Deprecate legacy provider interface

qiskit/providers/__init__.py

@@ -499,6 +499,11 @@ def status(self):
 Legacy Provider Interface Base Objects (:mod:`qiskit.providers`)
 ================================================================
 
+These abstract interfaces are deprecated and will be removed in a future
+release. The documentation here is left for reference purposes while they're
+still supported, but if you're creating or maintaining a provider you should
+be using the versioned interface.
+
 .. currentmodule:: qiskit.providers
 
 Base Objects

qiskit/providers/basebackend.py

@@ -17,6 +17,7 @@
 """
 
 from abc import ABC, abstractmethod
+import warnings
 
 from qiskit.version import VERSION as __version__
 from .models import BackendStatus
@@ -27,7 +28,7 @@ class BaseBackend(ABC):
 
     @abstractmethod
     def __init__(self, configuration, provider=None):
-        """Base class for backends.
+        """DEPRECATED Legacy base class for backends.
 
         This method should initialize the module and its configuration, and
         raise an exception if a component of the module is
@@ -40,6 +41,15 @@ def __init__(self, configuration, provider=None):
         Raises:
             QiskitError: if an error occurred when instantiating the backend.
         """
+        warnings.warn(
+            "The BaseBackend abstract interface is deprecated as of "
+            "the 0.18.0 release and will be removed in a future "
+            "release. Instead you should build your backends using "
+            "the BackendV1 abstract class (which is the current "
+            "latest version of the backend interface)",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self._configuration = configuration
         self._provider = provider
 

qiskit/providers/basejob.py

@@ -19,14 +19,15 @@
 from abc import ABC, abstractmethod
 from typing import Callable, Optional
 import time
+import warnings
 
 from .jobstatus import JobStatus, JOB_FINAL_STATES
 from .exceptions import JobTimeoutError
 from .basebackend import BaseBackend
 
 
 class BaseJob(ABC):
-    """Legacy Class to handle asynchronous jobs"""
+    """DEPRECATED Legacy Class to handle asynchronous jobs"""
 
     def __init__(self, backend: BaseBackend, job_id: str) -> None:
         """Initializes the asynchronous job.
@@ -36,6 +37,15 @@ def __init__(self, backend: BaseBackend, job_id: str) -> None:
             job_id: a unique id in the context of the backend used to run
                 the job.
         """
+        warnings.warn(
+            "The BaseJob abstract interface is deprecated as of "
+            "the 0.18.0 release and will be removed in a future "
+            "release. Instead you should build your backends using "
+            "the JobV1 abstract class (which is the current "
+            "latest version of the backend interface)",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self._job_id = job_id
         self._backend = backend
 

qiskit/providers/baseprovider.py

@@ -13,14 +13,24 @@
 """Base class for a backend provider."""
 
 from abc import ABC, abstractmethod
+import warnings
 
 from .exceptions import QiskitBackendNotFoundError
 
 
 class BaseProvider(ABC):
     """Base class for a Backend Provider."""
 
-    def __init__(self, *args, **kwargs):
+    def __init__(self, *args, **kwargs):  # pylint: disable=unused-argument
+        warnings.warn(
+            "The BaseProvider abstract interface is deprecated as of "
+            "the 0.18.0 release and will be removed in a future "
+            "release. Instead you should build your backends using "
+            "the ProviderV1 abstract class (which is the current "
+            "latest version of the provider interface).",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         pass
 
     def get_backend(self, name=None, **kwargs):

releasenotes/notes/deprecate-legacy-providers-383bf4142153297a.yaml

@@ -0,0 +1,14 @@
+---
+deprecations:
+  - |
+    The legacy providers interface, which consisted of the
+    :class:`qiskit.providers.BaseBackend`, :class:`qiskit.providers.BaseJob`,
+    and :class:`qiskit.providers.BaseProvider` abstract classes, has been
+    deprecated and will be removed in a future release. Instead you should use
+    the versioned interface, which the current abstract class versions are
+    :class:`qiskit.providers.BackendV1`, :class:`qiskit.providers.JobV1`, and
+    :class:`qiskit.providers.ProvidersV1`. The V1 objects are mostly backwards
+    compatible to ease migration from the legacy interface to the versioned
+    one. However, expect future versions of the abstract interfaces to diverge
+    more. You can refer to the :mod:`qiskit.providers` documentation for
+    more high level details about the versioned interface.

test/python/providers/test_fake_backends.py

@@ -78,14 +78,15 @@ def test_circuit_on_fake_legacy_backend(self, backend, optimization_level):
                 "Unable to run fake_backend %s without qiskit-aer"
                 % backend.configuration().backend_name
             )
-        job = execute(
-            self.circuit,
-            backend,
-            optimization_level=optimization_level,
-            seed_simulator=42,
-            seed_transpiler=42,
-        )
-        result = job.result()
+        with self.assertWarns(DeprecationWarning):
+            job = execute(
+                self.circuit,
+                backend,
+                optimization_level=optimization_level,
+                seed_simulator=42,
+                seed_transpiler=42,
+            )
+            result = job.result()
         counts = result.get_counts()
         max_count = max(counts.items(), key=operator.itemgetter(1))[0]
         self.assertEqual(max_count, "11")