Merge pull request qiskit-community/qiskit-aqua#680 from chunfuchen/f…

…ix/check_op_is_empty

Check operator is empty before using it

qiskit/aqua/operators/matrix_operator.py

@@ -23,6 +23,7 @@
 from scipy import linalg as scila
 from qiskit import QuantumCircuit  # pylint: disable=unused-import
 
+from qiskit.aqua import AquaError
 from .base_operator import BaseOperator
 
 logger = logging.getLogger(__name__)
@@ -240,7 +241,12 @@ def evaluate_with_result(self, result, statevector_mode=True,
         Returns:
             float: the mean value
             float: the standard deviation
+        Raises:
+            AquaError: if Operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
+
         del use_simulator_operator_mode  # unused
         avg, std_dev = 0.0, 0.0
         quantum_state = np.asarray(result.get_statevector(circuit_name_prefix + 'psi'))
@@ -257,7 +263,12 @@ def evaluate_with_statevector(self, quantum_state):
         Returns:
             float: the mean value
             float: the standard deviation
+        Raises:
+            AquaError: if Operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
+
         avg = np.vdot(quantum_state, self._matrix.dot(quantum_state))
         return avg, 0.0
 
@@ -322,8 +333,13 @@ def evolve(self, state_in, evo_time=0, num_time_slices=0, expansion_mode='trotte
             numpy.array: Return the matrix vector multiplication result.
         Raises:
             ValueError: Invalid arguments
+            AquaError: if Operator is empty
         """
         from .op_converter import to_weighted_pauli_operator
+
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
+
         # pylint: disable=no-member
         if num_time_slices < 0 or not isinstance(num_time_slices, int):
             raise ValueError('Number of time slices should be a non-negative integer.')

qiskit/aqua/operators/weighted_pauli_operator.py

@@ -66,7 +66,6 @@ def __init__(self, paulis, basis=None, z2_symmetries=None, atol=1e-12, name=None
             [(pauli[1], [i]) for i, pauli in enumerate(paulis)] if basis is None else basis
         # combine the paulis and remove those with zero weight
         self.simplify()
-        self._z2_symmetries = z2_symmetries
         self._aer_paulis = None
         self._atol = atol
 
@@ -594,7 +593,11 @@ def evaluate_with_statevector(self, quantum_state):
         Returns:
             float: the mean value
             float: the standard deviation
+        Raises:
+            AquaError: if Operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
         # convert to matrix first?
         from .op_converter import to_matrix_operator
         mat_op = to_matrix_operator(self)
@@ -627,10 +630,14 @@ def construct_evaluation_circuit(self, wave_function, statevector_mode,
                                   circuit_name_prefix + Pauli string
 
         Raises:
+            AquaError: if Operator is empty
             AquaError: Can not find quantum register with `q` as the name and do not provide
                        quantum register explicitly
             AquaError: The provided qr is not in the wave_function
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
+
         from qiskit.aqua.utils.run_circuits import find_regs_by_name
 
         if qr is None:
@@ -685,7 +692,12 @@ def evaluation_instruction(self, statevector_mode, use_simulator_operator_mode=F
 
         Returns:
             dict: Pauli-instruction pair.
+
+        Raises:
+            AquaError: if Operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
         instructions = {}
         qr = QuantumRegister(self.num_qubits)
         qc = QuantumCircuit(qr)
@@ -729,7 +741,12 @@ def evaluate_with_result(self, result, statevector_mode, use_simulator_operator_
         Returns:
             float: the mean value
             float: the standard deviation
+
+        Raises:
+            AquaError: if Operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, check the operator.")
 
         avg, std_dev, variance = 0.0, 0.0, 0.0
         if statevector_mode:
@@ -837,7 +854,11 @@ def evolve(self, state_in=None, evo_time=0, num_time_slices=1, quantum_registers
             QuantumCircuit: The constructed circuit.
         Raises:
             AquaError: quantum_registers must be in the provided state_in circuit
+            AquaError: if operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, can not evolve.")
+
         if state_in is not None and quantum_registers is not None:
             if not state_in.has_register(quantum_registers):
                 raise AquaError("quantum_registers must be in the provided state_in circuit.")
@@ -877,8 +898,10 @@ def evolve_instruction(self, evo_time=0, num_time_slices=1,
         Raises:
             ValueError: Number of time slices should be a non-negative integer
             NotImplementedError: expansion mode not supported
-
+            AquaError: if operator is empty
         """
+        if self.is_empty():
+            raise AquaError("Operator is empty, can not build evolve instruction.")
         # pylint: disable=no-member
         if num_time_slices <= 0 or not isinstance(num_time_slices, int):
             raise ValueError('Number of time slices should be a non-negative integer.')