Improve sum op equals (#7681)

* Enable PauliSumOp comparison with single PauliOp

* Also fix the commutative case where equals is called on PauliOp

* Add reno release note for PauliSumOp equals

Co-authored-by: Ikko Hamamura <ikkoham@users.noreply.github.com>

qiskit/opflow/primitive_ops/pauli_op.py

@@ -87,10 +87,16 @@ def adjoint(self) -> "PauliOp":
         return PauliOp(self.primitive, coeff=self.coeff.conjugate())
 
     def equals(self, other: OperatorBase) -> bool:
-        if not isinstance(other, PauliOp) or not self.coeff == other.coeff:
-            return False
+        if isinstance(other, PauliOp) and self.coeff == other.coeff:
+            return self.primitive == other.primitive
 
-        return self.primitive == other.primitive
+        # pylint: disable=cyclic-import
+        from .pauli_sum_op import PauliSumOp
+
+        if isinstance(other, PauliSumOp):
+            return other == self
+
+        return False
 
     def _expand_dim(self, num_qubits: int) -> "PauliOp":
         return PauliOp(Pauli("I" * num_qubits).expand(self.primitive), coeff=self.coeff)

qiskit/opflow/primitive_ops/pauli_sum_op.py

@@ -148,6 +148,11 @@ def adjoint(self) -> "PauliSumOp":
     def equals(self, other: OperatorBase) -> bool:
         self_reduced, other_reduced = self.reduce(), other.reduce()
 
+        if isinstance(other_reduced, PauliOp):
+            other_reduced = PauliSumOp(
+                SparsePauliOp(other_reduced.primitive, coeffs=[other_reduced.coeff])
+            )
+
         if not isinstance(other_reduced, PauliSumOp):
             return False
 

releasenotes/notes/paulisumop-may-equal-pauliop-af86de94020fba22.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    :class:`~qiskit.opflow.PauliSumOp` equality tests now handle the case when
+    one of the compared items is a single :class:`~qiskit.opflow.PauliOp`.
+    For example, ``0 * X + I == I`` now evaluates to True, whereas it was
+    False prior to this release.

test/python/opflow/test_pauli_sum_op.py

@@ -137,6 +137,8 @@ def test_equals(self):
 
         self.assertNotEqual((X ^ X) + (Y ^ Y), X + Y)
         self.assertEqual((X ^ X) + (Y ^ Y), (Y ^ Y) + (X ^ X))
+        self.assertEqual(0 * X + I, I)
+        self.assertEqual(I, 0 * X + I)
 
         theta = ParameterVector("theta", 2)
         pauli_sum0 = theta[0] * (X + Z)