Python - Testing an abstract base class

Question

I am looking for ways / best practices on testing methods defined in an abstract base class. One thing I can think of directly is performing the test on all concrete subclas

Answer 1

Here is what I have found: If you set __abstractmethods__ attribute to be an empty set you'll be able to instantiate abstract class. This behaviour is specified in PEP 3119:

If the resulting __abstractmethods__ set is non-empty, the class is considered abstract, and attempts to instantiate it will raise TypeError.

So you just need to clear this attribute for the duration of tests.

>>> import abc
>>> class A(metaclass = abc.ABCMeta):
...     @abc.abstractmethod
...     def foo(self): pass

You cant instantiate A:

>>> A()
Traceback (most recent call last):
TypeError: Can't instantiate abstract class A with abstract methods foo

If you override __abstractmethods__ you can:

>>> A.__abstractmethods__=set()
>>> A() #doctest: +ELLIPSIS
<....A object at 0x...>

It works both ways:

>>> class B(object): pass
>>> B() #doctest: +ELLIPSIS
<....B object at 0x...>

>>> B.__abstractmethods__={"foo"}
>>> B()
Traceback (most recent call last):
TypeError: Can't instantiate abstract class B with abstract methods foo

You can also use unittest.mock (from 3.3) to override temporarily ABC behaviour.

>>> class A(metaclass = abc.ABCMeta):
...     @abc.abstractmethod
...     def foo(self): pass
>>> from unittest.mock import patch
>>> p = patch.multiple(A, __abstractmethods__=set())
>>> p.start()
{}
>>> A() #doctest: +ELLIPSIS
<....A object at 0x...>
>>> p.stop()
>>> A()
Traceback (most recent call last):
TypeError: Can't instantiate abstract class A with abstract methods foo