Fix Change Operator Size in QAOA (qiskit-community/qiskit-aqua#929)

* Fix qiskit-community/qiskit-aqua#928

* trigger CLA check

* Really fix.

* Add test for changing Operator size.

* Fix style

Co-authored-by: Manoel Marques <manoel@us.ibm.com>
Co-authored-by: Julien Gacon <jules.gacon@googlemail.com>

Author: 3 people

test/optimization/test_qaoa.py

@@ -26,7 +26,7 @@
 from qiskit.aqua.components.optimizers import COBYLA
 from qiskit.aqua.algorithms import QAOA
 from qiskit.aqua import QuantumInstance, aqua_globals
-from qiskit.aqua.operators import WeightedPauliOperator
+from qiskit.aqua.operators import WeightedPauliOperator, X, Z
 
 W1 = np.array([
     [0, 1, 0, 1],
@@ -90,6 +90,21 @@ def test_qaoa(self, w, prob, m, solutions, convert_to_matrix_op):
         self.log.debug('solution objective: %s', max_cut.max_cut_value(x, w))
         self.assertIn(''.join([str(int(i)) for i in graph_solution]), solutions)
 
+    def test_change_operator_size(self):
+        """ QAOA test """
+        backend = BasicAer.get_backend('statevector_simulator')
+        optimizer = COBYLA(maxiter=2)
+        qubit_op, _ = max_cut.get_operator(W1)
+        qubit_op = qubit_op.to_opflow().to_matrix_op()
+
+        seed = 0
+        aqua_globals.random_seed = seed
+        qaoa = QAOA(qubit_op, optimizer, P1)
+        quantum_instance = QuantumInstance(backend, seed_simulator=seed, seed_transpiler=seed)
+        qaoa.run(quantum_instance)
+        qaoa.operator = (X ^ qubit_op ^ Z)
+        qaoa.run()
+
 
 if __name__ == '__main__':
     unittest.main()