问题:是否可以在Python中创建抽象类?

如何在Python中使类或方法抽象?

我尝试__new__()像这样重新定义:

class F:
    def __new__(cls):
        raise Exception("Unable to create an instance of abstract class %s" %cls)

但是现在,如果我创建一个像这样G继承的F类:

class G(F):
    pass

那么我也无法实例化G,因为它调用了其超类的__new__方法。

有没有更好的方法来定义抽象类?

How can I make a class or method abstract in Python?

I tried redefining __new__() like so:

class F:
    def __new__(cls):
        raise Exception("Unable to create an instance of abstract class %s" %cls)

but now if I create a class G that inherits from F like so:

class G(F):
    pass

then I can’t instantiate G either, since it calls its super class’s __new__ method.

Is there a better way to define an abstract class?


回答 0

使用该abc模块创建抽象类。使用abstractmethod装饰器来声明方法抽象,并根据您的Python版本使用以下三种方式之一声明类抽象。

在Python 3.4及更高版本中,您可以从继承ABC。在Python的早期版本中,您需要将类的元类指定为ABCMeta。指定元类在Python 3和Python 2中具有不同的语法。三种可能性如下所示:

# Python 3.4+
from abc import ABC, abstractmethod
class Abstract(ABC):
    @abstractmethod
    def foo(self):
        pass
# Python 3.0+
from abc import ABCMeta, abstractmethod
class Abstract(metaclass=ABCMeta):
    @abstractmethod
    def foo(self):
        pass
# Python 2
from abc import ABCMeta, abstractmethod
class Abstract:
    __metaclass__ = ABCMeta

    @abstractmethod
    def foo(self):
        pass

无论使用哪种方式,都将无法实例化具有抽象方法的抽象类,但将能够实例化提供这些方法的具体定义的子类:

>>> Abstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Abstract with abstract methods foo
>>> class StillAbstract(Abstract):
...     pass
... 
>>> StillAbstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class StillAbstract with abstract methods foo
>>> class Concrete(Abstract):
...     def foo(self):
...         print('Hello, World')
... 
>>> Concrete()
<__main__.Concrete object at 0x7fc935d28898>

Use the abc module to create abstract classes. Use the abstractmethod decorator to declare a method abstract, and declare a class abstract using one of three ways, depending upon your Python version.

In Python 3.4 and above, you can inherit from ABC. In earlier versions of Python, you need to specify your class’s metaclass as ABCMeta. Specifying the metaclass has different syntax in Python 3 and Python 2. The three possibilities are shown below:

# Python 3.4+
from abc import ABC, abstractmethod
class Abstract(ABC):
    @abstractmethod
    def foo(self):
        pass
# Python 3.0+
from abc import ABCMeta, abstractmethod
class Abstract(metaclass=ABCMeta):
    @abstractmethod
    def foo(self):
        pass
# Python 2
from abc import ABCMeta, abstractmethod
class Abstract:
    __metaclass__ = ABCMeta

    @abstractmethod
    def foo(self):
        pass

Whichever way you use, you won’t be able to instantiate an abstract class that has abstract methods, but will be able to instantiate a subclass that provides concrete definitions of those methods:

>>> Abstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Abstract with abstract methods foo
>>> class StillAbstract(Abstract):
...     pass
... 
>>> StillAbstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class StillAbstract with abstract methods foo
>>> class Concrete(Abstract):
...     def foo(self):
...         print('Hello, World')
... 
>>> Concrete()
<__main__.Concrete object at 0x7fc935d28898>

回答 1

老式的方法(PEP 3119之前的方法)只是raise NotImplementedError在调用抽象方法的抽象类中进行。

class Abstract(object):
    def foo(self):
        raise NotImplementedError('subclasses must override foo()!')

class Derived(Abstract):
    def foo(self):
        print 'Hooray!'

>>> d = Derived()
>>> d.foo()
Hooray!
>>> a = Abstract()
>>> a.foo()
Traceback (most recent call last): [...]

