Silently change hv to hν in _parse_formula and add a hash method

setup.py

@@ -8,7 +8,7 @@
 
 setup(
     name="pyvalem",
-    version="2.5.9",
+    version="2.5.10",
     description="A package for managing simple chemical species and states",
     long_description=long_description,
     long_description_content_type="text/x-rst",

src/pyvalem/formula.py

@@ -200,9 +200,10 @@ class Formula:
 
     Notes
     -----
-    The ``__repr__`` method is overloaded to provide a *canonicalised* representation
-    of the formula. The idea is that two formulas representing the same physical entity
-    will have the same ``repr(formula)`` representation.
+    The ``__repr__`` method is intended to be a *canonicalised* representation
+    of the formula, but no check is made to ensure that a single formula is used
+    to describe a unique species (e.g. "HD" and "DH" and "H(2H)" all have
+    different __repr__ representations.
 
     Examples
     --------
@@ -321,6 +322,10 @@ def _parse_formula(self, formula):
         if formula == "e":
             # Quietly convert e to e-.
             self.formula = formula = "e-"
+
+        if self.formula == "hv":
+            self.formula = "hν"
+
         if formula in special_cases:
             for attr, val in special_cases[formula].items():
                 setattr(self, attr, val)
@@ -506,12 +511,13 @@ def _get_charge_reps(charge):
         return "", "", ""
 
     def __repr__(self):
-        if self.formula == "hv":
-            return "hν"
         return self.formula
 
+    def __hash__(self):
+        return hash(self.formula)
+
     def __eq__(self, other):
-        return repr(self.formula) == repr(other.formula)
+        return self.formula == other.formula
 
     def _stoichiometric_formula_atomic_number(self):
         """Return a list of atoms/isotopes and their stoichiometries.

tests/test_formula.py

@@ -163,6 +163,16 @@ def test_parse_fail(self):
         self.assertRaises(FormulaParseError, Formula, "H3O^+")
         self.assertRaises(FormulaParseError, Formula, "H_2S")
 
+    def test_formula_hash(self):
+        f1 = Formula("hv")
+        f2 = Formula("Ar")
+        test_dict = {f1: 0, f2: "a"}
+        test_set = set((f1, f1, f2))
+
+        f3 = Formula("hν")
+        self.assertEqual(hash(f1), hash(f3))
+        self.assertEqual(repr(f1), repr(f3))
+
 
 if __name__ == "__main__":
     unittest.main()