message enhancement in MatrixOp init

qiskit/aqua/algorithms/quantum_algorithm.py

@@ -60,7 +60,7 @@ def run(self,
             AquaError: If a quantum instance or backend has not been provided
         """
         if quantum_instance is None and self.quantum_instance is None:
-            raise AquaError("Quantum device or backend "
+            raise AquaError("QuantumInstance or backend "
                             "is needed since you are running quantum algorithm.")
         if isinstance(quantum_instance, (BaseBackend, Backend)):
             self.set_backend(quantum_instance, **kwargs)

qiskit/aqua/operators/primitive_ops/circuit_op.py

@@ -35,9 +35,8 @@ class CircuitOp(PrimitiveOp):
     """
 
     def __init__(self,
-                 primitive: Union[Instruction, QuantumCircuit] = None,
-                 coeff: Optional[Union[int, float, complex,
-                                       ParameterExpression]] = 1.0) -> None:
+                 primitive: Union[Instruction, QuantumCircuit],
+                 coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> None:
         """
         Args:
             primitive: The QuantumCircuit which defines the

qiskit/aqua/operators/primitive_ops/matrix_op.py

@@ -12,7 +12,7 @@
 
 """ MatrixOp Class """
 
-from typing import Union, Optional, Set, Dict, List, cast
+from typing import Union, Optional, Set, Dict, List, cast, get_type_hints
 import logging
 import numpy as np
 from scipy.sparse import spmatrix
@@ -40,8 +40,8 @@ class MatrixOp(PrimitiveOp):
     """
 
     def __init__(self,
-                 primitive: Union[list, np.ndarray, spmatrix, Operator] = None,
-                 coeff: Optional[Union[int, float, complex, ParameterExpression]] = 1.0) -> None:
+                 primitive: Union[list, np.ndarray, spmatrix, Operator],
+                 coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> None:
         """
         Args:
             primitive: The matrix-like object which defines the behavior of the underlying function.
@@ -51,16 +51,18 @@ def __init__(self,
             TypeError: invalid parameters.
             ValueError: invalid parameters.
         """
+        primitive_orig = primitive
         if isinstance(primitive, spmatrix):
             primitive = primitive.toarray()
 
         if isinstance(primitive, (list, np.ndarray)):
             primitive = Operator(primitive)
 
         if not isinstance(primitive, Operator):
-            raise TypeError(
-                'MatrixOp can only be instantiated with MatrixOperator, '
-                'not {}'.format(type(primitive)))
+            type_hints = get_type_hints(MatrixOp.__init__).get('primitive')
+            valid_cls = [cls.__name__ for cls in type_hints.__args__]
+            raise TypeError(f"MatrixOp can only be instantiated with {valid_cls}, "
+                            f"not '{primitive_orig.__class__.__name__}'")
 
         if not primitive.input_dims() == primitive.output_dims():
             raise ValueError('Cannot handle non-square matrices yet.')

qiskit/aqua/operators/primitive_ops/pauli_op.py

@@ -39,7 +39,7 @@ class PauliOp(PrimitiveOp):
     """
 
     def __init__(self,
-                 primitive: Union[Pauli] = None,
+                 primitive: Union[Pauli],
                  coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> None:
         """
             Args:

qiskit/aqua/operators/primitive_ops/primitive_op.py

@@ -48,8 +48,9 @@ class PrimitiveOp(OperatorBase):
     @staticmethod
     # pylint: disable=unused-argument
     def __new__(cls,
-                primitive: Union[Instruction, QuantumCircuit, List,
-                                 np.ndarray, spmatrix, MatrixOperator, Pauli] = None,
+                primitive:
+                Optional[Union[Instruction, QuantumCircuit, List,
+                               np.ndarray, spmatrix, MatrixOperator, Pauli]] = None,
                 coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> 'PrimitiveOp':
         """ A factory method to produce the correct type of PrimitiveOp subclass
         based on the primitive passed in. Primitive and coeff arguments are passed into
@@ -85,9 +86,10 @@ def __new__(cls,
                         'factory constructor'.format(type(primitive)))
 
     def __init__(self,
-                 primitive: Union[Instruction, QuantumCircuit, List,
-                                  np.ndarray, spmatrix, MatrixOperator, Pauli] = None,
-                 coeff: Optional[Union[int, float, complex, ParameterExpression]] = 1.0) -> None:
+                 primitive:
+                 Optional[Union[Instruction, QuantumCircuit, List,
+                                np.ndarray, spmatrix, MatrixOperator, Pauli]] = None,
+                 coeff: Union[int, float, complex, ParameterExpression] = 1.0) -> None:
         """
             Args:
                 primitive: The operator primitive being wrapped.

test/aqua/operators/test_op_construction.py

@@ -35,6 +35,7 @@
     CircuitStateFn, VectorStateFn, DictStateFn, OperatorStateFn, ListOp, ComposedOp, TensoredOp,
     SummedOp
 )
+from qiskit.aqua.operators import MatrixOperator
 
 
 # pylint: disable=invalid-name
@@ -909,6 +910,31 @@ def test_to_circuit_op(self):
                 sfn.to_circuit_op().eval().primitive.data, vector
             )
 
+    def test_invalid_primitive(self):
+        """Test invalid MatrixOp construction"""
+        msg = "MatrixOp can only be instantiated with " \
+              "['list', 'ndarray', 'spmatrix', 'Operator'], not "
+
+        with self.assertRaises(TypeError) as cm:
+            _ = MatrixOp('invalid')
+
+        self.assertEqual(str(cm.exception), msg + "'str'")
+
+        with self.assertRaises(TypeError) as cm:
+            _ = MatrixOp(MatrixOperator(np.eye(2)))
+
+        self.assertEqual(str(cm.exception), msg + "'MatrixOperator'")
+
+        with self.assertRaises(TypeError) as cm:
+            _ = MatrixOp(None)
+
+        self.assertEqual(str(cm.exception), msg + "'NoneType'")
+
+        with self.assertRaises(TypeError) as cm:
+            _ = MatrixOp(2.0)
+
+        self.assertEqual(str(cm.exception), msg + "'float'")
+
 
 class TestOpMethods(QiskitAquaTestCase):
     """Basic method tests."""