def ddc_key_to_uri(key: str):
    """Convert a ddc key to a uri.

    Parameters
    ----------
    key: str
        the ddc key, eg. `123.123`, `123` or `001`

    Returns
    -------
    str
        a URI build from the ddc key as string, e.g. `ddc:123.123`
    """
    return f"ddc:{key}"