def unpack_archive(filename, extract_dir=None, format=None):
    """Unpack an archive.

    `filename` is the name of the archive.

    `extract_dir` is the name of the target directory, where the archive
    is unpacked. If not provided, the current working directory is used.

    `format` is the archive format: one of "zip", "tar", or "gztar". Or any
    other registered format. If not provided, unpack_archive will use the
    filename extension and see if an unpacker was registered for that
    extension.

    In case none is found, a ValueError is raised.
    """
    if extract_dir is None:
        extract_dir = os.getcwd()

    if format is not None:
        try:
            format_info = _UNPACK_FORMATS[format]
        except KeyError:
            raise ValueError("Unknown unpack format '{0}'".format(format))

        func = format_info[1]
        func(filename, extract_dir, **dict(format_info[2]))
    else:
        # we need to look at the registered unpackers supported extensions
        format = _find_unpack_format(filename)
        if format is None:
            raise ReadError("Unknown archive format '{0}'".format(filename))

        func = _UNPACK_FORMATS[format][1]
        kwargs = dict(_UNPACK_FORMATS[format][2])
        func(filename, extract_dir, **kwargs)