它没有与使用abc模块相同的好属性。您仍然可以实例化抽象基类本身,直到在运行时调用抽象方法,您才会发现错误。

但是,如果您要处理的是几套简单的类,也许只有一些抽象方法,则此方法比尝试阅读abc文档要容易一些。

The old-school (pre-PEP 3119) way to do this is just to raise NotImplementedError in the abstract class when an abstract method is called.

class Abstract(object):
    def foo(self):
        raise NotImplementedError('subclasses must override foo()!')

class Derived(Abstract):
    def foo(self):
        print 'Hooray!'

>>> d = Derived()
>>> d.foo()
Hooray!
>>> a = Abstract()
>>> a.foo()
Traceback (most recent call last): [...]

This doesn’t have the same nice properties as using the abc module does. You can still instantiate the abstract base class itself, and you won’t find your mistake until you call the abstract method at runtime.

But if you’re dealing with a small set of simple classes, maybe with just a few abstract methods, this approach is a little easier than trying to wade through the abc documentation.


回答 2

这是一种非常简单的方法,而无需处理ABC模块。

__init__要成为抽象类的类的方法中,可以检查self的“类型”。如果self的类型是基类,则调用方将尝试实例化基类,因此引发异常。这是一个简单的例子:

class Base():
    def __init__(self):
        if type(self) is Base:
            raise Exception('Base is an abstract class and cannot be instantiated directly')
        # Any initialization code
        print('In the __init__  method of the Base class')

class Sub(Base):
    def __init__(self):
        print('In the __init__ method of the Sub class before calling __init__ of the Base class')
        super().__init__()
        print('In the __init__ method of the Sub class after calling __init__ of the Base class')

subObj = Sub()
baseObj = Base()

运行时,它将生成:

In the __init__ method of the Sub class before calling __init__ of the Base class
In the __init__  method of the Base class
In the __init__ method of the Sub class after calling __init__ of the Base class
Traceback (most recent call last):
  File "/Users/irvkalb/Desktop/Demo files/Abstract.py", line 16, in <module>
    baseObj = Base()
  File "/Users/irvkalb/Desktop/Demo files/Abstract.py", line 4, in __init__
    raise Exception('Base is an abstract class and cannot be instantiated directly')
Exception: Base is an abstract class and cannot be instantiated directly

这表明您可以实例化从基类继承的子类,但不能直接实例化基类。

Here’s a very easy way without having to deal with the ABC module.

In the __init__ method of the class that you want to be an abstract class, you can check the “type” of self. If the type of self is the base class, then the caller is trying to instantiate the base class, so raise an exception. Here’s a simple example:

class Base():
    def __init__(self):
        if type(self) is Base:
            raise Exception('Base is an abstract class and cannot be instantiated directly')
        # Any initialization code
        print('In the __init__  method of the Base class')

class Sub(Base):
    def __init__(self):
        print('In the __init__ method of the Sub class before calling __init__ of the Base class')
        super().__init__()
        print('In the __init__ method of the Sub class after calling __init__ of the Base class')

subObj = Sub()
baseObj = Base()

When run, it produces:

In the __init__ method of the Sub class before calling __init__ of the Base class
In the __init__  method of the Base class
In the __init__ method of the Sub class after calling __init__ of the Base class
Traceback (most recent call last):
  File "/Users/irvkalb/Desktop/Demo files/Abstract.py", line 16, in <module>
    baseObj = Base()
  File "/Users/irvkalb/Desktop/Demo files/Abstract.py", line 4, in __init__
    raise Exception('Base is an abstract class and cannot be instantiated directly')
Exception: Base is an abstract class and cannot be instantiated directly

This shows that you can instantiate a subclass that inherits from a base class, but you cannot instantiate the base class directly.


