Gala unit system (#87)

* optional_deps
* register gala UnitSystem -> galax Unitsystem converter
* gala optional dependencies
* rearrange
* test dimensionless

Signed-off-by: nstarman <nstarman@users.noreply.github.com>

pyproject.toml

@@ -39,6 +39,12 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
+compat-gala = [
+  "gala",
+]
+compat-all = [
+  "galax[compat-gala]",
+]
 test = [
   "hypothesis[numpy]",
   "pytest >=6",
@@ -57,6 +63,7 @@ docs = [
   "furo>=2023.08.17",
 ]
 all = [
+  "galax[compat-all]",
   "galax[test]",
   "galax[docs]",
   "galax[dev]",

src/galax/potential/_potential/utils.py

@@ -10,7 +10,13 @@
 from astropy.units import Quantity
 from jax import Array
 
-from galax.units import UnitSystem, dimensionless, galactic, solarsystem
+from galax.units import (
+    DimensionlessUnitSystem,
+    UnitSystem,
+    dimensionless,
+    galactic,
+    solarsystem,
+)
 
 
 @singledispatch
@@ -141,3 +147,29 @@ def _convert_from_representation(
             )
         )
     return _convert_from_baserep(value, units=units)
+
+
+##############################################################################
+# Gala compatibility
+# TODO: move this to an interoperability module
+
+# isort: split
+from galax.utils._optional_deps import HAS_GALA  # noqa: E402
+
+if HAS_GALA:
+    from gala.units import (
+        DimensionlessUnitSystem as GalaDimensionlessUnitSystem,
+        UnitSystem as GalaUnitSystem,
+    )
+
+    @converter_to_usys.register
+    def _from_gala(value: GalaUnitSystem, /) -> UnitSystem:
+        usys = UnitSystem(*value._core_units)  # noqa: SLF001
+        usys._registry = value._registry  # noqa: SLF001
+        return usys
+
+    @converter_to_usys.register
+    def _from_gala_dimensionless(
+        value: GalaDimensionlessUnitSystem, /
+    ) -> DimensionlessUnitSystem:
+        return dimensionless

src/galax/utils/_optional_deps.py

@@ -0,0 +1,7 @@
+"""Optional dependencies."""
+
+__all__ = ["HAS_GALA"]
+
+from importlib.util import find_spec
+
+HAS_GALA = find_spec("gala") is not None

tests/unit/potential/test_utils.py

@@ -12,6 +12,7 @@
     galactic,
     solarsystem,
 )
+from galax.utils._optional_deps import HAS_GALA
 
 
 class TestConverterToUtils:
@@ -45,6 +46,23 @@ def test_from_name(self):
         with pytest.raises(NotImplementedError):
             converter_to_usys("invalid_value")
 
+    @pytest.mark.skipif(not HAS_GALA, reason="requires gala")
+    def test_from_gala(self):
+        """Test conversion from gala."""
+        # -------------------------------
+        # UnitSystem
+        from gala.units import UnitSystem as GalaUnitSystem
+
+        value = GalaUnitSystem(u.km, u.s, u.Msun, u.radian)
+        assert converter_to_usys(value) == UnitSystem(*value._core_units)
+
+        # -------------------------------
+        # DimensionlessUnitSystem
+        from gala.units import DimensionlessUnitSystem as GalaDimensionlessUnitSystem
+
+        value = GalaDimensionlessUnitSystem()
+        assert converter_to_usys(value) == dimensionless
+
 
 # ============================================================================