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
?
An abstract method is declared by abstract keyword, such methods cannot have a body.
You can not specify a body to abstract methods. Abstract methods means they are incomplete.
Abstract Classes and Methods Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class). Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).
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
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