abc.abstractmethod + property

Question

According to the docs it should work to combine @property and @abc.abstractmethod so the following should work in python3.3:

import abc

class FooBase(metaclass=abc.ABCMeta):

    @property
    @abc.abstractmethod
    def greet(self):
        """ must be implemented in order to instantiate """
        pass

    @property
    def greet_comparison(self):
        """ must be implemented in order to instantiate """
        return 'hello'

class Foo(FooBase):
    def greet(self):
        return 'hello'

test the implementation:

In [6]: foo = Foo()
In [7]: foo.greet
Out[7]: <bound method Foo.greet of <__main__.Foo object at 0x7f935a971f10>>

In [8]: foo.greet()
Out[8]: 'hello'

so it is obviously not a property, because then it should work like that:

In [9]: foo.greet_comparison
Out[9]: 'hello'

Maybe I'm to stupid or it simply doesn't work, somebody has an idea?

Answer 1

If you want greet to be a property, you still need to use the @property decorator in your implementation:

class Foo(FooBase):
    @property
    def greet(self):
        return 'hello'

All that an ABC metaclass does is test wether or not you have provided the same name in the concrete class; it doesn't care if it is a method or a property or a regular attribute.

Because it doesn't care, it doesn't magically apply property decorators either. This is a good thing; perhaps in a specific implementation a static attribute is enough to satisfy the requirement, and a property would be overkill.

The purpose of a ABC metaclass is to help you detect gaps in your implementation; it never was intended to enforce the types of the attributes.

Note that before Python 3.3 you can not combine a @property with an @abstractmethod. You would have to use an @abstractproperty decorator instead. There is an ambiguity there when your property needs more than a simple getter in that case; Python 3.3 covers that case much better (see issue 11610 for the painful details).