Pulls the joke from the service based on the argument.
    
    It is expected that all services used will return a string
    when successful or None otherwise.

def load_joke(service_num=1):
    """Pulls the joke from the service based on the argument.
    
    It is expected that all services used will return a string
    when successful or None otherwise.
    """
    result = {
      1 : ronswanson.get_joke(),
      2 : chucknorris.get_joke(),
      3 : catfacts.get_joke(),
      4 : dadjokes.get_joke(),
    }.get(service_num)

    return result