Fix multiple appends of '_dg' in name attribute on calling inverse() (#…

…6563)

* Fix multiple appends of '_dg' in name attribute.

If inverse() method of Gate is called multiple times, each call appended
'_dg' to the end of the name attribute, i.e. 'gate_name_dg_dg'. This commit
fixes it by checking if the name already contains '_dg'. If so, the '_dg'
is removed from the name. If not, '_dg' is added.

* Add tests and release notes.

* Update bugfix-gate-inverse-name-3401093a18813ace.yaml

qiskit/circuit/instruction.py

@@ -337,18 +337,20 @@ def inverse(self):
 
         from qiskit.circuit import QuantumCircuit, Gate  # pylint: disable=cyclic-import
 
+        if self.name.endswith("_dg"):
+            name = self.name[:-3]
+        else:
+            name = self.name + "_dg"
         if self.num_clbits:
             inverse_gate = Instruction(
-                name=self.name + "_dg",
+                name=name,
                 num_qubits=self.num_qubits,
                 num_clbits=self.num_clbits,
                 params=self.params.copy(),
             )
 
         else:
-            inverse_gate = Gate(
-                name=self.name + "_dg", num_qubits=self.num_qubits, params=self.params.copy()
-            )
+            inverse_gate = Gate(name=name, num_qubits=self.num_qubits, params=self.params.copy())
 
         inverse_gate.definition = QuantumCircuit(
             *self.definition.qregs,

releasenotes/notes/bugfix-gate-inverse-name-3401093a18813ace.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed bug in assigning name attribute to ``inverse()`` function of 
+    :class:`~qiskit.circuit.Gate` (inherited from :class:`~qiskit.circuit.Instruction`).
+    Earlier, the inverse() function returned a gate with label None. Now 
+    the label of inverse is built using the label calling it.
+    `#6563 <https://github.com/Qiskit/qiskit-terra/pull/6563>` for more details.

test/python/circuit/test_instructions.py

@@ -350,6 +350,17 @@ def test_inverse_with_global_phase(self):
         gate_inverse = circ.to_instruction()
         self.assertEqual(gate.inverse().definition, gate_inverse.definition)
 
+    def test_inverse_with_label(self):
+        """test inverting gate initialized with label attribute."""
+        q = QuantumRegister(2)
+        qc = QuantumCircuit(q, name="circ")
+        qc.cx(0, 1)
+        qc_gate = qc.to_gate()
+        qc_gate_inverse = qc_gate.inverse()
+        self.assertEqual(qc_gate.name + "_dg", qc_gate_inverse.name)
+        qc_gate_inverse_inverse = qc_gate_inverse.inverse()
+        self.assertEqual(qc_gate_inverse_inverse.name, qc_gate.name)
+
     def test_no_broadcast(self):
         """See https://github.com/Qiskit/qiskit-terra/issues/2777
         When creating custom instructions, do not broadcast parameters"""