def not_0(upper, lower):
    """Fills both values to whichever is not equal to 0, or leaves in place."""
    # Note that I compare the values to zero istead of using if not lower
    # This is because the values coming in can be anything, including an
    # empty list, which should be considered nonzero
    if upper == 0:
        return lower, lower
    elif lower == 0:
        return upper, upper
    else:
        return upper, lower