def generate_product_id(smooth, constant=None, initial_product_id='0000'):
    """

    Parameters
    ----------
    smooth: int
    constant: None or not None
    initial_product_id: str

    Returns
    -------
    str
        new 4 chars product id.
         - 1st char: 'Z' if smooth >= 1, or 'C' if constant is not Nonee
         - 2-4 chars : smooth size or 00 if constant, in decimal int


    """
    if constant is not None:
        res_id = 'C%0.3d' % constant
    else:
        res_id = 'Z%0.3d' % smooth

    return res_id