What should I put in the body of an abstract method?

Question

Say I have the following abstract class Foo:

import abc

class Foo(abc.ABC):

    @abc.abstractmethod
    def bar(self):
        raise NotImplementedError

What should I put in the body of the bar method?

I see a lot of code that has raise NotImplementedError, as shown above. However, this seems redundant, since any subclass that does not implement bar will raise the TypeError: Can't instantiate abstract class Foo with abstract methods bar when it is instantiated.

Is it Pythonic to leave bar empty, as follows:

import abc

class Foo(abc.ABC):

    @abc.abstractmethod
    def bar(self):
        ...

This is what is done in the Python docs for Abstract Base Classes, but I'm not sure if that's just a placeholder or an actual example of how to write code.

If it's ok to leave bar with only three dots (...), when should I use NotImplementedError?

Answer 1

The documentation does aim to give you an example. You don't have to follow it.

You could provide a default; subclasses are still free to use super() to call your implementation. This is what most of the collections.abc classes do; see the source code.

Size for example, returns 0 for __len__:

class Sized(metaclass=ABCMeta):
    # ...
    @abstractmethod
    def __len__(self):
        return 0