Abstract attribute (not property)?

Question

What\'s the best practice to define an abstract instance attribute, but not as a property?

I would like to write something like:

class AbstractFoo(me         


        

Answer 1

I've searched around for this for awhile but didn't see anything I like. As you probably know if you do:

class AbstractFoo(object):
    @property
    def bar(self):
        raise NotImplementedError(
                "Subclasses of AbstractFoo must set an instance attribute "
                "self._bar in it's __init__ method")

class Foo(AbstractFoo):
    def __init__(self):
        self.bar = "bar"

f = Foo()

You get an AttributeError: can't set attribute which is annoying.

To get around this you can do:

class AbstractFoo(object):

    @property
    def bar(self):
        try:
            return self._bar
        except AttributeError:
            raise NotImplementedError(
                "Subclasses of AbstractFoo must set an instance attribute "
                "self._bar in it's __init__ method")

class OkFoo(AbstractFoo):
    def __init__(self):
        self._bar = 3

class BadFoo(AbstractFoo):
    pass

a = OkFoo()
b = BadFoo()
print a.bar
print b.bar  # raises a NotImplementedError

This avoids the AttributeError: can't set attribute but if you just leave off the abstract property all together:

class AbstractFoo(object):
    pass

class Foo(AbstractFoo):
    pass

f = Foo()
f.bar

You get an AttributeError: 'Foo' object has no attribute 'bar' which is arguably almost as good as the NotImplementedError. So really my solution is just trading one error message from another .. and you have to use self._bar rather than self.bar in the init.