'Returns all immediate dependants for the given dependency.
>>> tracker = DependencyTracker()
>>> tracker.add(0, 1)
>>> tracker.add(0, 2)
>>> tracker.add(1, 3)
>>> tracker.get_dependants(0)
{1, 2}
>>> tracker.get_dependants(1)
{3}
>>> tracker.get_dependants(2)
set()
:param dependency:
The dependency to retrieve all dependants from.
:return:
A set of dependants.'

def get_dependants(self, dependency):

    try:
        return set(self._dependency_dict[dependency])
    except KeyError:
        return set()
