def url_encode(obj, charset='utf-8', encode_keys=False, sort=False, key=None,
               separator='&'):
    """URL encode a dict/`MultiDict`.  If a value is `None` it will not appear
    in the result string.  Per default only values are encoded into the target
    charset strings.  If `encode_keys` is set to ``True`` unicode keys are
    supported too.

    If `sort` is set to `True` the items are sorted by `key` or the default
    sorting algorithm.

    .. versionadded:: 0.5
        `sort`, `key`, and `separator` were added.

    :param obj: the object to encode into a query string.
    :param charset: the charset of the query string.
    :param encode_keys: set to `True` if you have unicode keys.
    :param sort: set to `True` if you want parameters to be sorted by `key`.
    :param separator: the separator to be used for the pairs.
    :param key: an optional function to be used for sorting.  For more details
                check out the :func:`sorted` documentation.
    """
    return separator.join(_url_encode_impl(obj, charset, encode_keys, sort, key))