回答 3

先前的大多数答案都是正确的,但这是Python 3.7的答案和示例是的,您可以创建一个抽象类和方法。提醒一下,有时一个类应该定义一个逻辑上属于一个类的方法,但是该类无法指定如何实现该方法。例如,在下面的“父母和婴儿”类中,他们都吃东西,但实施方式会有所不同,因为婴儿和父母吃的是不同种类的食物,并且进食的次数不同。因此,eat方法的子类将覆盖AbstractClass.eat。

from abc import ABC, abstractmethod

class AbstractClass(ABC):

    def __init__(self, value):
        self.value = value
        super().__init__()

    @abstractmethod
    def eat(self):
        pass

class Parents(AbstractClass):
    def eat(self):
        return "eat solid food "+ str(self.value) + " times each day"

class Babies(AbstractClass):
    def eat(self):
        return "Milk only "+ str(self.value) + " times or more each day"

food = 3    
mom = Parents(food)
print("moms ----------")
print(mom.eat())

infant = Babies(food)
print("infants ----------")
print(infant.eat())

输出:

moms ----------
eat solid food 3 times each day
infants ----------
Milk only 3 times or more each day

Most Previous answers were correct but here is the answer and example for Python 3.7. Yes, you can create an abstract class and method. Just as a reminder sometimes a class should define a method which logically belongs to a class, but that class cannot specify how to implement the method. For example, in the below Parents and Babies classes they both eat but the implementation will be different for each because babies and parents eat a different kind of food and the number of times they eat is different. So, eat method subclasses overrides AbstractClass.eat.

from abc import ABC, abstractmethod

class AbstractClass(ABC):

    def __init__(self, value):
        self.value = value
        super().__init__()

    @abstractmethod
    def eat(self):
        pass

class Parents(AbstractClass):
    def eat(self):
        return "eat solid food "+ str(self.value) + " times each day"

class Babies(AbstractClass):
    def eat(self):
        return "Milk only "+ str(self.value) + " times or more each day"

food = 3    
mom = Parents(food)
print("moms ----------")
print(mom.eat())

infant = Babies(food)
print("infants ----------")
print(infant.eat())

OUTPUT:

moms ----------
eat solid food 3 times each day
infants ----------
Milk only 3 times or more each day

回答 4

这将在python 3中工作

from abc import ABCMeta, abstractmethod

class Abstract(metaclass=ABCMeta):

    @abstractmethod
    def foo(self):
        pass

Abstract()
>>> TypeError: Can not instantiate abstract class Abstract with abstract methods foo

This one will be working in python 3

from abc import ABCMeta, abstractmethod

class Abstract(metaclass=ABCMeta):

    @abstractmethod
    def foo(self):
        pass

Abstract()
>>> TypeError: Can not instantiate abstract class Abstract with abstract methods foo

回答 5

如其他答案所述,是的,您可以使用abc模块在Python中使用抽象类。下面我举个实际的例子使用抽象@classmethod@property@abstractmethod(使用Python 3.6+)。对我而言,通常更容易从示例开始,我可以轻松地复制和粘贴;我希望这个答案对其他人也有用。

首先创建一个名为的基类Base

from abc import ABC, abstractmethod

class Base(ABC):

    @classmethod
    @abstractmethod
    def from_dict(cls, d):
        pass

    @property
    @abstractmethod
    def prop1(self):
        pass

    @property
    @abstractmethod
    def prop2(self):
        pass

    @prop2.setter
    @abstractmethod
    def prop2(self, val):
        pass

    @abstractmethod
    def do_stuff(self):
        pass

我们的Base类将始终具有from_dict classmethod,a property prop1(只读)和a property prop2(也可以设置)以及称为的函数do_stuff。现在构建的任何类都Base将必须为方法/属性实现所有这些。请注意,要使方法抽象,则需要两个装饰器- classmethod和abstract property

现在我们可以创建一个A这样的类:

class A(Base):
    def __init__(self, name, val1, val2):
        self.name = name
        self.__val1 = val1
        self._val2 = val2

    @classmethod
    def from_dict(cls, d):
        name = d['name']
        val1 = d['val1']
        val2 = d['val2']

        return cls(name, val1, val2)

    @property
    def prop1(self):
        return self.__val1

    @property
    def prop2(self):
        return self._val2

    @prop2.setter
    def prop2(self, value):
        self._val2 = value

    def do_stuff(self):
        print('juhu!')

    def i_am_not_abstract(self):
        print('I can be customized')

所有必需的方法/属性均已实现,我们当然可以添加不属于Base(here :)的其他功能i_am_not_abstract

现在我们可以做:

a1 = A('dummy', 10, 'stuff')
a2 = A.from_dict({'name': 'from_d', 'val1': 20, 'val2': 'stuff'})

a1.prop1
# prints 10

a1.prop2
# prints 'stuff'

