How to use Abstract Base Classes in Python?

Question

Following this tutorial I'm trying to use Abstract Base Classes in Python. So I constructed two files:

basis.py:

import abc

class PluginBase(object):
    __metaclass__ = abc.ABCMeta

    @abc.abstractmethod
    def load(self, input):
        return

and implementation.py:

import abc
from basis import PluginBase

class SubclassImplementation(PluginBase):

    def load(self, input):
        print input
        return input

if __name__ == '__main__':
    print 'Subclass:', issubclass(SubclassImplementation, PluginBase)
    print 'Instance:', isinstance(SubclassImplementation(), PluginBase)

running python implementation.py works fine, but I now want to use implementation.py as a module for something else. So I get into the command line and do:

>>> from implementation import SubclassImplementation as imp
>>> imp.load('lala')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method load() must be called with SubclassImplementation instance as first argument (got str instance instead)
>>>

What am I doing wrong here, and how can I make this work? All tips are welcome!

Answer 1

You do need to create an instance:

from implementation import SubclassImplementation as imp

imp().load('lala')

This is not a specific ABC problem either; this applies to all Python classes and methods, unless you make SubclassImplementation.load a classmethod.