def set_length_units(client, units, file_=None, convert=None):
    """Set the current length units for a model.

    This will search the model's available Unit Systems for the first one
    which contains the given length unit.

    Args:
        client (obj):
            creopyson Client.
        units (str):
            New length units.
        `file_` (str|list:str, optional):
            File name or List of file names;
            Defaults is currently active model.
        convert (bool, optional):
            Whether to convert the model's length values to the
            new units (True) or leave them the same value (False).
            Defaults is True.

    Returns:
        None

    """
    data = {"units": units}
    if file_ is not None:
        if isinstance(file_, (str)):
            data["file"] = file_
        elif isinstance(file_, (list)):
            data["files"] = file_
    else:
        active_file = client.file_get_active()
        if active_file:
            data["file"] = active_file["file"]
    if convert is not None:
        data["convert"] = convert
    return client._creoson_post("file", "set_length_units", data)