pythonpython-3.xmetaclassabstract-methods

Body of abstract method in Python 3.5


I have an abstract class, Model, with a few abstract methods, what should I put in the body of the methods?

  1. A return

    class Model(metaclass=ABCMeta):
        @abstractmethod
        def foo(self):
            return
    
  2. A pass

    class Model(metaclass=ABCMeta):
        @abstractmethod
        def foo(self):
            pass
    
  3. Raising a descriptive error

    class Model(metaclass=ABCMeta):
        @abstractmethod
        def foo(self):
            raise NotImplementedError("Class {class_name} doesn't implement {func_name} function"
                                  .format(class_name=self.__class__.__name__, func_name=self.foo.__name__))
    

Typically I would implement method 3 and raise an error, however it looks like it would be redundant, as Python raises an error for me:

>>> bar = module.Model()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Model with abstract methods foo

Between the options presented, which is best practice? Or is there another way I should handle this?


Solution

  • The best thing to put in the body of an abstractmethod (or abstractproperty) would be a docstring.

    Then you don't need pass or return or ... because a return None is implicitly included - and a docstring makes this construct "compile" without a SyntaxError:

    from abc import abstractmethod, ABCMeta
    
    class Model(metaclass=ABCMeta):
        @abstractmethod
        def foo(self):
            """This method foos the model."""
    

    The docstring should then explain what one can expect of this method, so that users and subclassers know what is intended.