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

qiskit/aqua/algorithms/minimum_eigen_solvers/qaoa/qaoa.py

@@ -123,6 +123,8 @@ def __init__(self,
     @VQE.operator.setter
     def operator(self, operator: Union[OperatorBase, LegacyBaseOperator]) -> None:
         """ Sets operator """
+        # Need to wipe the var_form in case number of qubits differs from operator.
+        self.var_form = None
         # Setting with VQE's operator property
         super(QAOA, self.__class__).operator.__set__(self, operator)
         self.var_form = QAOAVarForm(self.operator,

qiskit/aqua/algorithms/minimum_eigen_solvers/vqe.py

@@ -131,7 +131,9 @@ def __init__(self,
 
         self._max_evals_grouped = max_evals_grouped
         self._circuit_sampler = None
+        self._expectation = expectation
         self._expect_op = None
+        self._operator = None
 
         super().__init__(var_form=var_form,
                          optimizer=optimizer,
@@ -143,8 +145,6 @@ def __init__(self,
         self._optimizer.set_max_evals_grouped(max_evals_grouped)
         self._callback = callback
 
-        self._expectation = expectation
-        self._operator = None
         if operator is not None:
             self.operator = operator
         self.aux_operators = aux_operators

qiskit/aqua/algorithms/vq_algorithm.py

@@ -84,7 +84,7 @@ def var_form(self) -> Optional[Union[QuantumCircuit, VariationalForm]]:
         return self._var_form
 
     @var_form.setter
-    def var_form(self, var_form: Union[QuantumCircuit, VariationalForm]):
+    def var_form(self, var_form: Optional[Union[QuantumCircuit, VariationalForm]]):
         """ Sets variational form """
         if isinstance(var_form, QuantumCircuit):
             # store the parameters
@@ -93,6 +93,9 @@ def var_form(self, var_form: Union[QuantumCircuit, VariationalForm]):
         elif isinstance(var_form, VariationalForm):
             self._var_form_params = ParameterVector('θ', length=var_form.num_parameters)
             self._var_form = var_form
+        elif var_form is None:
+            self._var_form_params = None
+            self._var_form = var_form
         else:
             raise ValueError(
                 "Unsupported type '{}' of var_form".format(