Able to instantiate python class, in spite of it being Abstract (using abc)

Question

This is in reference to the answer for this question to "Use abc module of python to create abstract classes." (by @alexvassel and accepted as an answer).

I tried the suggestions, but strangely enough, in spite of following the suggestions to use the abc way, it doesn't work for me. Hence I am posting it as a question here:

Here is my Python code:

from abc import ABCMeta, abstractmethod

class Abstract(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def foo(self):
        print("tst")

a = Abstract()
a.foo()

When I execute this module, here is the output on my console:

pydev debugger: starting (pid: 20388)
tst

as opposed to that accepted answer

>>> TypeError: Can not instantiate abstract class Abstract with abstract methods foo

So what am I doing right or wrong? Why does work and not fail? Appreciate any expert insight into this.

Answer 1

In Python 3 use the metaclass argument when creating the abstract base class:

from abc import ABCMeta, abstractmethod

class Abstract(metaclass=ABCMeta):

    @abstractmethod
    def foo(self):
        print("tst")

a = Abstract()
a.foo()

Answer 2

In Python 2, you must assign the metaclass thusly:

import abc

class ABC(object):

    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def foo(self):
        return True

a = ABC()

Which raises a TypeError

Traceback (most recent call last):
  File "<pyshell#59>", line 1, in <module>
    a = ABC()
TypeError: Can't instantiate abstract class ABC with abstract methods foo

But in Python 3, assigning __metaclass__ as an attribute doesn't work (as you intend it, but the interpreter doesn't consider it an error, just a normal attribute as any other, which is why the above code would not raise an error). Metaclasses are now defined as a named argument to the class:

import abc

class ABC(metaclass=abc.ABCMeta):

    @abc.abstractmethod
    def foo(self):
        return True

a = ABC()

raises the TypeError:

Traceback (most recent call last):
  File "main.py", line 11, in 
    a = ABC()
TypeError: Can't instantiate abstract class ABC with abstract methods foo