Why can I instantiate an abstract class without implementing an abstractmethod?

Question

Trying to understand abstract classes, I created a simple model:

from abc import ABC, abstractmethod

class Publication(ABC):
    def __init__(self, title):
        self.title = title

    @abstractmethod
    def Description(self):
        pass

class Periodical(Publication):
    def __init__(self, title, publisher):
        super().__init__(title)
        self.publisher = publisher

class Book(Publication):
    def __init__(self, title, author):
        super().__init__(title)
        self.author = author

    def Description(self):
        print(f'Book: {self.title} ({self.author})')

class Magazine(Periodical):
    def __init__(self, title, publisher):
        super().__init__(title, publisher)

    def Description(self):
        print(f'Magazine: {self.title} ({self.publisher})')

class Newspaper(Periodical):
    def __init__(self, title, publisher):
        super().__init__(title, publisher)

    def Description(self):
        print(f'Newspaper: {self.title} ({self.publisher})')

book = Book('Thoughts', 'A. Einstein')
magazine = Magazine('Sailing', 'M. Polo')
newspaper = Newspaper('Daily Joke', 'Ms. Maisel')

book.Description()
magazine.Description()
newspaper.Description()

In Publication I define Description() as abstract method. If I don’t implement it, e.g. in class Newspaper, an error is thrown: TypeError: Can't instantiate abstract class Newspaper with abstract method Description. That’s what I intended.

But why is it possible to create Periodical from Publication without implementing Description()?

Asked By: Anumi

||

Source

Answer 1

You are not required to provide concrete implementations of abstract methods in a subclass. A subclass that does not do so simply remains an abstract base class itself.

Only when you try to instantiate a class is there a check for abstract methods that haven’t been override. If there are, instantiation fails. You couldn’t instantiate Periodical directly, but the class itself is just another abstract base class, with its subclasses Book, Magazine, and Newspaper being made concrete by implementing Description.

# Class is defined...
>>> Periodical
<class '__main__.Periodical'>

# ... but you can't instantiate it
>>> Periodical()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Periodical with abstract method Description

Answered By: chepner

Why can I instantiate an abstract class without implementing an abstractmethod?

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