def make_good_url(url=None, addition="/"):
    """Appends addition to url, ensuring the right number of slashes
    exist and the path doesn't get clobbered.

    >>> make_good_url('http://www.server.com/anywhere', 'else')
    'http://www.server.com/anywhere/else'
    >>> make_good_url('http://test.com/', '/somewhere/over/the/rainbow/')
    'http://test.com/somewhere/over/the/rainbow/'
    >>> make_good_url('None')
    'None/'
    >>> make_good_url()
    >>> make_good_url({})
    >>> make_good_url(addition='{}')

    :param url: URL
    :param addition: Something to add to the URL
    :return: New URL with addition"""

    if url is None:
        return None

    if isinstance(url, str) and isinstance(addition, str):
        return "%s/%s" % (url.rstrip('/'), addition.lstrip('/'))
    else:
        return None