Enable proper iteration over Statevector (Qiskit#8062)

* Enable proper iteration over Statevector

* Add Statevector.__len__ and add a release note

* Simplify release note

* Update test_statevctor_iter

Co-authored-by: Jake Lishman <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/quantum_info/states/statevector.py

@@ -218,6 +218,12 @@ def __getitem__(self, key):
         else:
             raise QiskitError("Key must be int or a valid binary string.")
 
+    def __iter__(self):
+        yield from self._data
+
+    def __len__(self):
+        return len(self._data)
+
     @property
     def data(self):
         """Return data."""

releasenotes/notes/statevector-enable-iter-4652d7ce87f4d459.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    :class:`.Statevector` will now allow direct iteration through its values
+    (such as ``for coefficient in statevector``) and
+    correctly report its length under ``len``.  Previously it would try and
+    and access out-of-bounds data and raise a :class:`.QiskitError`.  See
+    `#8039 <https://github.com/Qiskit/qiskit-terra/issues/8039>`__.

test/python/quantum_info/states/test_statevector.py

@@ -1164,6 +1164,29 @@ def test_statevector_draw_latex_regression(self):
         self.assertTrue(latex_string.startswith(" |0\\rangle"))
         self.assertNotIn("|1\\rangle", latex_string)
 
+    def test_statevctor_iter(self):
+        """Test iteration over a state vector"""
+        empty_vector = []
+        dummy_vector = [1, 2, 3]
+        empty_sv = Statevector([])
+        sv = Statevector(dummy_vector)
+
+        # Assert that successive iterations behave as expected, i.e., the
+        # iterator is reset upon each exhaustion of the corresponding
+        # collection of elements.
+        for _ in range(2):
+            self.assertEqual(empty_vector, list(empty_sv))
+            self.assertEqual(dummy_vector, list(sv))
+
+    def test_statevector_len(self):
+        """Test state vector length"""
+        empty_vector = []
+        dummy_vector = [1, 2, 3]
+        empty_sv = Statevector([])
+        sv = Statevector(dummy_vector)
+        self.assertEqual(len(empty_vector), len(empty_sv))
+        self.assertEqual(len(dummy_vector), len(sv))
+
 
 if __name__ == "__main__":
     unittest.main()