def get_locale_identifier(tup, sep='_'):
    """The reverse of :func:`parse_locale`.  It creates a locale identifier out
    of a ``(language, territory, script, variant)`` tuple.  Items can be set to
    ``None`` and trailing ``None``\s can also be left out of the tuple.

    >>> get_locale_identifier(('de', 'DE', None, '1999'))
    'de_DE_1999'

    .. versionadded:: 1.0

    :param tup: the tuple as returned by :func:`parse_locale`.
    :param sep: the separator for the identifier.
    """
    tup = tuple(tup[:4])
    lang, territory, script, variant = tup + (None,) * (4 - len(tup))
    return sep.join(filter(None, (lang, script, territory, variant)))