Implementing Abstract Properties in Python

Question

What's the best, most Pythonic, way to deal with abstract properties in Python? If I want to implement a base class which has a mixture of abstract properties, and concrete methods, I can do so similar to the following.

class BaseClass(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def property1(self):
        pass

    @abstractmethod
    def property2(self):
        pass

    @abstractmethod
    def property3(self):
        pass

    @abstractmethod
    def abstract_method(self):
        pass

    def concrete_method(self):
        return self.property1 + self.property2

However, when I then go to implement the inheriting class I need to implement each of those properties as getter method for a private property.

class Klass(BaseClass):
    def __init__(self, property1, property2, property3):
        self.__property1 = property1
        self.__property2 = property2
        self.__property3 = property3

    @property
    def property1(self):
        return self.__property1

    @property
    def property2(self):
        return self.__property2

    @property
    def property3(self):
        return self.__property3

Which seems both unnecessarily verbose, and makes the code more obscure than it needs to be.

I don't love the idea of implementing things concretely and raising a NotImplementedErrorif the inheriting class doesn't implement it's own version.

Is there a better way to do this?

I know this comment does not change anything now, since this has a correct and accepted answer, but just for correctness, aren't you missing the @property decorator from property1, etc. in BaseClass? The concrete_method implementation suggests you really want to use these as attributes rather than method calls. But back in 2016, you probably could not stack/nest decorators...@abstractproperty was invented for that — Andras Vanyolos
– Andras Vanyolos, Commented Feb 22, 2024 at 17:47

Andras Vanyolos · Accepted Answer · 2024-02-23 01:54:20Z

3

You are not required to implement properties as properties. All you need is for the name to exist on the class. So the following, using regular attributes, would work:

class Klass(BaseClass):
    property1 = None
    property2 = None
    property3 = None

    def __init__(self, property1, property2, property3):
        self.property1 = property1
        self.property2 = property2
        self.property3 = property3

    def abstract_method(self):
        # implementation for the abstract method

Note that there is a abstractproperty decorator that'd better document that you want to use those names as simple values, not methods to call:

class BaseClass(object):
    __metaclass__ = ABCMeta

    @abstractproperty
    def property1(self):
        pass

    @abstractproperty
    def property2(self):
        pass

    @abstractproperty
    def property3(self):
        pass

    @abstractmethod    
    def abstract_method(self):
        pass

    def concrete_method(self):
        return self.property1 + self.property2

edited Feb 23, 2024 at 1:54

Andras Vanyolos

4551 gold badge3 silver badges15 bronze badges

answered Feb 4, 2016 at 17:27

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Gree Tree Python Over a year ago

Thanks. I knew that the abstractproperty decorator existed, but it didn't make much sense to me to use it. This is a lot more readable.

Collectives™ on Stack Overflow

Implementing Abstract Properties in Python

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related