIterable(ABC):

          @abstractmethod
          def __iter__(self):
              while False:
                  yield None

          def get_iterator(self):
              return self.__iter__()

          @classmethod
          def __subclasshook__(cls, C):
              if cls is MyIterable:
                  if any("__iter__" in B.__dict__ for B in C.__mro__):
                      return True
              return NotImplemented

      MyIterable.register(Foo)

  The ABC `MyIterable` defines the standard iterable method, `__iter__()`, as an abstract method. The implementation given here can still be called from subclasses. The `get_iterator()` method is also part of the `MyIterable` abstract base class, but it does not have to be overridden in non-abstract derived classes.

  The `__subclasshook__()` class method defined here says that any class that has an `__iter__()` method in its `__dict__` (or in that of one of its base classes, accessed via the `__mro__` list) is considered a `MyIterable` too.

  Finally, the last line makes `Foo` a virtual subclass of `MyIterable`, even though it does not define an `__iter__()` method (it uses the old-style iterable protocol, defined in terms of `__len__()` and `__getitem__()`). Note that this will not make `get_iterator` available as a method of `Foo`, so it is provided separately.

The `abc` module also provides the following decorator:

@abc.abstractmethod

  Dekorator wskazujący abstrakcyjne sposoby postępowania.

  Using this decorator requires that the class’s metaclass is `ABCMeta` or is derived from it. A class that has a metaclass derived from `ABCMeta` cannot be instantiated unless all of its abstract methods and properties are overridden. The abstract methods can be called using any of the normal «super» call mechanisms. `abstractmethod()` may be used to declare abstract methods for properties and descriptors.

  Dynamically adding abstract methods to a class, or attempting to modify the abstraction status of a method or class once it is created, are only supported using the `update_abstractmethods()` function. The `abstractmethod()` only affects subclasses derived using regular inheritance; „virtual subclasses” registered with the ABC’s `register()` method are not affected.

  When `abstractmethod()` is applied in combination with other method descriptors, it should be applied as the innermost decorator, as shown in the following usage examples:

      class C(ABC):
          @abstractmethod
          def my_abstract_method(self, arg1):
              ...
          @classmethod
          @abstractmethod
          def my_abstract_classmethod(cls, arg2):
              ...
          @staticmethod
          @abstractmethod
          def my_abstract_staticmethod(arg3):
              ...

          @property
          @abstractmethod
          def my_abstract_property(self):
              ...
          @my_abstract_property.setter
          @abstractmethod
          def my_abstract_property(self, val):
              ...

          @abstractmethod
          def _get_x(self):
              ...
          @abstractmethod
          def _set_x(self, val):
              ...
          x = property(_get_x, _set_x)

  In order to correctly interoperate with the abstract base class machinery, the descriptor must identify itself as abstract using `__isabstractmethod__`. In general, this attribute should be `True` if any of the methods used to compose the descriptor are abstract. For example, Python’s built-in `property` does the equivalent of:

      class Descriptor:
          ...
          @property
          def __isabstractmethod__(self):
              return any(getattr(f, '__isabstractmethod__', False) for
                         f in (self._fget, self._fset, self._fdel))

  Informacja

  W przeciwieństwie do abstrakcyjnych sposobów postępowania Javy, te abstrakcyjne sposoby postępowania mogą mieć swoje wypełnienie. To wypełnienie może być wzywane przez mechanizm odwołań do zadania `super()`, z uogólnienia które przesłania taki sposób postępowania. To może być użyteczne jako punkt końcowy super-wywołania w ramach udogodnienia które używa współpracującego wielokrotnego-dziedziczenia.

The `abc` module also supports the following legacy decorators:

@abc.abstractclassmethod

  Added in version 3.2.

  Niezalecane od wersji 3.3: It is now possible to use `classmethod` with `abstractmethod()`, making this decorator redundant.

  A subclass of the built-in `classmethod()`, indicating an abstract classmethod. Otherwise it is similar to `abstractmethod()`.

  This special case is deprecated, as the `classmethod()` decorator is now correctly identified as abstract when applied to an abstract method:

      class C(ABC):
          @classmethod
          @abstractmethod
          def my_abstract_classmethod(cls, arg):
              ...

@abc.abstractstaticmethod

  Added in version 3.2.

  Niezalecane od wersji 3.3: It is now possible to use `staticmethod` with `abstractmethod()`, making this decorator redundant.

  A subclass of the built-in `staticmethod()`, indicating an abstract staticmethod. Otherwise it is similar to `abstractmethod()`.

  This special case is deprecated, as the `staticmethod()` decorator is now correctly identified as abstract when applied to an abstract method:

      class C(ABC):
          @staticmethod
          @abstractmethod
          def my_abstract_staticmethod(arg):
              ...

@abc.abstractproperty

  Niezalecane od wersji 3.3: It is now possible to use `property`, `property.getter()`, `property.setter()` and `property.deleter()` with `abstractmethod()`, making this decorator redundant.

  Podrzędne uogólnienie wbudowanego zadania `property()` wskazujące abstrakcyjną właściwość.

  This special case is deprecated, as the `property()` decorator is now correctly identified as abstract when applied to an abstract method:

      class C(ABC):
          @property
          @abstractmethod
          def my_abstract_property(self):
              ...

  The above example defines a read-only property; you can also define a read-write abstract property by appropriately marking one or more of the underlying methods as abstract:

      class C(ABC):
          @property
          def x(self):
              ...

          @x.setter
          @abstractmethod
          def x(self, val):
              ...

  If only some components are abstract, only those components need to be updated to create a concrete property in a subclass:

      class D(C):
          @C.x.setter
          def x(self, val):
              ...

The `abc` module also provides the following functions:

abc.get_cache_token()

  Returns the current abstract base class cache token.

  The token is an opaque object (that supports equality testing) identifying the current version of the abstract base class cache for virtual subclasses. The token changes with every call to `ABCMeta.register()` on any ABC.

  Added in version 3.4.

abc.update_abstractmethods(cls)

  A function to recalculate an abstract class’s abstraction status. This function should be called if a class’s abstract methods have been implemented or changed after it was created. Usually, this function should be called from within a class decorator.

  Returns cls, to allow usage as a class decorator.

  If cls is not an instance of `ABCMeta`, does nothing.

  Informacja

  This function assumes that cls’s superclasses are already updated. It does not update any subclasses.

  Added in version 3.10.

Przypisy

[1]

Programiści C++ powinni zauważyć, że koncepcja wirtualnego uogólnienia podstawowego z języka pytonowskiego nie jest tożsame z tym z C++»a.