无法根据需要设置prop1

a.prop1 = 100

将返回

AttributeError:无法设置属性

我们的from_dict方法也可以正常工作:

a2.prop1
# prints 20

如果我们现在这样定义第二个类B

class B(Base):
    def __init__(self, name):
        self.name = name

    @property
    def prop1(self):
        return self.name

并尝试实例化这样的对象:

b = B('iwillfail')

我们会得到一个错误

TypeError:无法使用抽象方法do_stuff,from_dict,prop2实例化抽象类B

列出Base我们未在其中实现的所有定义的事物B

As explained in the other answers, yes you can use abstract classes in Python using the abc module. Below I give an actual example using abstract @classmethod, @property and @abstractmethod (using Python 3.6+). For me it is usually easier to start off with examples I can easily copy&paste; I hope this answer is also useful for others.

Let’s first create a base class called Base:

from abc import ABC, abstractmethod

class Base(ABC):

    @classmethod
    @abstractmethod
    def from_dict(cls, d):
        pass

    @property
    @abstractmethod
    def prop1(self):
        pass

    @property
    @abstractmethod
    def prop2(self):
        pass

    @prop2.setter
    @abstractmethod
    def prop2(self, val):
        pass

    @abstractmethod
    def do_stuff(self):
        pass

Our Base class will always have a from_dict classmethod, a property prop1 (which is read-only) and a property prop2 (which can also be set) as well as a function called do_stuff. Whatever class is now built based on Base will have to implement all of these for methods/properties. Please note that for a method to be abstract, two decorators are required – classmethod and abstract property.

Now we could create a class A like this:

class A(Base):
    def __init__(self, name, val1, val2):
        self.name = name
        self.__val1 = val1
        self._val2 = val2

    @classmethod
    def from_dict(cls, d):
        name = d['name']
        val1 = d['val1']
        val2 = d['val2']

        return cls(name, val1, val2)

    @property
    def prop1(self):
        return self.__val1

    @property
    def prop2(self):
        return self._val2

    @prop2.setter
    def prop2(self, value):
        self._val2 = value

    def do_stuff(self):
        print('juhu!')

    def i_am_not_abstract(self):
        print('I can be customized')

All required methods/properties are implemented and we can – of course – also add additional functions that are not part of Base (here: i_am_not_abstract).

Now we can do:

a1 = A('dummy', 10, 'stuff')
a2 = A.from_dict({'name': 'from_d', 'val1': 20, 'val2': 'stuff'})

a1.prop1
# prints 10

a1.prop2
# prints 'stuff'

As desired, we cannot set prop1:

a.prop1 = 100

will return

AttributeError: can’t set attribute

Also our from_dict method works fine:

a2.prop1
# prints 20

If we now defined a second class B like this:

class B(Base):
    def __init__(self, name):
        self.name = name

    @property
    def prop1(self):
        return self.name

and tried to instantiate an object like this:

b = B('iwillfail')

we will get an error

TypeError: Can’t instantiate abstract class B with abstract methods do_stuff, from_dict, prop2

listing all the things defined in Base which we did not implement in B.


回答 6

这也有效并且很简单:

class A_abstract(object):

    def __init__(self):
        # quite simple, old-school way.
        if self.__class__.__name__ == "A_abstract": 
            raise NotImplementedError("You can't instantiate this abstract class. Derive it, please.")

class B(A_abstract):

        pass

b = B()

# here an exception is raised:
a = A_abstract()

also this works and is simple:

class A_abstract(object):

    def __init__(self):
        # quite simple, old-school way.
        if self.__class__.__name__ == "A_abstract": 
            raise NotImplementedError("You can't instantiate this abstract class. Derive it, please.")

class B(A_abstract):

        pass

b = B()

# here an exception is raised:
a = A_abstract()

回答 7

您还可以利用__new__方法来发挥自己的优势。你只是忘记了什么。__new__方法始终返回新对象,因此您必须返回其超类的new方法。进行如下操作。

class F:
    def __new__(cls):
        if cls is F:
            raise TypeError("Cannot create an instance of abstract class '{}'".format(cls.__name__))
        return super().__new__(cls)

使用新方法时,必须返回对象,而不是None关键字。那就是你所错过的。

You can also harness the __new__ method to your advantage. You just forgot something. The __new__ method always returns the new object so you must return its superclass’ new method. Do as follows.

class F:
    def __new__(cls):
        if cls is F:
            raise TypeError("Cannot create an instance of abstract class '{}'".format(cls.__name__))
        return super().__new__(cls)

When using the new method, you have to return the object, not the None keyword. That’s all you missed.


