Difference between abstract class and interface in Python

What is the difference between abstract class and interface in Python?

What you'll see sometimes is the following:

class Abstract1:
    """Some description that tells you it's abstract,
    often listing the methods you're expected to supply."""

    def aMethod(self):
        raise NotImplementedError("Should have implemented this")

Because Python doesn't have (and doesn't need) a formal Interface contract, the Java-style distinction between abstraction and interface doesn't exist. If someone goes through the effort to define a formal interface, it will also be an abstract class. The only differences would be in the stated intent in the docstring.

And the difference between abstract and interface is a hairsplitting thing when you have duck typing.

Java uses interfaces because it doesn't have multiple inheritance.

Because Python has multiple inheritance, you may also see something like this

class SomeAbstraction:
    pass  # lots of stuff - but missing something

class Mixin1:
    def something(self):
        pass  # one implementation

class Mixin2:
    def something(self):
        pass  # another

class Concrete1(SomeAbstraction, Mixin1):
    pass

class Concrete2(SomeAbstraction, Mixin2):
    pass

This uses a kind of abstract superclass with mixins to create concrete subclasses that are disjoint.

What is the difference between abstract class and interface in Python?

An interface, for an object, is a set of methods and attributes on that object.

In Python, we can use an abstract base class to define and enforce an interface.

Using an Abstract Base Class

For example, say we want to use one of the abstract base classes from the collections module:

import collections
class MySet(collections.Set):
    pass

If we try to use it, we get an TypeError because the class we created does not support the expected behavior of sets:

>>> MySet()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class MySet with abstract methods
__contains__, __iter__, __len__

So we are required to implement at least __contains__, __iter__, and __len__. Let's use this implementation example from the documentation:

class ListBasedSet(collections.Set):
    """Alternate set implementation favoring space over speed
    and not requiring the set elements to be hashable. 
    """
    def __init__(self, iterable):
        self.elements = lst = []
        for value in iterable:
            if value not in lst:
                lst.append(value)
    def __iter__(self):
        return iter(self.elements)
    def __contains__(self, value):
        return value in self.elements
    def __len__(self):
        return len(self.elements)

s1 = ListBasedSet('abcdef')
s2 = ListBasedSet('defghi')
overlap = s1 & s2

Implementation: Creating an Abstract Base Class

We can create our own Abstract Base Class by setting the metaclass to abc.ABCMeta and using the abc.abstractmethod decorator on relevant methods. The metaclass will be add the decorated functions to the __abstractmethods__ attribute, preventing instantiation until those are defined.

import abc

For example, "effable" is defined as something that can be expressed in words. Say we wanted to define an abstract base class that is effable, in Python 2:

class Effable(object):
    __metaclass__ = abc.ABCMeta
    @abc.abstractmethod
    def __str__(self):
        raise NotImplementedError('users must define __str__ to use this base class')

Or in Python 3, with the slight change in metaclass declaration:

class Effable(object, metaclass=abc.ABCMeta):
    @abc.abstractmethod
    def __str__(self):
        raise NotImplementedError('users must define __str__ to use this base class')

Now if we try to create an effable object without implementing the interface:

class MyEffable(Effable): 
    pass

and attempt to instantiate it:

>>> MyEffable()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class MyEffable with abstract methods __str__

We are told that we haven't finished the job.

Now if we comply by providing the expected interface:

class MyEffable(Effable): 
    def __str__(self):
        return 'expressable!'

we are then able to use the concrete version of the class derived from the abstract one:

>>> me = MyEffable()
>>> print(me)
expressable!

There are other things we could do with this, like register virtual subclasses that already implement these interfaces, but I think that is beyond the scope of this question. The other methods demonstrated here would have to adapt this method using the abc module to do so, however.

Conclusion

We have demonstrated that the creation of an Abstract Base Class defines interfaces for custom objects in Python.

Python >= 2.6 has Abstract Base Classes.

Abstract Base Classes (abbreviated ABCs) complement duck-typing by providing a way to define interfaces when other techniques like hasattr() would be clumsy. Python comes with many builtin ABCs for data structures (in the collections module), numbers (in the numbers module), and streams (in the io module). You can create your own ABC with the abc module.

There is also the Zope Interface module, which is used by projects outside of zope, like twisted. I'm not really familiar with it, but there's a wiki page here that might help.

In general, you don't need the concept of abstract classes, or interfaces in python (edited - see S.Lott's answer for details).

In a more basic way to explain: An interface is sort of like an empty muffin pan. It's a class file with a set of method definitions that have no code.

An abstract class is the same thing, but not all functions need to be empty. Some can have code. It's not strictly empty.

Why differentiate: There's not much practical difference in Python, but on the planning level for a large project, it could be more common to talk about interfaces, since there's no code. Especially if you're working with Java programmers who are accustomed to the term.

Python doesn't really have either concept.

It uses duck typing, which removed the need for interfaces (at least for the computer :-))

Python <= 2.5: Base classes obviously exist, but there is no explicit way to mark a method as 'pure virtual', so the class isn't really abstract.

Python >= 2.6: Abstract base classes do exist (http://docs.python.org/library/abc.html). And allow you to specify methods that must be implemented in subclasses. I don't much like the syntax, but the feature is there. Most of the time it's probably better to use duck typing from the 'using' client side.