How come an abstract base class in python can be instantiated?

Question

It is very surprising to me that I can instantiate an abstract class in python:

from abc import ABC

class Duck(ABC):
    def __init__(self, name):
        self.name = name

if __name__=="__main__":
    d = Duck("Bob")
    print(d.name)

The above code compiles just fine and prints out the expected result. Doesn't this sort of defeat the purpose of having an ABC?

Answer 1

If you have no abstract method, you will able to instantiate the class. If you have at least one, you will not.

Consider the following code:

from abc import ABC, abstractmethod

class Duck(ABC):
    def __init__(self, name):
        self.name = name

    @abstractmethod
    def implement_me(self):
        ...

if __name__=="__main__":
    d = Duck("Bob")
    print(d.name)

which returns:

TypeError: Can't instantiate abstract class Duck with abstract method implent_me

It is the combination of the ABC metaclass, and at least one abstractmethod that will lead to you not being able to instantiate a class. If you leave out one of the two, you will be able to do so.