"abc" --- Abstract Base Classes
"abc" --- Abstract Base Classes#
Código fuente: Lib/abc.py
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Este módulo proporciona la infraestructura para definir clases de base abstracta (CBAs) en Python, como se describe en PEP 3119; consulte en el PEP el porqué fue agregado a Python. (Véase también PEP 3141 y el módulo "numbers" con respecto a una jerarquía de tipos para números basados en CBAs.)
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.
Este módulo provee la metaclase "ABCMeta" para definir CBAs y una clase auxiliar "ABC" para definir CBAs alternativamente a través de herencia:
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
Metaclases para definir Clases de Base Abstracta (CBAs).
Utilice esta metaclase para crear una CBA. Una CBA puede ser heredada directamente y así, actuar como una clase mixta. También se puede registrar clases concretas no relacionadas (incluso clases integradas) y CBAs no relacionadas como "subclases virtuales" -- estas y sus descendientes serán consideradas subclases del CBA registrado por la función integrada "issubclass()", pero la CBA registrada no aparecerá en su MRO (Orden de Resolución de Métodos) ni las implementaciones de método definidas por la CBA registrada serán invocables (ni siquiera a través de "super()"). [1]
Classes created with a metaclass of "ABCMeta" have the following method:
register(subclass)
Registre la *subclase* como una "subclase virtual" de esta CBA.
Por ejemplo:
from abc import ABC
class MyABC(ABC):
pass
MyABC.register(tuple)
assert issubclass(tuple, MyABC)
assert isinstance((), MyABC)
Distinto en la versión 3.3: Retorna la subclase registrada, para
permitir su uso como decorador de clase.
Distinto en la versión 3.4: To detect calls to "register()", you
can use the "get_cache_token()" function.
También se puede redefinir este método en una clase de base abstracta:
subclasshook(subclass)
(Debe ser definido como un método de clase.)
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.
Para una demostración de estos conceptos, vea este ejemplo de la definición CBA:
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
Un decorador que indica métodos abstractos.
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))
Nota:
A diferencia de los métodos abstractos de Java, estos métodos
abstractos pueden tener una implementación. Esta implementación
se puede llamar a través del mecanismo "super()" de la clase que
lo invalida. Esto podría ser útil como un *end-point* para una
super llamada en un *framework* que use herencia múltiple
cooperativa.
The "abc" module also supports the following legacy decorators:
@abc.abstractclassmethod
Added in version 3.2.
Obsoleto desde la versión 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.
Obsoleto desde la versión 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
Obsoleto desde la versión 3.3: It is now possible to use "@property", "@property.getter", "@property.setter" and "@property.deleter" with "@abstractmethod", making this decorator redundant.
A subclass of the built-in "property", indicating an abstract property.
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):
...
En el ejemplo anterior se define una propiedad de solo lectura; también se puede definir una propiedad abstracta de lectura y escritura marcando adecuadamente uno o varios de los métodos subyacentes como abstractos:
class C(ABC):
@property
def x(self):
...
@x.setter
@abstractmethod
def x(self, val):
...
Si solo algunos componentes son abstractos, solo estos componentes necesitan ser actualizados para crear una propiedad concreta en una subclase:
class D(C):
@C.x.setter
def x(self, val):
...
The "abc" module also provides the following functions:
abc.get_cache_token()
Retorna el token de caché de la clase base abstracta actual.
El token es un objeto opaco (que admite pruebas de igualdad) que identifica la versión actual de la caché de clases de base abstractas para subclases virtuales. El token cambia con cada llamada a "ABCMeta.register()" en cualquier CBA.
Added in version 3.4.
abc.update_abstractmethods(cls)
Una función para recalcular el estado de abstracción de una clase abstracta. Se debe llamar a esta función si los métodos abstractos de una clase se han implementado o cambiado después de su creación. Por lo general, esta función debe llamarse desde dentro de un decorador de clases.
Retorna cls, para permitir su uso como decorador de clases.
Si cls no es una instancia de "ABCMeta", no hace nada.
Nota:
Esta función asume que las superclases de *cls* ya están
actualizadas. No actualiza subclases.
Added in version 3.10.
-[ Notas al pie ]-
[1] Los desarrolladores de C++ pueden notar que el concepto de clase base virtual de Python no es el mismo que en C++.