Add QAOAAnsatz.num_qubits to improve performance (#7203)

* Add QAOAAnsatz.num_qubits

* returns 0 if no cost operator is given

qiskit/circuit/library/n_local/qaoa_ansatz.py

@@ -246,6 +246,12 @@ def mixer_operator(self, mixer_operator) -> None:
         self._mixer = mixer_operator
         self._invalidate()
 
+    @property
+    def num_qubits(self) -> int:
+        if self._cost_operator is None:
+            return 0
+        return self._cost_operator.num_qubits
+
     def _build(self):
         if self._data is not None:
             return

test/python/circuit/library/test_qaoa_ansatz.py

@@ -13,13 +13,16 @@
 """Test QAOA ansatz from the library."""
 
 import numpy as np
+from ddt import ddt, data
+
 from qiskit.circuit import QuantumCircuit, Parameter
-from qiskit.circuit.library.n_local.qaoa_ansatz import QAOAAnsatz
 from qiskit.circuit.library import HGate, RXGate, YGate, RYGate, RZGate
+from qiskit.circuit.library.n_local.qaoa_ansatz import QAOAAnsatz
 from qiskit.opflow import I, Y, Z
 from qiskit.test import QiskitTestCase
 
 
+@ddt
 class TestQAOAAnsatz(QiskitTestCase):
     """Test QAOAAnsatz."""
 
@@ -141,3 +144,17 @@ def test_circuit_mixer(self):
         reps = 4
         circuit = QAOAAnsatz(cost_operator=Z ^ Z, mixer_operator=mixer, reps=reps)
         self.assertEqual(circuit.num_parameters, 3 * reps)
+
+    def test_empty_op(self):
+        """Test construction without cost operator"""
+        circuit = QAOAAnsatz(reps=1)
+        self.assertEqual(circuit.num_qubits, 0)
+        with self.assertRaises(ValueError):
+            circuit.decompose()
+
+    @data(1, 2, 3, 4)
+    def test_num_qubits(self, num_qubits):
+        """Test num_qubits with {num_qubits} qubits"""
+
+        circuit = QAOAAnsatz(cost_operator=I ^ num_qubits, reps=5)
+        self.assertEqual(circuit.num_qubits, num_qubits)