python – When to use raise NotImplementedError?

python – When to use raise NotImplementedError?

As the documentation states ^[docs],

In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added.

Note that although the main stated use case this error is the indication of abstract methods that should be implemented on inherited classes, you can use it anyhow youd like, like for indication of a TODO marker.

As Uriel says, it is meant for a method in an abstract class that should be implemented in child class, but can be used to indicate a TODO as well.

There is an alternative for the first use case: Abstract Base Classes. Those help creating abstract classes.

Heres a Python 3 example:

class C(abc.ABC):
    @abc.abstractmethod
    def my_abstract_method(self, ...):
        ...

When instantiating C, youll get an error because my_abstract_method is abstract. You need to implement it in a child class.

TypeError: Cant instantiate abstract class C with abstract methods my_abstract_method

Subclass C and implement my_abstract_method.

class D(C):
    def my_abstract_method(self, ...):
        ...

Now you can instantiate D.

C.my_abstract_method does not have to be empty. It can be called from D using super().

An advantage of this over NotImplementedError is that you get an explicit Exception at instantiation time, not at method call time.

python – When to use raise NotImplementedError?

Consider if instead it was:

class RectangularRoom(object):
    def __init__(self, width, height):
        pass

    def cleanTileAtPosition(self, pos):
        pass

    def isTileCleaned(self, m, n):
        pass

and you subclass and forget to tell it how to isTileCleaned() or, perhaps more likely, typo it as isTileCLeaned(). Then in your code, youll get a None when you call it.

• Will you get the overridden function you wanted? Definitely not.
• Is None valid output? Who knows.
• Is that intended behavior? Almost certainly not.
• Will you get an error? It depends.

raise NotImplmentedError forces you to implement it, as it will throw an exception when you try to run it until you do so. This removes a lot of silent errors. Its similar to why a bare except is almost never a good idea: because people make mistakes and this makes sure they arent swept under the rug.

Note: Using an abstract base class, as other answers have mentioned, is better still, as then the errors are frontloaded and the program wont run until you implement them (with NotImplementedError, it will only throw an exception if actually called).