Feature: length __len__ in AtomicSystem

PQAnalysis/atomic_system/atomic_system.py

@@ -680,6 +680,17 @@ def __getitem__(
             topology=self.topology[keys]
         )
 
+    def __len__(self) -> int:
+        """
+        Returns the number of atoms in the AtomicSystem.
+
+        Returns
+        -------
+        int
+            The number of atoms in the AtomicSystem.
+        """
+        return self.n_atoms
+
     def __str__(self) -> str:
         """
         Returns the string representation of the AtomicSystem.

tests/atomicSystem/test_atomic_system.py

@@ -356,6 +356,20 @@ def test_n_atoms(self, caplog):
             system
         )
 
+    def test__len__(self):
+        system = AtomicSystem()
+        assert len(system) == 0
+
+        system = AtomicSystem(pos=np.array([[0, 0, 0], [1, 1, 1]]))
+        assert len(system) == 2
+
+        system = AtomicSystem(
+            pos=np.array([[0, 0, 0], [1, 1, 1]]),
+            atoms=[Atom('C'), Atom('H')],
+            cell=Cell(0.75, 0.75, 0.75)
+        )
+        assert len(system) == 2
+
     def test__str__(self):
         """
         Test the __str__ method of the AtomicSystem class.