convert a standalone unicode string or unicode strings in a
    mapping or iterable into byte strings.

def convert(data):
    """
    convert a standalone unicode string or unicode strings in a
    mapping or iterable into byte strings.
    """
    if isinstance(data, unicode):
        return data.encode('utf-8')
    elif isinstance(data, str):
        return data
    elif isinstance(data, collections.Mapping):
        return dict(map(convert, data.iteritems()))
    elif isinstance(data, collections.Iterable):
        return type(data)(map(convert, data))
    else:
        return data