'Access endpoint for problem responses report, ensuring that
UsageKey.from_string returns a problem key that the endpoint
can work with.
msg: message to display if assertion fails.'

def _access_problem_responses_endpoint(self, msg):

    mock_problem_key = Mock(return_value=u'')
    mock_problem_key.course_key = self.course.id
    with patch.object(UsageKey, 'from_string') as patched_method:
        patched_method.return_value = mock_problem_key
        self._access_endpoint('get_problem_responses', {}, 200, msg)
