Little script to verify https://xkcd.com/2585/

xkcd2585.py

from functools import partial

from pint import UnitRegistry, Unit, Quantity

ureg = UnitRegistry()


def rounded_conversion(from_: Quantity, to: Unit) -> Quantity:
    return Quantity(round(from_.to(to).magnitude), to)


def rounded_conversion_error(from_: Quantity, to: Unit) -> Quantity:
    return rounded_conversion(from_, to).to(from_.units).to(from_.units)


def maximum_rounded_conversion_error(from_: Quantity, options: list[Unit]) -> Unit:
    return max(options, key=partial(rounded_conversion_error, from_))


def worst_conversion(from_: Quantity, options: list[Unit]) -> Quantity:
    worst_unit = maximum_rounded_conversion_error(from_, options)
    return rounded_conversion(from_, worst_unit)


OPTIONS = [
    ureg.mph,
    ureg.m / ureg.s,
    ureg.knots,
    ureg.fathoms / ureg.s,
    ureg.furlongs / ureg.min,
    ureg.km / ureg.hour,
    ureg.furlongs / ureg.hour,
    ureg.yards / ureg.s,
]


value = 17. * ureg.mph
for _ in range(24):
    value = worst_conversion(value, OPTIONS)
    print(value)

print(value.to(ureg.mph))