astropy: Incorrect/inconsistent equality of custom units (value rounding error)

If the quantities have integer values (or float values that represent integer value), then it would be nice to have exact conversions though, whenever possible.

But I agree; we should keep the code as is, because it would require more thought to do this right than just hacking the _to() function. E.g. it might be worthwhile to try to also getting the Hz thing right.

Nevertheless, I thought of several ways to accomplish exact conversion, for my immediate goal of mm <-> um <-> nm. The (enormous) kludge that I settled on, makes use of the fact that SI unit conversions have short human readable representations by design. I’m only showing it here because I promised, and just maybe it could help someone:

def save_unit_to(unit_from: u.Unit, unit_to: u.Unit):
    """Like unit_from.to(unit_to), but u.um to u.nm == 1000., not 999.999.."""
    to1 = unit_from.to(unit_to)
    sto1 = str(to1)
    to2 = 1 / unit_to.to(unit_from)
    sto2 = str(to2)
    if len(sto2) < len(sto1):
        return to2
    return to1

with as test:

def test_save_to():
    for ufrom, uto, fac in [
        (u.um, u.nm, 1000.),
        (u.mm, u.um, 1000.),
        (u.nm, u.um, 0.001),
        (u.um, u.mm, 0.001),
    ]:
        assert save_unit_to(ufrom, uto) == fac

Maybe someone will say “Noooooooo…” and implement it properly.