make it a full mappings

gufe/chemicalsystem.py

@@ -1,6 +1,7 @@
 # This code is part of OpenFE and is licensed under the MIT license.
 # For details, see https://github.com/OpenFreeEnergy/gufe
 
+from collections import abc
 from typing import Dict, Optional
 
 import numpy as np
@@ -9,7 +10,7 @@
 from .component import Component
 
 
-class ChemicalSystem(Serializable):
+class ChemicalSystem(Serializable, abc.Mapping):
     """A node of an alchemical network.
 
     Attributes
@@ -130,6 +131,12 @@ def total_charge(self):
     def __getitem__(self, item):
         return self.components[item]
 
+    def __iter__(self):
+        return iter(self.components)
+
+    def __len__(self):
+        return len(self.components)
+
     @classmethod
     def as_protein_smallmolecule_solvent(cls):
         """ """

gufe/tests/test_chemicalsystem.py

@@ -54,6 +54,9 @@ def test_ligand_construction(solv_comp, toluene_ligand_comp):
     )
 
     assert len(state.components) == 2
+    assert len(state) == 2
+
+    assert list(state) == ['solvent', 'ligand']
 
     assert state.components['solvent'] == solv_comp
     assert state.components['ligand'] == toluene_ligand_comp
@@ -71,6 +74,9 @@ def test_complex_construction(prot_comp, solv_comp, toluene_ligand_comp):
     )
 
     assert len(state.components) == 3
+    assert len(state) == 3
+
+    assert list(state) == ['protein', 'solvent', 'ligand']
 
     assert state.components['protein'] == prot_comp
     assert state.components['solvent'] == solv_comp