def _check_unpack_options(extensions, function, extra_args):
    """Checks what gets registered as an unpacker."""
    # first make sure no other unpacker is registered for this extension
    existing_extensions = {}
    for name, info in _UNPACK_FORMATS.items():
        for ext in info[0]:
            existing_extensions[ext] = name

    for extension in extensions:
        if extension in existing_extensions:
            msg = '%s is already registered for "%s"'
            raise RegistryError(msg % (extension,
                                       existing_extensions[extension]))

    if not isinstance(function, collections.Callable):
        raise TypeError('The registered function must be a callable')
Checks what gets registered as an unpacker.