Returns numerical interval based on optionals num_pre, num_post values

def _get_num_interval(config, num_pre, num_post):
    '''
    Returns numerical interval based on optionals num_pre, num_post values
    '''
    post = int(num_post) if num_post else 0
    pre = int(num_pre) if num_pre is not None else _get_last_snapshot(config)['id']
    return pre, post