def _get_units_prod(array_in, array_out):
    """Helper method calculate units after a product operation
    
    Parameters
    ----------
        array_in : np.ndarray
            Array before product operation
        array_out : np.ndarray, float
            Array after product operation
    Returns
    -------
        n : int
            Factor to multiply units
    """
    try:
        size_out = array_out.size
    except AttributeError:
        # Use 1 if float is returned
        size_out = 1
    return array_in.size/size_out