Add namespace redirect hook for qiskit-aer

qiskit/__init__.py

@@ -31,6 +31,15 @@
 sys.modules["qiskit._accelerate.results"] = qiskit._accelerate.results
 
 
+# Extend namespace for backwards compat
+from qiskit import namespace
+
+# Add hook to redirect imports from qiskit.providers.aer* to qiskit_aer*
+# this is necessary for backwards compatibility for users when qiskit-aer
+# and qiskit-terra shared the qiskit namespace
+new_meta_path_finder = namespace.QiskitElementImport("qiskit.providers.aer", "qiskit_aer")
+sys.meta_path = [new_meta_path_finder] + sys.meta_path
+
 # qiskit errors operator
 from qiskit.exceptions import QiskitError, MissingOptionalLibraryError
 
@@ -52,6 +61,9 @@
 # Allow extending this namespace. Please note that currently this line needs
 # to be placed *before* the wrapper imports or any non-import code AND *before*
 # importing the package you want to allow extensions for (in this case `backends`).
+
+# TODO: Remove when we drop support for importing qiskit-aer < 0.11.0 and the
+# qiskit-ibmq-provider package is retired/archived.
 __path__ = pkgutil.extend_path(__path__, __name__)
 
 # Please note these are global instances, not modules.
@@ -61,11 +73,11 @@
 
 # Moved to after IBMQ and Aer imports due to import issues
 # with other modules that check for IBMQ (tools)
-from qiskit.execute_function import execute  # noqa
-from qiskit.compiler import transpile, assemble, schedule, sequence  # noqa
+from qiskit.execute_function import execute
+from qiskit.compiler import transpile, assemble, schedule, sequence
 
-from .version import __version__  # noqa
-from .version import QiskitVersion  # noqa
+from .version import __version__
+from .version import QiskitVersion
 
 
 __qiskit_version__ = QiskitVersion()
@@ -83,6 +95,13 @@ def __bool__(self):
                 from qiskit.providers import aer
 
                 self.aer = aer.Aer
+                warnings.warn(
+                    "The qiskit.Aer entry point will be deprecated in a future release and "
+                    "subsequently removed. Instead you should use this "
+                    "directly from the root of the qiskit-aer package.",
+                    PendingDeprecationWarning,
+                    stacklevel=2,
+                )
             except ImportError:
                 return False
         return True
@@ -93,6 +112,13 @@ def __getattr__(self, attr):
                 from qiskit.providers import aer
 
                 self.aer = aer.Aer
+                warnings.warn(
+                    "The qiskit.Aer entry point will be deprecated in a future release and "
+                    "subsequently removed. Instead you should use this "
+                    "directly from the root of the qiskit-aer package.",
+                    PendingDeprecationWarning,
+                    stacklevel=2,
+                )
             except ImportError as ex:
                 raise MissingOptionalLibraryError(
                     "qiskit-aer", "Aer provider", "pip install qiskit-aer"

qiskit/namespace.py

@@ -0,0 +1,69 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2020.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+# pylint: disable=unused-argument
+
+"""Module for utilities to manually construct qiskit namespace"""
+
+import sys
+from importlib.abc import MetaPathFinder, Loader
+import importlib
+
+
+def _new_namespace(fullname, old_namespace, new_package):
+    names = fullname.split(".")
+    new_namespace_names = new_package.split(".")
+    old_namespace_names = old_namespace.split(".")
+    fullname = ".".join(new_namespace_names + names[len(old_namespace_names) :])
+    return fullname
+
+
+class QiskitLoader(Loader):
+    """Load qiskit element as a namespace package."""
+
+    def __init__(self, new_package, old_namespace):
+        super().__init__()
+        self.new_package = new_package
+        self.old_namespace = old_namespace
+
+    def module_repr(self, module):
+        return repr(module)
+
+    def load_module(self, fullname):
+        old_name = fullname
+        fullname = _new_namespace(fullname, self.old_namespace, self.new_package)
+        module = importlib.import_module(fullname)
+        sys.modules[old_name] = module
+        return module
+
+
+class QiskitElementImport(MetaPathFinder):
+    """Meta importer to enable unified qiskit namespace."""
+
+    def __init__(self, old_namespace, new_package):
+        super().__init__()
+        self.old_namespace = old_namespace
+        self.new_package = new_package
+
+    def find_spec(self, fullname, path=None, target=None):
+        """Return the ModuleSpec for Qiskit element."""
+        if fullname.startswith(self.old_namespace):
+            try:
+                importlib.import_module(
+                    _new_namespace(fullname, self.old_namespace, self.new_package)
+                )
+                return importlib.util.spec_from_loader(
+                    fullname, QiskitLoader(self.new_package, self.old_namespace), origin="qiskit"
+                )
+            except ModuleNotFoundError:
+                return None
+        return None

qiskit/providers/__init__.py

@@ -692,4 +692,6 @@ def status(self):
 
 
 # Allow extending this namespace.
+# TODO: Remove when we drop support for importing qiskit-aer < 0.11.0 and the
+# qiskit-ibmq-provider package is retired/archived.
 __path__ = pkgutil.extend_path(__path__, __name__)

qiskit/version.py

@@ -104,6 +104,8 @@ def _load_versions(self):
         import pkg_resources
 
         try:
+            # TODO: Update to use qiskit_aer instead when we remove the
+            # namespace redirect
             from qiskit.providers import aer
 
             self._version_dict["qiskit-aer"] = aer.__version__