回答 8

我发现了可接受的答案,所有其他答案都很奇怪,因为它们传递self给了抽象类。没有实例化抽象类,因此不能具有self

所以尝试一下,它可以工作。

from abc import ABCMeta, abstractmethod


class Abstract(metaclass=ABCMeta):
    @staticmethod
    @abstractmethod
    def foo():
        """An abstract method. No need to write pass"""


class Derived(Abstract):
    def foo(self):
        print('Hooray!')


FOO = Derived()
FOO.foo()

I find the accepted answer, and all the others strange, since they pass self to an abstract class. An abstract class is not instantiated so can’t have a self.

So try this, it works.

from abc import ABCMeta, abstractmethod


class Abstract(metaclass=ABCMeta):
    @staticmethod
    @abstractmethod
    def foo():
        """An abstract method. No need to write pass"""


class Derived(Abstract):
    def foo(self):
        print('Hooray!')


FOO = Derived()
FOO.foo()

回答 9

 from abc import ABCMeta, abstractmethod

 #Abstract class and abstract method declaration
 class Jungle(metaclass=ABCMeta):
     #constructor with default values
     def __init__(self, name="Unknown"):
     self.visitorName = name

     def welcomeMessage(self):
         print("Hello %s , Welcome to the Jungle" % self.visitorName)

     # abstract method is compulsory to defined in child-class
     @abstractmethod
     def scarySound(self):
         pass
 from abc import ABCMeta, abstractmethod

 #Abstract class and abstract method declaration
 class Jungle(metaclass=ABCMeta):
     #constructor with default values
     def __init__(self, name="Unknown"):
     self.visitorName = name

     def welcomeMessage(self):
         print("Hello %s , Welcome to the Jungle" % self.visitorName)

     # abstract method is compulsory to defined in child-class
     @abstractmethod
     def scarySound(self):
         pass

回答 10

在您的代码段中,您还可以通过为__new__子类中的方法提供一个实现来解决此问题,类似地:

def G(F):
    def __new__(cls):
        # do something here

但这是一个hack,除非您知道自己在做什么,否则我建议您不要这样做。对于几乎所有情况,我都建议您使用该abc模块,而我之前的其他人都建议使用该模块。

同样,当您创建一个新的(基)类时,使其成为子类object,如下所示:class MyBaseClass(object):。我不知道它是否还有那么大的意义,但它有助于保持代码的样式一致性

In your code snippet, you could also resolve this by providing an implementation for the __new__ method in the subclass, likewise:

def G(F):
    def __new__(cls):
        # do something here

But this is a hack and I advise you against it, unless you know what you are doing. For nearly all cases I advise you to use the abc module, that others before me have suggested.

Also when you create a new (base) class, make it subclass object, like this: class MyBaseClass(object):. I don’t know if it is that much significant anymore, but it helps retain style consistency on your code


回答 11

只是@TimGilbert的老式答案的快速补充…您可以使抽象基类的init()方法抛出异常,这将阻止实例化它,不是吗?

>>> class Abstract(object):
...     def __init__(self):
...         raise NotImplementedError("You can't instantiate this class!")
...
>>> a = Abstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
NotImplementedError: You can't instantiate this class! 

Just a quick addition to @TimGilbert’s old-school answer…you can make your abstract base class’s init() method throw an exception and that would prevent it from being instantiated, no?

>>> class Abstract(object):
...     def __init__(self):
...         raise NotImplementedError("You can't instantiate this class!")
...
>>> a = Abstract()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
NotImplementedError: You can't instantiate this class! 

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