Body of abstract method in Python 3.5 [duplicate]

Question

This question already has an answer here:

• What should I put in the body of an abstract method in Python 1 answer

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?

Answer 1

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 should implement how to foo the model."""

The docstring should then explain what should be implemented here so that subclassers know what is/was intended.