Abstract variables in abc

Python Help
1

the ability to create abstract variables so you can encourage the user to override a class variable so you can encourage a user to override it

2

Use ABC.

from abc import ABC, abstractmethod

class BaseClass(ABC):
    # Abstract method (must be overridden in subclass)
    @abstractmethod
    def do_something(self):
        pass


class SubClass(BaseClass):
    # Implementing the abstract method
    def do_something(self):
        print("Doing something!")

# Usage
sub = SubClass()
sub.do_something()  # Doing something!
3

Moving to the Help category as this is not framed as a clear, detailed proposal and may already be solved by the abc module.

4

More to the point you can define abstract properties:

from abc import ABC, abstractmethod

class BaseClass(ABC):
    # Abstract property (must be defined in subclass)
    @property
    @abstractmethod
    def required_property(self):
        pass

    @classmethod
    def provided_method(cls):
        print(cls.required_property)


class SubClass(BaseClass):
    # Implementing the abstract property
    required_property = "I am from subclass"

# Usage
SubClass.provided_method()  # I am from subclass


class BadSubClass(BaseClass):
    pass

BadSubClass()  # TypeError: Can't instantiate abstract class
1 Like
5

This replaces the property with a str value. abc is satisfied, because it doesn’t care what gets bound to the name required_property, as long as something does.

6

Ah, I see you are correct.

I was being lazy with syntax at the expense of semantics.

You can implement the abstract property more precisely like this:

class SubClass(BaseClass):
    # Implementing the abstract property
    @property
    def required_property(cls):
        return "I am from subclass"

being a property you can still access it succinctly:

SubClass().required_property

However, since they want a class variable, and class properties are (I’ve just discovered) deprecated, it might be best to override the property and make a class attribute anyway.

Unless ABC provides a way to make an abstract attribute I can’t think of a better way.