Allow for circuit measure methods to return a new circuit with inplac…

…e kwarg (#3755)

* all inplace kwarg to measure_all

* fix style

* fix lint

* Update quantumcircuit.py

* add inplace to other measure methods

* add release note

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

qiskit/circuit/quantumcircuit.py

@@ -1106,42 +1106,91 @@ def _create_creg(self, length, name):
             new_creg = ClassicalRegister(length, name)
         return new_creg
 
-    def measure_active(self):
+    def measure_active(self, inplace=True):
         """Adds measurement to all non-idle qubits. Creates a new ClassicalRegister with
         a size equal to the number of non-idle qubits being measured.
+
+        Returns a new circuit with measurements if `inplace=False`.
+
+        Parameters:
+            inplace (bool): All measurements inplace or return new circuit.
+
+        Returns:
+            QuantumCircuit: Returns circuit with measurements when `inplace = False`.
         """
         from qiskit.converters.circuit_to_dag import circuit_to_dag
-        dag = circuit_to_dag(self)
-        qubits_to_measure = [qubit for qubit in self.qubits if qubit not in dag.idle_wires()]
-        new_creg = self._create_creg(len(qubits_to_measure), 'measure')
-        self.add_register(new_creg)
-        self.barrier()
-        self.measure(qubits_to_measure, new_creg)
-
-    def measure_all(self):
+        if inplace:
+            circ = self
+        else:
+            circ = self.copy()
+        dag = circuit_to_dag(circ)
+        qubits_to_measure = [qubit for qubit in circ.qubits if qubit not in dag.idle_wires()]
+        new_creg = circ._create_creg(len(qubits_to_measure), 'measure')
+        circ.add_register(new_creg)
+        circ.barrier()
+        circ.measure(qubits_to_measure, new_creg)
+
+        if not inplace:
+            return circ
+        else:
+            return None
+
+    def measure_all(self, inplace=True):
         """Adds measurement to all qubits. Creates a new ClassicalRegister with a
         size equal to the number of qubits being measured.
+
+        Returns a new circuit with measurements if `inplace=False`.
+
+        Parameters:
+            inplace (bool): All measurements inplace or return new circuit.
+
+        Returns:
+            QuantumCircuit: Returns circuit with measurements when `inplace = False`.
         """
-        new_creg = self._create_creg(len(self.qubits), 'measure')
-        self.add_register(new_creg)
-        self.barrier()
-        self.measure(self.qubits, new_creg)
+        if inplace:
+            circ = self
+        else:
+            circ = self.copy()
 
-    def remove_final_measurements(self):
+        new_creg = circ._create_creg(len(circ.qubits), 'measure')
+        circ.add_register(new_creg)
+        circ.barrier()
+        circ.measure(circ.qubits, new_creg)
+
+        if not inplace:
+            return circ
+        else:
+            return None
+
+    def remove_final_measurements(self, inplace=True):
         """Removes final measurement on all qubits if they are present.
         Deletes the ClassicalRegister that was used to store the values from these measurements
         if it is idle.
+
+        Returns a new circuit without measurements if `inplace=False`.
+
+        Parameters:
+            inplace (bool): All measurements removed inplace or return new circuit.
+
+        Returns:
+            QuantumCircuit: Returns circuit with measurements removed when `inplace = False`.
         """
         # pylint: disable=cyclic-import
         from qiskit.transpiler.passes import RemoveFinalMeasurements
         from qiskit.converters import circuit_to_dag
-        dag = circuit_to_dag(self)
+
+        if inplace:
+            circ = self
+        else:
+            circ = self.copy()
+
+        dag = circuit_to_dag(circ)
         remove_final_meas = RemoveFinalMeasurements()
         new_dag = remove_final_meas.run(dag)
 
