def say_hello(name='World'):
    """Say hello to someone

    Parameters
    ----------
    name : string
        A string containing the name of the person who is to greeted.

    Returns
    -------
    string : string
        The greetings string.

    """
    if not isinstance(name, str):
        raise ValueError("I need a string.")
    string = 'Hello {}!'.format(name)
    return string