Abstract attributes in Python

Question

What is the shortest / most elegant way to implement the following Scala code with an abstract attribute in Python?

abstract class Controller {

    val path: String

}

A subclass of Controller is enforced to define "path" by the Scala compiler. A subclass would look like this:

class MyController extends Controller {

    override val path = "/home"

}

Answer 1

Bastien Léonard's answer mentions the abstract base class module and Brendan Abel's answer deals with non-implemented attributes raising errors. To ensure that the class is not implemented outside of the module, you could prefix the base name with an underscore which denotes it as private to the module (i.e. it is not imported).

i.e.

class _Controller(object):
    path = '' # There are better ways to declare attributes - see other answers

class MyController(_Controller):
    path = '/Home'

Answer 2

Have a look at the abc (Abtract Base Class) module: http://docs.python.org/library/abc.html

However, in my opinion the simplest and most common solution is to raise an exception when an instance of the base class is created, or when its property is accessed.

Answer 3

Python has a built-in exception for this, though you won't encounter the exception until runtime.

class Base(object):
    @property
    def path(self):
        raise NotImplementedError


class SubClass(Base):
    path = 'blah'