'Retrieves an assignment using the assignment\'s ID. Requesters can only
retrieve their own assignments, and only assignments whose related HIT
has not been disposed.
The returned ResultSet will have the following attributes:
Request
This element is present only if the Request ResponseGroup
is specified.
Assignment
The assignment. The response includes one Assignment object.
HIT
The HIT associated with this assignment. The response
includes one HIT object.'

def get_assignment(self, assignment_id, response_groups=None):

    params = {'AssignmentId': assignment_id}
    if response_groups:
        self.build_list_params(params, response_groups, 'ResponseGroup')
    return self._process_request('GetAssignment', params, [('Assignment', Assignment), ('HIT', HIT)])
