Check operator is empty before using it

CHANGELOG.md

@@ -18,6 +18,12 @@ Changelog](http://keepachangelog.com/en/1.0.0/).
 [UNRELEASED](https://github.com/Qiskit/qiskit-aqua/compare/0.6.0...HEAD)
 ========================================================================
 
+Fixed
+-------
+
+-   A bug where `UCCSD` might generate an empty operator and try to evolve it. (#680)
+
+
 [0.6.0](https://github.com/Qiskit/qiskit-aqua/compare/0.5.5...0.6.0) - 2019-08-22
 =================================================================================
 

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.')

qiskit/chemistry/aqua_extensions/components/variational_forms/uccsd.py

@@ -169,6 +169,24 @@ def __init__(self, num_qubits, depth, num_orbitals, num_particles,
 
         self._logging_construct_circuit = True
 
+    @property
+    def single_excitations(self):
+        """
+        Getter of single excitation list
+        Returns:
+            list[list[int]]: single excitation list
+        """
+        return self._single_excitations
+
+    @property
+    def double_excitations(self):
+        """
+        Getter of double excitation list
+        Returns:
+            list[list[int]]: double excitation list
+        """
+        return self._double_excitations
+
     def _build_hopping_operators(self):
         if logger.isEnabledFor(logging.DEBUG):
             TextProgressBar(sys.stderr)
@@ -179,7 +197,20 @@ def _build_hopping_operators(self):
                                           self._num_particles, self._qubit_mapping,
                                           self._two_qubit_reduction, self._z2_symmetries),
                                num_processes=aqua_globals.num_processes)
-        hopping_ops = [qubit_op for qubit_op in results if qubit_op is not None]
+        hopping_ops = []
+        s_e_list = []
+        d_e_list = []
+        for op, index in results:
+            if op is not None and not op.is_empty():
+                hopping_ops.append(op)
+                if len(index) == 2:  # for double excitation
+                    s_e_list.append(index)
+                else:  # for double excitation
+                    d_e_list.append(index)
+
+        self._single_excitations = s_e_list
+        self._double_excitations = d_e_list
+
         num_parameters = len(hopping_ops) * self._depth
         return hopping_ops, num_parameters
 
@@ -215,7 +246,7 @@ def _build_hopping_operator(index, num_orbitals, num_particles, qubit_mapping,
         if qubit_op is None:
             logger.debug('Excitation (%s) is skipped since it is not commuted '
                          'with symmetries', ','.join([str(x) for x in index]))
-        return qubit_op
+        return qubit_op, index
 
     def construct_circuit(self, parameters, q=None):
         """