-        # Set self's cregs and instructions to match the new DAGCircuit's
-        self.data.clear()
-        self.cregs = list(new_dag.cregs.values())
+        # Set circ cregs and instructions to match the new DAGCircuit's
+        circ.data.clear()
+        circ.cregs = list(new_dag.cregs.values())
 
         for node in new_dag.topological_op_nodes():
             qubits = []
@@ -1155,7 +1204,12 @@ def remove_final_measurements(self):
             # Get arguments for classical condition (if any)
             inst = node.op.copy()
             inst.condition = node.condition
-            self.append(inst, qubits, clbits)
+            circ.append(inst, qubits, clbits)
+
+        if not inplace:
+            return circ
+        else:
+            return None
 
     @staticmethod
     def from_qasm_file(path):

releasenotes/notes/copy-measure-methods-562f0c1096f93145.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    QuantumCircuit methods `measure_active`, `measure_all`, and
+    `remove_final_measurements` now take the `inplace` keyword
+    argument that, when, `False` return a modified copy of the
+    circuit.

test/python/circuit/test_circuit_operations.py

@@ -180,6 +180,29 @@ def test_measure_active(self):
 
         self.assertEqual(expected, circuit)
 
+    def test_measure_active_copy(self):
+        """Test measure_active copy
+        Applies measurements only to non-idle qubits. Creates a ClassicalRegister of size equal to
+        the amount of non-idle qubits to store the measured values.
+        """
+        qr = QuantumRegister(4)
+        cr = ClassicalRegister(2, 'measure')
+
+        circuit = QuantumCircuit(qr)
+        circuit.h(qr[0])
+        circuit.h(qr[2])
+        new_circuit = circuit.measure_active(inplace=False)
+
+        expected = QuantumCircuit(qr)
+        expected.h(qr[0])
+        expected.h(qr[2])
+        expected.add_register(cr)
+        expected.barrier()
+        expected.measure([qr[0], qr[2]], [cr[0], cr[1]])
+
+        self.assertEqual(expected, new_circuit)
+        self.assertFalse('measure' in circuit.count_ops().keys())
+
     def test_measure_active_repetition(self):
         """Test measure_active in a circuit with a 'measure' creg.
         measure_active should be aware that the creg 'measure' might exists.
@@ -212,6 +235,22 @@ def test_measure_all(self):
 
         self.assertEqual(expected, circuit)
 
+    def test_measure_all_copy(self):
+        """Test measure_all with inplace=False
+        """
+        qr = QuantumRegister(2)
+        cr = ClassicalRegister(2, 'measure')
+
+        circuit = QuantumCircuit(qr)
+        new_circuit = circuit.measure_all(inplace=False)
+
+        expected = QuantumCircuit(qr, cr)
+        expected.barrier()
+        expected.measure(qr, cr)
+
+        self.assertEqual(expected, new_circuit)
+        self.assertFalse('measure' in circuit.count_ops().keys())
+
     def test_measure_all_repetition(self):
         """Test measure_all in a circuit with a 'measure' creg.
         measure_all should be aware that the creg 'measure' might exists.
@@ -241,6 +280,22 @@ def test_remove_final_measurements(self):
 
         self.assertEqual(expected, circuit)
 
+    def test_remove_final_measurements_copy(self):
+        """Test remove_final_measurements on copy
+        Removes all measurements at end of circuit.
+        """
+        qr = QuantumRegister(2)
+        cr = ClassicalRegister(2, 'measure')
+
+        circuit = QuantumCircuit(qr, cr)
+        circuit.measure(qr, cr)
+        new_circuit = circuit.remove_final_measurements(inplace=False)
+
+        expected = QuantumCircuit(qr)
+
+        self.assertEqual(expected, new_circuit)
+        self.assertTrue('measure' in circuit.count_ops().keys())
+
     def test_remove_final_measurements_multiple_measures(self):
         """Test remove_final_measurements only removes measurements at the end of the circuit
         remove_final_measurements should not remove measurements in the beginning or middle of the