pint unit-aware math: conversions, offset units and formatting: working versions (tested 2026-10-16)

pint==0.26.1
pytest==9.1.1

import pint

ureg = pint.UnitRegistry()
Q_ = ureg.Quantity


def to_celsius(fahrenheit: float):
    return Q_(fahrenheit, "degF").to("degC")


def to_kelvin(celsius: float):
    return Q_(celsius, "degC").to("kelvin")


def add_temperature(celsius: float, delta_celsius: float):
    # absolute + delta is well defined; absolute + absolute is not (see tests)
    return (Q_(celsius, "degC") + Q_(delta_celsius, "delta_degC")).to("degC")


def speed_mps(distance_km: float, hours: float):
    return (Q_(distance_km, "km") / Q_(hours, "hour")).to("m/s")


def unit_ratio(a_meters: float, b_centimeters: float) -> float:
    # a plain dividend keeps the compound unit, so reduce it explicitly
    return (Q_(a_meters, "m") / Q_(b_centimeters, "cm")).to("").magnitude

import pint
import pytest

from units import Q_, add_temperature, speed_mps, to_celsius, to_kelvin, unit_ratio


def test_offset_units_convert_only_via_to():
    assert to_celsius(212.0).m_as("degC") == pytest.approx(100.0, abs=1e-6)
    assert to_kelvin(100.0).m_as("kelvin") == pytest.approx(373.15)


def test_mixing_dimensions_raises():
    with pytest.raises(pint.errors.DimensionalityError):
        Q_(1, "m") + Q_(1, "s")
    with pytest.raises(pint.errors.DimensionalityError):
        Q_(1, "m").to("s")


def test_absolute_temperatures_cannot_be_added():
    with pytest.raises(pint.errors.OffsetUnitCalculusError):
        Q_(25, "degC") + Q_(25, "degC")
    assert add_temperature(25.0, 5.0).m_as("degC") == pytest.approx(30.0)


def test_compound_units_need_explicit_conversion():
    assert speed_mps(60.0, 2.0).m_as("m/s") == pytest.approx(8.3333333, rel=1e-6)
    assert speed_mps(60.0, 2.0).m_as("km/h") == pytest.approx(30.0)


def test_dimensionless_ratio_is_not_reduced_for_free():
    r = Q_(2, "m") / Q_(50, "cm")
    assert r.dimensionless is True
    assert r.magnitude == pytest.approx(0.04)
    assert unit_ratio(2.0, 50.0) == pytest.approx(4.0)


def test_formatting_specs():
    q = Q_(1234.5678, "meter")
    assert f"{q:.2f}" == "1234.57 meter"
    assert f"{q:~P}" == "1234.5678 m"
    assert f"{q:.2f~}" == "1234.57 m"

[pytest]
pythonpath = .