feat: add Pint integration for unit conversions (#273)

* feat: add Pint integration for unit conversions

* Better test coverage

* style: pre-commit fixes

* lint

* style: pre-commit fixes

* More lint

* more lint

* Construct explicitly

* Add license statement to pint.py

* Add trivial docstring to test_pint.py

* style: pre-commit fixes

* Apply suggestions from code review

Co-authored-by: Eduardo Rodrigues <[email protected]>

* Advertise pint on README

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Eduardo Rodrigues <[email protected]>

Author: 3 people

.pre-commit-config.yaml

@@ -30,6 +30,7 @@ repos:
       - id: mypy
         files: src
         args: []
+        additional_dependencies: [pint]
 
   - repo: https://github.com/codespell-project/codespell
     rev: v2.4.1

README.rst

@@ -71,7 +71,7 @@ The package is also available on `conda-forge`_, and installable with
 Getting started
 ---------------
 
-The package contains 2 modules, ``constants`` and ``units``,
+The package contains 2 core modules, ``constants`` and ``units``,
 whose names are self-explanatory.
 It may be more readable to import quantities explicitly from each of the modules
 though everything is available from the top-level as ``from hepunits import ...``.
@@ -137,6 +137,30 @@ to ensure an explicit conversion to the desired unit dividing by it (GeV in the
     0.0005
 
 
+Pint integration
+~~~~~~~~~~~~~~~~
+The package can interoperate with `Pint`_, which provides a more full-featured units
+and quantities system. Pint is an optional dependency of ``hepunits``.
+When Pint is installed, ``hepunits`` units and constants can be used to create Pint
+quantities, and Pint quantities can be converted to ``hepunits`` units, as shown below.
+
+.. code-block:: pycon
+
+    >>> import pint
+    >>> import hepunits
+    >>> from hepunits.pint import to_clhep, from_clhep
+    >>> ureg = pint.UnitRegistry()
+    >>> g = 9.8 * ureg.meter / ureg.second**2
+    >>> g
+    <Quantity(9.8, 'meter / second ** 2')>
+    >>> to_clhep(g)
+    9.800000000000001e-15
+    >>> from_clhep(hepunits.c_light, ureg.meter / ureg.second)
+    <Quantity(299792458.0, 'meter / second')>
+    >>> from_clhep(hepunits.c_light, ureg.fathom / ureg.fortnight)
+    <Quantity(1.98287528e+14, 'fathom / fortnight')>
+
+.. _Pint: https://pint.readthedocs.io/
 
 .. |Scikit-HEP| image:: https://scikit-hep.org/assets/images/Scikit--HEP-Project-blue.svg
    :target: https://scikit-hep.org

pyproject.toml

@@ -42,14 +42,17 @@ dynamic = ["version"]
 all = [
     "pytest-cov>=2.8.0",
     "pytest>=6",
+    "pint",
 ]
 dev = [
     "pytest-cov>=2.8.0",
     "pytest>=6",
+    "pint",
 ]
 test = [
     "pytest-cov>=2.8.0",
     "pytest>=6",
+    "pint",
 ]
 
 [project.urls]

