>>> import abc
>>> class Shape(metaclass=abc.ABCMeta):
...     @abc.abstractmethod
...     def method_to_implement(self, input):
...         return
...
>>> class Foo(Shape):
...     pass
...
>>> foo = Foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class Foo with abstract methods method_to_implement