def unquote_etag(etag):
    """Unquote a single etag:

    >>> unquote_etag('W/"bar"')
    ('bar', True)
    >>> unquote_etag('"bar"')
    ('bar', False)

    :param etag: the etag identifier to unquote.
    :return: a ``(etag, weak)`` tuple.
    """
    if not etag:
        return None, None
    etag = etag.strip()
    weak = False
    if etag.startswith(('W/', 'w/')):
        weak = True
        etag = etag[2:]
    if etag[:1] == etag[-1:] == '"':
        etag = etag[1:-1]
    return etag, weak
