Implementing CouplingMap.__eq__ (#9766)

* implementing Coupling.__eq__

* Update qiskit/transpiler/coupling.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* adding comments to test, and adding release note

* black formatting

* updating CouplingMap.__eq__ to check edge list equality instead of isomorphism

* updating release notes

* updating test doc string

* Update test/python/transpiler/test_coupling.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update test/python/transpiler/test_coupling.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update test/python/transpiler/test_coupling.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Update test/python/transpiler/test_coupling.py

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/transpiler/coupling.py

@@ -412,6 +412,22 @@ def __str__(self):
             string += "]"
         return string
 
+    def __eq__(self, other):
+        """Check if the graph in ``other`` has the same node labels and edges as the graph in
+        ``self``.
+
+        This function assumes that the graphs in :class:`.CouplingMap` instances are connected.
+
+        Args:
+            other (CouplingMap): The other coupling map.
+
+        Returns:
+            bool: Whether or not other is isomorphic to self.
+        """
+        if not isinstance(other, CouplingMap):
+            return False
+        return set(self.graph.edge_list()) == set(other.graph.edge_list())
+
     def draw(self):
         """Draws the coupling map.
 

releasenotes/notes/coupling-map-eq-b0507b703d62a5f3.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    The :meth:`.CouplingMap.__eq__`` method has been updated to check that the edge lists of the
+    underlying graphs contain the same elements. Under the assumption that the underlying graphs are
+    connected, this check additionally ensures that the graphs have the same number of nodes with
+    the same labels. Any code using ``CouplingMap() == CouplingMap()`` to check object equality
+    should be updated to ``CouplingMap() is CouplingMap()``.

test/python/transpiler/test_coupling.py

@@ -448,6 +448,25 @@ def test_implements_iter(self):
         expected = [(0, 1), (1, 0), (1, 2), (2, 1)]
         self.assertEqual(sorted(coupling), expected)
 
+    def test_equality(self):
+        """Test that equality checks that the graphs have the same nodes, node labels, and edges."""
+
+        # two coupling maps with 4 nodes and the same edges
+        coupling0 = CouplingMap([(0, 1), (0, 2), (2, 3)])
+        coupling1 = CouplingMap([(0, 1), (0, 2), (2, 3)])
+        self.assertEqual(coupling0, coupling1)
+
+        # coupling map with 5 nodes not equal to the previous 2
+        coupling2 = CouplingMap([(0, 1), (0, 2), (2, 4)])
+        self.assertNotEqual(coupling0, coupling2)
+
+        # coupling map isomorphic to coupling0, but with cyclically shifted labels
+        coupling3 = CouplingMap([(1, 2), (1, 3), (3, 0)])
+        self.assertNotEqual(coupling0, coupling3)
+
+        # additional test for comparison to a non-CouplingMap object
+        self.assertNotEqual(coupling0, 1)
+
 
 class CouplingVisualizationTest(QiskitVisualizationTestCase):
     @unittest.skipUnless(optionals.HAS_GRAPHVIZ, "Graphviz not installed")