Question

When designing classes abstract methods can be very helpful. From what I know, Python does not have a mechanism for enforcing an inherited class to implement the abstract method. In my code (see example below) I enter a failed assertion in the base class to cause a runtime error if not implemented. Is this unpythonic?

class Dog(Animal):
  def speak(self):
   return "bark"

class Animal():
  def speak(self):
   assert(False) #abstract

Answer 1

Python actually does have abstract classes with abstact methods:

>>> import abc
>>> 
>>> class IFoo(object):
...     __metaclass__ = abc.ABCMeta
...     
...     @abc.abstractmethod
...     def foo(self):
...         pass
... 
>>> foo = IFoo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cant instantiate abstract class IFoo with abstract methods foo
>>> class FooDerived(IFoo):
...     pass
... 
>>> foo = FooDerived()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cant instantiate abstract class FooDerived with abstract methods foo
>>> class FooImplements(FooDerived):
...     def foo(self):
...         print "foo'ed"
... 
>>> foo = FooImplements()
>>> foo.foo()
foo'ed
>>> 

On the other hand, the fundamental question of “is this pythonic” is a bit harder to say. If your intent is to provide the abstract base class so that you can later check to make sure that values inherit from it, then no, that’s not particularly pythonic, even though it’s possible to make arbitrary types abstract subclasses of your baseclass. On the other hand, it’s perfectly fine to provide an abstract baseclass that implements some functionality based upon the implementation provided in concrete subclasses. For example, collections.Sequence and collections.Mapping do just this for list like and dict like classes; subclasses can provide __getitem__ and can get __contains__ and others for free.

For certain, you should never use assert() except to document the expectations of code; If it’s actually possible for the assert to fail, you shouldn’t be using an assert. Optimized python (python -O script.py) does not check assertions.

Edit: more exposition:

If you are checking the type of a value:

def foo(bar):
    if not isinstance(bar, AbstractBaz):
        raise ValueError, ("bar must be an instance of AbstractBaz, "
                           "got %s" % type(bar))

If for some reason you can’t use @abstractmethod, but still want that effect, you should raise NotImplementedError. You might want to do this because you actually do want instances of that class, some of which might not need to implement optional functionality. You still should account for the possibility that the function was called through super(). To a first approximation, that might look like this.

class Foo(object):
    def bar(self, baz):
        if self.bar.im_func == Foo.bar.im_func:
            raise NotImplementedError, "Subclasses must implement bar"