Deprecate BasicSimulator.configuration

qiskit/compiler/assembler.py

@@ -189,6 +189,16 @@ def assemble(
     )
 
 
+# Note for future: this method is used in `BasicSimulator` and may need to be kept past the
+# `assemble` removal deadline (2.0). If it is kept (potentially in a different location),
+# we will need an alternative for the backend.configuration() access that currently takes
+# place in L566 (`parse_circuit_args`) and L351 (`parse_common_args`)
+# because backend.configuration() is also set for removal in 2.0.
+# The ultimate goal will be to move away from relying on any kind of `assemble` implementation
+# because of how tightly coupled it is to these legacy data structures. But as a transition step,
+# given that we would only have to support the subcase of `BasicSimulator`, we could probably just
+# inline the relevant config values that are already hardcoded in the basic simulator configuration
+# generator.
 def _assemble(
     experiments: Union[
         QuantumCircuit,
@@ -229,7 +239,7 @@ def _assemble(
     experiments = experiments if isinstance(experiments, list) else [experiments]
     pulse_qobj = any(isinstance(exp, (ScheduleBlock, Schedule, Instruction)) for exp in experiments)
     with warnings.catch_warnings():
-        # The Qobj is deprecated
+        # The Qobj class is deprecated, the backend.configuration() method is too
         warnings.filterwarnings("ignore", category=DeprecationWarning, module="qiskit")
         qobj_id, qobj_header, run_config_common_dict = _parse_common_args(
             backend,
@@ -554,9 +564,12 @@ def _parse_circuit_args(
     run_config_dict = {"parameter_binds": parameter_binds, **run_config}
     if parametric_pulses is None:
         if backend:
-            run_config_dict["parametric_pulses"] = getattr(
-                backend.configuration(), "parametric_pulses", []
-            )
+            with warnings.catch_warnings():
+                # TODO (2.0): See comment on L192 regarding backend.configuration removal
+                warnings.filterwarnings("ignore", category=DeprecationWarning, module="qiskit")
+                run_config_dict["parametric_pulses"] = getattr(
+                    backend.configuration(), "parametric_pulses", []
+                )
         else:
             run_config_dict["parametric_pulses"] = []
     else:

qiskit/providers/basic_provider/basic_simulator.py

@@ -48,6 +48,7 @@
 from qiskit.qobj import QasmQobj, QasmQobjConfig, QasmQobjExperiment
 from qiskit.result import Result
 from qiskit.transpiler import Target
+from qiskit.utils.deprecation import deprecate_func
 
 from .basic_provider_job import BasicProviderJob
 from .basic_provider_tools import single_gate_matrix
@@ -212,6 +213,14 @@ def _build_basic_target(self) -> Target:
                 )
         return target
 
+    @deprecate_func(
+        since="1.3.0",
+        removal_timeline="in Qiskit 2.0.0",
+        additional_msg="The `BackendConfiguration` class is part of the deprecated `BackendV1` "
+        "workflow, and no longer necessary for `BackendV2`. The individual configuration elements "
+        "can be retrieved directly from the backend or from the contained `Target` instance "
+        "(`backend.target)`).",
+    )
     def configuration(self) -> BackendConfiguration:
         """Return the simulator backend configuration.
 
@@ -532,7 +541,8 @@ def run(
                     "initial_statevector": np.array([1, 0, 0, 1j]) / math.sqrt(2),
                 }
         """
-        # TODO: replace assemble with new run flow
+        # TODO: replace assemble with new run flow. If this is not achieved before 2.0,
+        # see removal note on `def _assemble`, L192 of qiskit/compiler/assembler.py
         from qiskit.compiler.assembler import _assemble
 
         out_options = {}

releasenotes/notes/deprecate-basic-simulator-configuration-9d782925196993e9.yaml

@@ -0,0 +1,32 @@
+---
+deprecations_providers:
+  - |
+    The :meth:`.BasicSimulator.configuration` method is deprecated and will be removed in 2.0.0. 
+    This method returned a legacy ``providers.models.BackendConfiguration`` instance which is part
+    of the deprecated ``BackendV1`` model. This model has been replaced with :class:`.BackendV2`, 
+    where the constraints are stored directly in the backend instance or the underlying :class:`.Target`
+    (``backend.target``).
+
+    Here is a quick guide for accessing the most common ``BackendConfiguration`` attributes in the 
+    :class:`BackendV2` model:""
+
+      BackendV1 model (deprecated) ------------> BackendV2 model
+      ----------------------------               ---------------
+      backend.configuration().backend_name       backend.name
+      backend.configuration().backend_version    backend.backend_version
+      backend.configuration().n_qubits           backend.num_qubits
+      backend.configuration().num_qubits         backend.num_qubits
+      backend.configuration().basis_gates        backend.target.operation_names (*)
+      backend.configuration().coupling_map       backend.target.build_coupling_map()
+      backend.configuration().local              No representation
+      backend.configuration().simulator          No representation
+      backend.configuration().conditional        No representation
+      backend.configuration().open_pulse         No representation     
+      backend.configuration().memory             No representation      
+      backend.configuration().max_shots          No representation      
+
+    (*) Note that ``backend.target.operation_names`` includes ``basis_gates`` and additional 
+    non-gate instructions, in some implementations it might be necessary to filter the output.
+
+    See `this guide <https://docs.quantum.ibm.com/api/qiskit/providers#migrating-from-backendv1-to-backendv2>`__
+    for more information on migrating to the ``BackendV2`` model.

test/python/providers/basic_provider/__init__.py

@@ -20,7 +20,9 @@ class BasicProviderBackendTestMixin:
 
     def test_configuration(self):
         """Test backend.configuration()."""
-        configuration = self.backend.configuration()
+        with self.assertWarns(DeprecationWarning):
+            # The method is deprecated
+            configuration = self.backend.configuration()
         return configuration
 
     def test_run_circuit(self):