Add an error check for Sampler (Qiskit#8678)

* add an error check for sampler

* update

* Update qiskit/primitives/base_sampler.py

Co-authored-by: Julien Gacon <[email protected]>

* remove a duplicate check

* update an error messgage

Co-authored-by: Julien Gacon <[email protected]>

qiskit/primitives/base_sampler.py

@@ -105,6 +105,7 @@
 from qiskit.utils.deprecation import deprecate_arguments, deprecate_function
 
 from .sampler_result import SamplerResult
+from .utils import final_measurement_mapping
 
 
 class BaseSampler(ABC):
@@ -383,6 +384,23 @@ def run(
                     f"The number of values ({len(parameter_value)}) does not match "
                     f"the number of parameters ({circuit.num_parameters}) for the {i}-th circuit."
                 )
+
+        for i, circuit in enumerate(circuits):
+            if circuit.num_clbits == 0:
+                raise QiskitError(
+                    f"The {i}-th circuit does not have any classical bit. "
+                    "Sampler requires classical bits, plus measurements "
+                    "on the desired qubits."
+                )
+
+            mapping = final_measurement_mapping(circuit)
+            if set(range(circuit.num_clbits)) != set(mapping.values()):
+                raise QiskitError(
+                    f"Some classical bits of the {i}-th circuit are not used for measurements."
+                    f" the number of classical bits ({circuit.num_clbits}),"
+                    f" the used classical bits ({set(mapping.values())})."
+                )
+
         run_opts = copy(self.run_options)
         run_opts.update_options(**run_options)
 

qiskit/primitives/sampler.py

@@ -158,12 +158,6 @@ def _run(
     def _preprocess_circuit(circuit: QuantumCircuit):
         circuit = init_circuit(circuit)
         q_c_mapping = final_measurement_mapping(circuit)
-        if set(range(circuit.num_clbits)) != set(q_c_mapping.values()):
-            raise QiskitError(
-                "some classical bits are not used for measurements."
-                f" the number of classical bits {circuit.num_clbits},"
-                f" the used classical bits {set(q_c_mapping.values())}."
-            )
         c_q_mapping = sorted((c, q) for q, c in q_c_mapping.items())
         qargs = [q for _, q in c_q_mapping]
         circuit = circuit.remove_final_measurements(inplace=False)

releasenotes/notes/add-sampler-error-check-38426fb186db44d4.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    Added some error checks to :meth:`~qiskit.primitives.BaseSampler.run`.
+    It raises an error if there is no classical bit or some classical bits
+    are not used for measurements.
+

test/python/primitives/test_sampler.py

@@ -572,19 +572,33 @@ def test_run_2qubit(self):
         np.testing.assert_allclose(values, [0, 0, 0, 1])
 
     def test_run_errors(self):
-        """Test for errors"""
+        """Test for errors with run method"""
         qc1 = QuantumCircuit(1)
         qc1.measure_all()
         qc2 = RealAmplitudes(num_qubits=1, reps=1)
         qc2.measure_all()
+        qc3 = QuantumCircuit(1)
+        qc4 = QuantumCircuit(1, 1)
 
         sampler = Sampler()
-        with self.assertRaises(QiskitError):
-            sampler.run([qc1], [[1e2]]).result()
-        with self.assertRaises(QiskitError):
-            sampler.run([qc2], [[]]).result()
-        with self.assertRaises(QiskitError):
-            sampler.run([qc2], [[1e2]]).result()
+        with self.subTest("set parameter values to a non-parameterized circuit"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc1], [[1e2]])
+        with self.subTest("missing all parameter values for a parameterized circuit"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc2], [[]])
+        with self.subTest("missing some parameter values for a parameterized circuit"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc2], [[1e2]])
+        with self.subTest("too many parameter values for a parameterized circuit"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc2], [[1e2]] * 100)
+        with self.subTest("no classical bits"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc3], [[]])
+        with self.subTest("no measurement"):
+            with self.assertRaises(QiskitError):
+                _ = sampler.run([qc4], [[]])
 
     def test_run_empty_parameter(self):
         """Test for empty parameter"""