VQE change backends

qiskit/aqua/algorithms/minimum_eigen_solvers/vqe.py

@@ -150,6 +150,7 @@ def __init__(self,
         self._max_evals_grouped = max_evals_grouped
         self._circuit_sampler = None  # type: Optional[CircuitSampler]
         self._expectation = expectation
+        self._user_passed_expectation = self._expectation is not None
         self._include_custom = include_custom
         self._expect_op = None
         self._operator = None
@@ -184,26 +185,28 @@ def operator(self, operator: Union[OperatorBase, LegacyBaseOperator]) -> None:
         self._operator = operator
         self._expect_op = None
         self._check_operator_varform()
+        # Expectation was never created, try to create one
         if self._expectation is None:
             self._try_set_expectation_value_from_factory()
 
-    def _try_set_expectation_value_from_factory(self):
+    def _try_set_expectation_value_from_factory(self) -> None:
         if self.operator and self.quantum_instance:
-            self.expectation = ExpectationFactory.build(operator=self.operator,
-                                                        backend=self.quantum_instance,
-                                                        include_custom=self._include_custom)
+            self._set_expectation(ExpectationFactory.build(operator=self.operator,
+                                                           backend=self.quantum_instance,
+                                                           include_custom=self._include_custom))
+
+    def _set_expectation(self, exp: ExpectationBase) -> None:
+        self._expectation = exp
+        self._expect_op = None
 
     @QuantumAlgorithm.quantum_instance.setter
     def quantum_instance(self, quantum_instance: Union[QuantumInstance, BaseBackend]) -> None:
         """ set quantum_instance """
         super(VQE, self.__class__).quantum_instance.__set__(self, quantum_instance)
 
-        if self._circuit_sampler is None:
-            self._circuit_sampler = CircuitSampler(self._quantum_instance)
-        else:
-            self._circuit_sampler.quantum_instance = self._quantum_instance
-
-        if self._expectation is None:
+        self._circuit_sampler = CircuitSampler(self._quantum_instance)
+        # Expectation was never created or it was not passed by user, try to create one
+        if self._expectation is None or not self._user_passed_expectation:
             self._try_set_expectation_value_from_factory()
 
     @property
@@ -214,8 +217,8 @@ def expectation(self) -> ExpectationBase:
 
     @expectation.setter
     def expectation(self, exp: ExpectationBase) -> None:
-        self._expectation = exp
-        self._expect_op = None
+        self._set_expectation(exp)
+        self._user_passed_expectation = self._expectation is not None
 
     @property
     def aux_operators(self) -> Optional[List[Optional[OperatorBase]]]:
@@ -328,8 +331,8 @@ def construct_expectation(self,
         else:
             wave_function = self.var_form.construct_circuit(parameter)
 
-        # If ExpectationValue was never created, create one now.
-        if self.expectation is None:
+        # Expectation was never created, try to create one
+        if self._expectation is None:
             self._try_set_expectation_value_from_factory()
 
         # If setting the expectation failed, raise an Error:

releasenotes/notes/expectation_change_backend-5483d30095f8bd82.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Changing backends in VQE from statevector to qasm_simulator or real device
+    was causing an error due to CircuitSampler incompatible reuse. VQE was changed
+    to always create a new CircuitSampler and create a new  expectation in case not
+    entered by user.
+    Refer to
+    `#1153 <https://github.com/Qiskit/qiskit-aqua/issues/1153>` for more details.

test/aqua/test_vqe.py

@@ -315,6 +315,22 @@ def test_ibmq(self):
         self.assertIsNotNone(result.cost_function_evals)
         self.assertIsNotNone(result.optimizer_time)
 
+    def test_backend_change(self):
+        """Test that VQE works when backend changes."""
+        vqe = VQE(operator=self.h2_op,
+                  var_form=TwoLocal(rotation_blocks=['ry', 'rz'], entanglement_blocks='cz'),
+                  optimizer=SLSQP(maxiter=2),
+                  quantum_instance=BasicAer.get_backend('statevector_simulator'))
+        result0 = vqe.run()
+        try:
+            vqe.set_backend(BasicAer.get_backend('qasm_simulator'))
+        except Exception as ex:  # pylint: disable=broad-except
+            self.fail("Failed to change backend. Error: '{}'".format(str(ex)))
+            return
+
+        result1 = vqe.run()
+        self.assertEqual(len(result0.optimal_point), len(result1.optimal_point))
+
 
 if __name__ == '__main__':
     unittest.main()