src/hepunits/pint.py

@@ -0,0 +1,95 @@
+# Licensed under a 3-clause BSD style license, see LICENSE.
+"""
+Conversion routines between CLHEP, the HEP System of Units, and Pint units.
+
+CLHEP adopts the approach where all the quantities are stored in the base unit
+system, effectively dimensionless. Pint offers to store both the magnitude and
+the dimensionality of the unit, which is helpful in deducing and/or validating
+the resulting unit of formulas. This module offers conversion routines between
+Pint's default base unit system and CLHEP.
+"""
+
+from __future__ import annotations
+
+try:
+    import pint
+except ImportError as exc:
+    msg = "Pint is required to use hepunits.pint."
+    raise ImportError(msg) from exc
+
+# TODO: support more unit conversions
+_clhep_base_units = {
+    "[length]": "millimeter",
+    "[time]": "nanosecond",
+    "[mass]": "MeV * millimeter**-2 * nanosecond**2",
+    "[current]": "elementary_charge / nanosecond",
+}
+
+
+def _unit_from(val: pint.Quantity | pint.Unit) -> pint.Unit:
+    """Extract the dimensionality from a Pint Quantity or Unit."""
+    # Grabbing the type is a quick way to be in the correct unit registry
+    # see e.g. https://github.com/hgrecco/pint/issues/2207
+    unit = type(val.units) if isinstance(val, pint.Quantity) else type(val)
+    out = unit("dimensionless")
+    for dim, exponent in val.dimensionality.items():
+        if dim not in _clhep_base_units:
+            msg = f"Unsupported dimension {dim} in {val}"
+            raise ValueError(msg)
+        out *= unit(_clhep_base_units[dim]) ** exponent
+
+    return out
+
+
+def to_clhep(val: pint.Quantity | pint.Unit) -> float:
+    """
+    Convert a Pint Quantity or Unit to CLHEP base units.
+
+    Parameters
+    ----------
+    val : pint.Quantity or pint.Unit
+        The value to convert.
+
+    Returns
+    -------
+    float
+        The value in CLHEP base units (dimensionless).
+
+    Examples
+    --------
+    >>> ureg = pint.UnitRegistry()
+    >>> g = 9.8 * ureg.meter / ureg.second**2
+    >>> g
+    <Quantity(9.8, 'meter / second ** 2')>
+    >>> to_clhep(g)
+    9.800000000000001e-15
+    """
+    clhep_unit = _unit_from(val)
+    q = pint.Quantity(1.0, val) if isinstance(val, pint.Unit) else val
+    return q.to(clhep_unit).magnitude  # type: ignore[no-any-return]
+
+
+def from_clhep(val: float, unit: pint.Unit) -> pint.Quantity:
+    """
+    Convert a value in CLHEP base units to a Pint Quantity.
+
+    Parameters
+    ----------
+    val : float
+        The value in CLHEP base units (dimensionless).
+    unit : pint.Unit
+        The desired output unit.
+
+    Returns
+    -------
+    pint.Quantity
+        The value in the desired unit.
+
+    Examples
+    --------
+    >>> ureg = pint.UnitRegistry()
+    >>> from_clhep(hepunits.c_light, ureg.meter / ureg.second)
+    <Quantity(299792458.0, 'meter / second')>
+    """
+    clhep_unit = _unit_from(unit)
+    return pint.Quantity(val, clhep_unit).to(unit)

tests/test_missing_all.py

@@ -14,7 +14,6 @@ def filter_module(item: str) -> bool:
 @pytest.mark.parametrize(
     "module",
     [
-        hepunits,
         hepunits.units,
         hepunits.constants,
         hepunits.constants.constants,

tests/test_pint.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+# Licensed under a 3-clause BSD style license, see LICENSE.
+"""
+Tests for the hepunits.pint module.
+"""
+
+import pint
+import pytest
+from pytest import approx
+
+import hepunits
+from hepunits.pint import from_clhep, to_clhep
+
+
+def test_pint_constants():
+    ureg = pint.UnitRegistry()
+
+    # These three constants set the relationship between mass, length, time, and charge
+    # TODO: check all 7 SI defining constants (hence also supporting more unit conversions)
+    pint_c = to_clhep(1 * ureg.speed_of_light)
+    assert pint_c == approx(hepunits.c_light, rel=1e-15)
+    pint_h = to_clhep(1 * ureg.planck_constant)
+    assert pint_h == approx(hepunits.h_Planck, rel=1e-15)
+    pint_e = to_clhep(1 * ureg.elementary_charge)
+    assert pint_e == approx(1.0, rel=1e-15)
+
+
+def test_pint_roundtrip():
+    ureg = pint.UnitRegistry()
+
+    assert to_clhep(3 * ureg.mm) == approx(3.0)
+    assert to_clhep(3 * ureg.cm) == approx(30.0)
+    assert to_clhep(2 * ureg.ohm) == approx(2.0 * hepunits.ohm)
+    assert to_clhep(ureg.coulomb) == approx(hepunits.coulomb)
+
+    assert from_clhep(hepunits.c_light, ureg.meter / ureg.second).m == approx(
+        (1.0 * ureg.c).to(ureg.meter / ureg.second).m, rel=1e-15
+    )
+    assert from_clhep(hepunits.tesla, ureg.tesla).m == approx(
+        (1 * ureg.tesla).m, rel=1e-15
+    )
+
+
+def test_unsupported_dimension():
+    ureg = pint.UnitRegistry()
+    with pytest.raises(ValueError, match="Unsupported dimension"):
+        to_clhep(1 * ureg.kelvin)