How can I make Python/Sphinx document object attributes only declared in __init__?

I have Python classes with object attributes which are only declared as part of running the constructor, like so:

class Foo(object):
    def __init__(self, base):
        self.basepath = base

        temp = []
        for run in os.listdir(self.basepath):
            if self.foo(run):
                temp.append(run)
        self.availableruns = tuple(sorted(temp))

If I now use either help(Foo) or attempt to document Foo in Sphinx, the self.basepath and self.availableruns attributes are not shown. That's a problem for users of our API.

I've tried searching for a standard way to ensure that these "dynamically declared" attributes can be found (and preferably docstring'd) by the parser, but no luck so far. Any suggestions? Thanks.