Python 2.7 Combine abc.abstractmethod and classmethod

Question

How do I create a decorator for an abstract class method in Python 2.7?

Yes, this is similar to this question, except I would like to combine abc.abstractmet

Answer 1

Here's a working example derived from the source code in Python 3.3's abc module:

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

Here's what it looks like when running:

>>> 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

Note that the final example fails because cls() won't instantiate. ABCMeta prevents premature instantiation of classes that haven't defined all of the required abstract methods.

Another way to trigger a failure when the from_int() abstract class method is called is to have it raise an exception:

class DemoABC:

    __metaclass__ = ABCMeta

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

The design ABCMeta makes no effort to prevent any abstract method from being called on an uninstantiated class, so it is up to you to trigger a failure by invoking cls() as classmethods usually do or by raising a NotImplementedError. Either way, you get a nice, clean failure.

It is probably tempting to write a descriptor to intercept a direct call to an abstract class method, but that would be at odds with the overall design of ABCMeta (which is all about checking for required methods prior to instantiation rather than when methods are called).