Enforcing Abstract Property in Python Classes

How to create Python abstract property in abstract classes?

In the following code, I created a base abstract class Base. I want all the classes inheriting from Base to provide the name property, so I made it an @abstractmethod.

Then, I created a subclass of Base, called Base1, which is meant to provide some functionality, but remain abstract. There is no name property in Base1, but Python still allows instantiation of that class without any error. How can I enforce the requirement for an abstract property in such cases?

from abc import ABCMeta, abstractmethod

class Base(object):
    # class Base(metaclass = ABCMeta): <- Python 3
    __metaclass__ = ABCMeta
    def __init__(self, str_dir_config):
        self.str_dir_config = str_dir_config
    
    @abstractmethod
    def _do_stuff(self, signals):
        pass
    
    @property    
    @abstractmethod
    def name(self):
        """This property will be supplied by the inheriting classes individually."""
        pass
    

class Base1(Base):
    __metaclass__ = ABCMeta
    """This class does not provide the name property and should raise an error."""
    def __init__(self, str_dir_config):
        super(Base1, self).__init__(str_dir_config)
        # super().__init__(str_dir_config) <- Python 3
    
    def _do_stuff(self, signals):
        print("Base_1 does stuff")
        # print("Base_1 does stuff") <- Python 3

class C(Base1):
    @property
    def name(self):
        return "class C"
    

if __name__ == "__main__":
    b1 = Base1("ABC")

How can I enforce the name property requirement in subclasses of Base?

Hey! Great question. This issue is quite common when dealing with abstract classes in Python. Since Python 3.3, a bug was fixed, allowing the @property decorator to work correctly with @abstractmethod. Here’s how you can implement it:

For Python 3.3+:

from abc import ABC, abstractmethod

class C(ABC):
    @property
    @abstractmethod
    def my_abstract_property(self):
        pass

For Python 2, you’ll need to use @abstractproperty instead:

from abc import ABCMeta, abstractproperty

class C:
    __metaclass__ = ABCMeta

    @abstractproperty
    def my_abstract_property(self):
        pass

The important point to note here is that the order of the decorators matters@property must always come before @abstractmethod. This ensures that Python recognizes it correctly as an abstract property.

Totally agree with Priyanka’s explanation about decorator order! To expand on this, if you’re working in Python 3.x and need to enforce that all subclasses of Base implement the name property, you should define the Base class as abstract and ensure it includes the @property and @abstractmethod decorators together.

Here’s a simple example of how the Base class can be structured:

from abc import ABCMeta, abstractmethod

class Base:
    __metaclass__ = ABCMeta
    
    @property
    @abstractmethod
    def name(self):
        """This property must be implemented in all subclasses."""
        pass

With this setup, if any subclass of Base does not implement the name property, it will raise an error. This approach ensures consistency across all subclasses.

Absolutely! Both of you make great points. For anyone using Python 3.3+ and wanting a foolproof way to implement and enforce a python abstract property, here’s a concise example:

from abc import ABC, abstractmethod

class Base(ABC):
    @property
    @abstractmethod
    def name(self):
        """Define an abstract property that must be overridden."""
        pass

The key advantage of this approach in Python 3.x is that the ABC class simplifies the use of metaclasses and ensures that all subclasses adhere to the rules defined by Base. If a subclass fails to implement the name property, Python will raise a TypeError, signaling the developer to correct it.

For example:

class Derived(Base):
    pass  # TypeError: Can't instantiate abstract class Derived with abstract methods name

The added safety and readability make this the preferred way to enforce an abstract property.