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devel / comp.lang.python / Instantiating abstract classes

Instantiating abstract classes

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From: __pete...@web.de (Peter Otten)
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Subject: Instantiating abstract classes
Date: Fri, 21 May 2021 20:00:23 +0200
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by: Peter Otten - Fri, 21 May 2021 18:00 UTC

Usually an abstract class cannot be instantiated:

>>> from abc import ABCMeta, abstractmethod
>>> from fractions import Fraction
>>> class A(Fraction, metaclass=ABCMeta):
@abstractmethod
def frobnicate(self): pass

>>> A()
Traceback (most recent call last):
File "<pyshell#287>", line 1, in <module>
A()
File "C:\Program Files\Python39-32\lib\fractions.py", line 93, in __new__
self = super(Fraction, cls).__new__(cls)
TypeError: Can't instantiate abstract class A with abstract method
frobnicate

However, if I derive from a builtin class that mechanism doesn't work:

>>> class A(int, metaclass=ABCMeta):
@abstractmethod
def frobnicate(self): pass

>>> A()
0

Is this a bug, or an implementation accident, or the expected behavior?

Subject	Author
Instantiating abstract classes	Peter Otten