Question

如何在Python 2.7中为抽象类方法创建装饰器？

是的，这与this question类似，但我想将abc.abstractmethod和classmethod结合起来，而不是staticmethod。此外，看起来abc.abstractclassmethod是added in Python 3 (I think?)，但我使用的是Google App Engine，因此我目前仅限于使用Python 2.7

提前致谢。

Answer 1

这是一个从Python 3.3的 abc 模块中的源代码派生的工作示例：

from abc import ABCMeta

class abstractclassmethod(classmethod):

    __isabstractmethod__ = True

    def __init__(self, callable):
        callable.__isabstractmethod__ = True
        super(abstractclassmethod, self).__init__(callable)

class DemoABC:

    __metaclass__ = ABCMeta

    @abstractclassmethod
    def from_int(cls, n):
        return cls()

class DemoConcrete(DemoABC):

    @classmethod
    def from_int(cls, n):
        return cls(2*n)

    def __init__(self, n):
        print 'Initializing with', n

这是运行时的样子：

>>> d = DemoConcrete(5)             # Succeeds by calling a concrete __init__()
Initializing with 5

>>> d = DemoConcrete.from_int(5)    # Succeeds by calling a concrete from_int()
Initializing with 10

>>> DemoABC()                       # Fails because from_int() is abstract    
Traceback (most recent call last):
  ...
TypeError: Can't instantiate abstract class DemoABC with abstract methods from_int

>>> DemoABC.from_int(5)             # Fails because from_int() is not implemented
Traceback (most recent call last):
  ...
TypeError: Can't instantiate abstract class DemoABC with abstract methods from_int

请注意，最终示例失败，因为cls()无法实例化。 ABCMeta 可防止未定义所有必需抽象方法的类过早实例化。

调用 from_int（）抽象类方法时触发失败的另一种方法是让它引发异常：

class DemoABC:

    __metaclass__ = ABCMeta

    @abstractclassmethod
    def from_int(cls, n):
        raise NotImplementedError

设计 ABCMeta 不会阻止在未实例化的类上调用任何抽象方法，因此您可以通过调用cls()来触发失败，因为类方法通常会这样做或者通过引发 NotImplementedError 。无论哪种方式，你都会得到一个漂亮，干净的失败。

编写描述符来拦截对抽象类方法的直接调用可能很诱人，但这与 ABCMeta 的总体设计不一致（这与检查所需方法有关）在实例化之前而不是在调用方法时。）

Answer 2

另一种可能的解决方法：

class A:
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def some_classmethod(cls):
        """IMPORTANT: this is class method, override it with @classmethod!"""
        pass

class B(A):
    @classmethod
    def some_classmethod(cls):
        print cls

现在，在实现'some_classmethod'之前，仍然无法从A实例化，并且如果使用classmethod实现它，它就可以工作。

Answer 3

您可以升级到 Python 3 。

从 Python 3.3 开始，它是possible to combine @classmethod和@abstractmethod：

import abc
class Foo(abc.ABC):
    @classmethod
    @abc.abstractmethod
    def my_abstract_classmethod(...):
        pass

感谢@gerrit向我指出这一点。

Answer 4

我最近遇到了同样的问题。也就是说，我需要抽象的类方法，但由于其他项目限制而无法使用Python 3。我想出的解决方案如下：

abcExtend.py：

import abc

class instancemethodwrapper(object):
    def __init__(self, callable):
        self.callable = callable
        self.__dontcall__ = False

    def __getattr__(self, key):
        return getattr(self.callable, key)

    def __call__(self, *args, **kwargs):
        if self.__dontcall__:
            raise TypeError('Attempted to call abstract method.')
        return self.callable(*args,**kwargs)

class newclassmethod(classmethod):
    def __init__(self, func):
        super(newclassmethod, self).__init__(func)
        isabstractmethod = getattr(func,'__isabstractmethod__',False)
        if isabstractmethod:
            self.__isabstractmethod__ = isabstractmethod

    def __get__(self, instance, owner):
        result = instancemethodwrapper(super(newclassmethod, self).__get__(instance, owner))
        isabstractmethod = getattr(self,'__isabstractmethod__',False)
        if isabstractmethod:
            result.__isabstractmethod__ = isabstractmethod
            abstractmethods = getattr(owner,'__abstractmethods__',None)
            if abstractmethods and result.__name__ in abstractmethods:
                result.__dontcall__ = True
        return result

class abstractclassmethod(newclassmethod):
    def __init__(self, func):
        func = abc.abstractmethod(func)
        super(abstractclassmethod,self).__init__(func)

用法：

from abcExtend import abstractclassmethod

class A(object):
    __metaclass__ = abc.ABCMeta    
    @abstractclassmethod
    def foo(cls):
        return 6

class B(A):
    pass

class C(B):
    @classmethod
    def foo(cls):
        return super(C,cls).foo() + 1

try:
    a = A()
except TypeError:
    print 'Instantiating A raises a TypeError.'

try:
    A.foo()
except TypeError:
    print 'Calling A.foo raises a TypeError.'

try:
    b = B()
except TypeError:
    print 'Instantiating B also raises a TypeError because foo was not overridden.'

try:
    B.foo()
except TypeError:
    print 'As does calling B.foo.'

#But C can be instantiated because C overrides foo
c = C()

#And C.foo can be called
print C.foo()

这里有一些pyunit测试，可以进行更详尽的演示。

testAbcExtend.py：

import unittest
import abc
oldclassmethod = classmethod
from abcExtend import newclassmethod as classmethod, abstractclassmethod

class Test(unittest.TestCase):
    def setUp(self):
        pass

    def tearDown(self):
        pass

    def testClassmethod(self):
        class A(object):
            __metaclass__ = abc.ABCMeta            
            @classmethod
            @abc.abstractmethod
            def foo(cls):
                return 6

        class B(A):
            @classmethod
            def bar(cls):
                return 5

        class C(B):
            @classmethod
            def foo(cls):
                return super(C,cls).foo() + 1

        self.assertRaises(TypeError,A.foo)
        self.assertRaises(TypeError,A)
        self.assertRaises(TypeError,B)
        self.assertRaises(TypeError,B.foo)
        self.assertEqual(B.bar(),5)
        self.assertEqual(C.bar(),5)
        self.assertEqual(C.foo(),7)

    def testAbstractclassmethod(self):
        class A(object):
            __metaclass__ = abc.ABCMeta    
            @abstractclassmethod
            def foo(cls):
                return 6

        class B(A):
            pass

        class C(B):
            @oldclassmethod
            def foo(cls):
                return super(C,cls).foo() + 1

        self.assertRaises(TypeError,A.foo)
        self.assertRaises(TypeError,A)
        self.assertRaises(TypeError,B)
        self.assertRaises(TypeError,B.foo)
        self.assertEqual(C.foo(),7)
        c = C()
        self.assertEqual(c.foo(),7)

if __name__ == "__main__":
    #import sys;sys.argv = ['', 'Test.testName']
    unittest.main()

我还没有评估此解决方案的性能成本，但到目前为止它已经用于我的目的。

Python 2.7结合abc.abstractmethod和classmethod

4 个答案: