Add special casing for from_name

src/particle/particle/particle.py

@@ -63,6 +63,14 @@ class InvalidParticle(RuntimeError):
     _NON_UNIQUE_PDGIDS[-pdgid1] = -pdgid2
     _NON_UNIQUE_PDGIDS[-pdgid2] = -pdgid1
 
+# lookup for hash and from_name which representation is the preferred one
+# this results in the "bag-of-quarks" representation
+_PREFERRED_PDGID = {}
+for pdgid1, pdgid2 in _NON_UNIQUE_PDGIDS.items():
+    sign = -1 if pdgid1 < 0 else 1
+    _PREFERRED_PDGID[pdgid1] = sign * min(abs(pdgid1), abs(pdgid2))
+    _PREFERRED_PDGID[pdgid2] = _PREFERRED_PDGID[pdgid1]
+
 
 def _isospin_converter(isospin: str) -> float | None:
     vals: dict[str | None, float | None] = {
@@ -627,8 +635,8 @@ def __eq__(self, other: object) -> bool:
         return self.pdgid == other
 
     def __hash__(self) -> int:
-        if self.pdgid in _NON_UNIQUE_PDGIDS:
-            return hash(min(_NON_UNIQUE_PDGIDS[self.pdgid], self.pdgid))
+        if self.pdgid in _PREFERRED_PDGID:
+            return hash(_PREFERRED_PDGID[self.pdgid])
         return hash(self.pdgid)
 
     # Shared with PDGID
@@ -1011,6 +1019,13 @@ def from_name(cls: type[Self], name: str) -> Self:
         ParticleNotFound
             If no particle matches the input name uniquely and exactly.
         """
+        # special handling for the particles with two possible pdgids
+        if name in {"p", "n", "p~", "n~"}:
+
+            def find_preferred_id(p: Self) -> bool:
+                return int(p.pdgid) in _PREFERRED_PDGID.values()
+
+            return next(filter(find_preferred_id, cls.finditer(name=name)))
         try:
             (particle,) = cls.finditer(
                 name=name

tests/particle/test_particle.py

@@ -711,10 +711,10 @@ def test_evtgen_name(name, pid):  # noqa: ARG001
 @pytest.mark.parametrize(
     ("pdgid1", "pdgid2"),
     [
-        pytest.param(2212, 1000010010, id="proton"),
-        pytest.param(2112, 1000000010, id="neutron"),
-        pytest.param(-2212, -1000010010, id="anti-proton"),
-        pytest.param(-2112, -1000000010, id="anti-neutron"),
+        pytest.param(2212, 1000010010, id="p"),
+        pytest.param(2112, 1000000010, id="n"),
+        pytest.param(-2212, -1000010010, id="p~"),
+        pytest.param(-2112, -1000000010, id="n~"),
     ],
 )
 def test_eq_non_unique_pdgids(pdgid1, pdgid2):
@@ -725,3 +725,20 @@ def test_eq_non_unique_pdgids(pdgid1, pdgid2):
     assert p1.pdgid != p2.pdgid
     assert p1 == p2
     assert hash(p1) == hash(p2)
+
+
+@pytest.mark.parametrize(
+    ("name", "pdgid"),
+    [
+        pytest.param("p", 2212, id="p"),
+        pytest.param("n", 2112, id="n"),
+        pytest.param("p~", -2212, id="p~"),
+        pytest.param("n~", -2112, id="n~"),
+    ],
+)
+def test_from_name_non_unique_pdgids(name, pdgid):
+    """The proton and the neutron have two pdgid representations, make sure they still compare equal"""
+
+    p = Particle.from_name(name)
+    assert p.name == name
+    assert p.pdgid == pdgid