Fixes assymetric == between Element and Species. Fixes #3504.

Author: Shyue Ping Ong

pymatgen/core/periodic_table.py

@@ -454,7 +454,7 @@ def ground_state_term_symbol(self):
         return J_sorted_terms[-1][0]
 
     def __eq__(self, other: object) -> bool:
-        return isinstance(other, Element) and self.Z == other.Z
+        return isinstance(self, Element) and isinstance(other, Element) and self.Z == other.Z
 
     def __hash__(self) -> int:
         return self.Z
@@ -895,7 +895,12 @@ class Species(MSONable, Stringify):
 
     STRING_MODE = "SUPERSCRIPT"
 
-    def __init__(self, symbol: SpeciesLike, oxidation_state: float | None = None, spin: float | None = None) -> None:
+    def __init__(
+        self,
+        symbol: SpeciesLike,
+        oxidation_state: float | None = None,
+        spin: float | None = None,
+    ) -> None:
         """
         Args:
             symbol (str): Element symbol optionally incl. oxidation state. E.g. Fe, Fe2+, O2-.
@@ -1138,7 +1143,9 @@ def get_shannon_radius(
         return data[f"{radius_type}_radius"]
 
     def get_crystal_field_spin(
-        self, coordination: Literal["oct", "tet"] = "oct", spin_config: Literal["low", "high"] = "high"
+        self,
+        coordination: Literal["oct", "tet"] = "oct",
+        spin_config: Literal["low", "high"] = "high",
     ) -> float:
         """Calculate the crystal field spin based on coordination and spin
         configuration. Only works for transition metal species.

tests/core/test_periodic_table.py

@@ -383,6 +383,9 @@ def test_eq(self):
         assert self.specie4 != self.specie3
         assert self.specie1 != Element("Fe")
         assert Element("Fe") != self.specie1
+        assert Element("Fe") == Element("Fe")
+        assert Species("Fe", 0) != Element("Fe")
+        assert Element("Fe") != Species("Fe", 0)
 
     def test_cmp(self):
         assert self.specie1 < self.specie2, "Fe2+ should be < Fe3+"
@@ -423,7 +426,10 @@ def test_get_crystal_field_spin(self):
 
         for elem in ("Li+", "Ge4+", "H+"):
             symbol = Species(elem).symbol
-            with pytest.raises(AttributeError, match=f"Invalid element {symbol} for crystal field calculation"):
+            with pytest.raises(
+                AttributeError,
+                match=f"Invalid element {symbol} for crystal field calculation",
+            ):
                 Species(elem).get_crystal_field_spin()
         with pytest.raises(AttributeError, match="Invalid oxidation state 10 for element Fe"):
             Species("Fe", 10).get_crystal_field_spin()