Make constants in cherab.core.utility.constants accessible to Python

CHANGELOG.md

@@ -17,6 +17,7 @@ New:
 * Add Integrator2D base class for integration of two-dimensional functions. (#472)
 * Support Raysect 0.9. (#486)
 * Test against Python 3.9, 3.10, 3.11, 3.12, 3.13 and latest released Numpy. Drop Python 3.7, 3.8 and older Numpy from tests. (#486)
+* Make values in `cherab.core.utility.constants` accessible to Python. (#509)
 
 Release 1.5.0 (27 Aug 2024)
 -------------------

cherab/core/utility/constants.pyx

@@ -15,6 +15,11 @@
 #
 # See the Licence for the specific language governing permissions and limitations
 # under the Licence.
+import sys
+from types import ModuleType
+
+from libc.math cimport M_PI
+
 
 cdef:
 
@@ -35,3 +40,47 @@ cdef:
     double RYDBERG_CONSTANT_EV = 13.605693122994
     double VACUUM_PERMITTIVITY = 8.8541878128e-12
     double BOHR_MAGNETON = 5.78838180123e-5  # in eV/T
+
+
+# Make the constants available to Python too.
+# To ensure the Python and Cython constants do not got out of sync the exported
+# Python attributes of the module are made read only using module getattr.
+cdef dict _CONSTANTS = {
+    # c stdlib
+    "RECIP_2_PI": RECIP_2_PI,
+    "RECIP_4_PI": RECIP_4_PI,
+    "DEGREES_TO_RADIANS": DEGREES_TO_RADIANS,
+    "RADIANS_TO_DEGREES": RADIANS_TO_DEGREES,
+    # NIST 2018
+    "ATOMIC_MASS": ATOMIC_MASS,
+    "ELEMENTARY_CHARGE": ELEMENTARY_CHARGE,
+    "SPEED_OF_LIGHT": SPEED_OF_LIGHT,
+    "PLANCK_CONSTANT": PLANCK_CONSTANT,
+    "HC_EV_NM": HC_EV_NM,
+    "ELECTRON_CLASSICAL_RADIUS": ELECTRON_CLASSICAL_RADIUS,
+    "ELECTRON_REST_MASS": ELECTRON_REST_MASS,
+    "RYDBERG_CONSTANT_EV": RYDBERG_CONSTANT_EV,
+    "VACUUM_PERMITTIVITY": VACUUM_PERMITTIVITY,
+    "BOHR_MAGNETON": BOHR_MAGNETON,
+}
+
+
+def __getattr__(name):
+    if name not in _CONSTANTS:
+        raise AttributeError()
+    return _CONSTANTS[name]
+
+
+def __dir__():
+    return list(_CONSTANTS.keys())
+
+
+class ReadOnlyModule(ModuleType):
+    def __setattr__(self, attr, value):
+        raise AttributeError("Constants are read-only")
+
+    def __delattr__(self, attr):
+        raise AttributeError("Constants are read-only")
+
+
+sys.modules[__name__].__class__ = ReadOnlyModule

cherab/core/utility/tests/test_constants.py

@@ -0,0 +1,84 @@
+# Copyright 2026 Oak Ridge National Laboratory
+#
+# Licensed under the EUPL, Version 1.1 or – as soon they will be approved by the
+# European Commission - subsequent versions of the EUPL (the "Licence");
+# You may not use this work except in compliance with the Licence.
+# You may obtain a copy of the Licence at:
+#
+# https://joinup.ec.europa.eu/software/page/eupl5
+#
+# Unless required by applicable law or agreed to in writing, software distributed
+# under the Licence is distributed on an "AS IS" basis, WITHOUT WARRANTIES OR
+# CONDITIONS OF ANY KIND, either express or implied.
+#
+# See the Licence for the specific language governing permissions and limitations
+# under the Licence.
+
+import math
+import unittest
+from cherab.core.utility import constants
+
+
+class TestConstants(unittest.TestCase):
+    def setUp(self):
+        self._expected_constants = dict(
+            # sourced c standard maths library
+            # CPython wraps libc's math so uses the same constants as cython's
+            # cimport of libc.math.
+            RECIP_2_PI=1 / (2 * math.pi),
+            RECIP_4_PI=1 / (4 * math.pi),
+            DEGREES_TO_RADIANS=math.pi / 180,
+            RADIANS_TO_DEGREES=180 / math.pi,
+
+            # sourced from NIST, CODATA 2018=https://physics.nist.gov/cuu/Constants/Table/allascii.txt
+            ATOMIC_MASS=1.66053906660e-27,
+            ELEMENTARY_CHARGE=1.602176634e-19,
+            SPEED_OF_LIGHT=299792458.0,
+            PLANCK_CONSTANT=6.62607015e-34,
+            HC_EV_NM=1239.8419738620933,  # (Planck constant in eV s) x (speed of light in nm/s)
+            ELECTRON_CLASSICAL_RADIUS=2.8179403262e-15,
+            ELECTRON_REST_MASS=9.1093837015e-31,
+            RYDBERG_CONSTANT_EV=13.605693122994,
+            VACUUM_PERMITTIVITY=8.8541878128e-12,
+            BOHR_MAGNETON=5.78838180123e-5,  # in eV/T
+        )
+
+    def test_all_exported(self):
+        """
+        Test Cython constants exported as Python floats.
+        """
+        for name, value in self._expected_constants.items():
+            self.assertEqual(value, getattr(constants, name))
+
+    def test_exported_literal(self):
+        """
+        Test a constant accessed by literal name.
+        """
+        self.assertEqual(self._expected_constants['ATOMIC_MASS'], constants.ATOMIC_MASS)
+
+    def test_exported_names(self):
+        """
+        Test all exported names are as expected by this test class.
+        """
+        self.assertEqual(sorted(self._expected_constants.keys()), sorted(dir(constants)))
+
+    def test_readonly(self):
+        """
+        Test that constants can't be modified or removed and new constants can't be added.
+        """
+        with self.assertRaises(AttributeError):
+            constants.ATOMIC_MASS = 1.66e-27
+
+    def test_nonew(self):
+        """
+        Test that attempting to assign a new constant from Python errors.
+        """
+        with self.assertRaises(AttributeError):
+            constants.TAU = math.tau
+
+    def test_nodel(self):
+        """
+        Test that attempting to delete and constant from Python errors.
+        """
+        with self.assertRaises(AttributeError):
+            del constants.RYDBERG_CONSTANT_EV