Fix a transpile issue of BackendSampler (#9236)

* fix an issue of transpile of BackendSampler

* optimize

* add reno

* updated test

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/primitives/backend_sampler.py

@@ -72,7 +72,7 @@ def __init__(
         self._transpile_options = Options()
         self._bound_pass_manager = bound_pass_manager
         self._preprocessed_circuits: list[QuantumCircuit] | None = None
-        self._transpiled_circuits: list[QuantumCircuit] | None = None
+        self._transpiled_circuits: list[QuantumCircuit] = []
         self._skip_transpilation = skip_transpilation
 
     def __new__(  # pylint: disable=signature-differs
@@ -108,8 +108,8 @@ def transpiled_circuits(self) -> list[QuantumCircuit]:
         """
         if self._skip_transpilation:
             self._transpiled_circuits = list(self._circuits)
-        elif self._transpiled_circuits is None:
-            # Only transpile if have not done so yet
+        elif len(self._transpiled_circuits) < len(self._circuits):
+            # transpile only circuits that are not transpiled yet
             self._transpile()
         return self._transpiled_circuits
 
@@ -174,10 +174,10 @@ def _postprocessing(self, result: Result, circuits: list[QuantumCircuit]) -> Sam
     def _transpile(self):
         from qiskit.compiler import transpile
 
-        self._transpiled_circuits = cast(
-            "list[QuantumCircuit]",
+        start = len(self._transpiled_circuits)
+        self._transpiled_circuits.extend(
             transpile(
-                self.preprocessed_circuits,
+                self.preprocessed_circuits[start:],
                 self.backend,
                 **self.transpile_options.__dict__,
             ),

releasenotes/notes/fix-backend-sampler-890cbcf913667b08.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug of :class:`~.BackendSampler` that raises an error
+    if :meth:`~.BackendSampler.run` method is called two times sequentially.

test/python/primitives/test_backend_sampler.py

@@ -319,6 +319,22 @@ def test_primitive_job_size_limit_backend_v1(self):
         self.assertDictAlmostEqual(result.quasi_dists[0], {0: 1}, 0.1)
         self.assertDictAlmostEqual(result.quasi_dists[1], {1: 1}, 0.1)
 
+    def test_sequential_run(self):
+        """Test sequential run."""
+        qc = QuantumCircuit(1)
+        qc.measure_all()
+        qc2 = QuantumCircuit(1)
+        qc2.x(0)
+        qc2.measure_all()
+        sampler = BackendSampler(backend=FakeNairobi())
+        result = sampler.run([qc]).result()
+        self.assertDictAlmostEqual(result.quasi_dists[0], {0: 1}, 0.1)
+        result2 = sampler.run([qc2]).result()
+        self.assertDictAlmostEqual(result2.quasi_dists[0], {1: 1}, 0.1)
+        result3 = sampler.run([qc, qc2]).result()
+        self.assertDictAlmostEqual(result3.quasi_dists[0], {0: 1}, 0.1)
+        self.assertDictAlmostEqual(result3.quasi_dists[1], {1: 1}, 0.1)
+
 
 if __name__ == "__main__":
     unittest.main()