Wrap qiskit.Aer and qiskit.IBMQ with lazy loading object

This commit migrates the qiskit.Aer and qiskit.IBMQ module attributes to
be lazy loading instances of lazy loading wrapper classes. The intent
here is to avoid importing from qiskit-aer or qiskit-ibmq-provider from
qiskit-terra, while this is safe while the packages share a shared
namespace we've been actively working to remove the use of namespace
packaging (see Qiskit#5089, Qiskit#4767, and Qiskit#559) and the circular
dependency caused by re-exporting these attributes is blocking progress
on this. By using a lazy loading wrapper class we avoid an import type
circular dependency and opportunistically use qiskit-aer and/or
qiskit-ibmq-provider at runtime only if they're present after everything
is imported. This also may have some benefit for the overall import
performance of qiskit (being tracked in Qiskit#5100) as it removes qiskit-aer
and qiskit-ibmq-provider from the import path unless they're being used.
Although the presence of qiskit.__qiskit_version__ might prevent any
performance improvements as it still imports all the elements to get
version information (and will be tackled in a separate PR).

Fixes Qiskit#5532

qiskit/__init__.py

@@ -15,7 +15,6 @@
 
 """Main Qiskit public functionality."""
 
-
 import pkgutil
 import sys
 import warnings
@@ -49,28 +48,6 @@
 
 _config = _user_config.get_config()
 
-# Try to import the Aer provider if installed.
-try:
-    from qiskit.providers.aer import Aer
-except ImportError:
-    suppress_warnings = os.environ.get('QISKIT_SUPPRESS_PACKAGING_WARNINGS', '')
-    if suppress_warnings.upper() != 'Y':
-        if not _config.get('suppress_packaging_warnings') or suppress_warnings.upper() == 'N':
-            warnings.warn('Could not import the Aer provider from the qiskit-aer '
-                          'package. Install qiskit-aer or check your installation.',
-                          RuntimeWarning)
-# Try to import the IBMQ provider if installed.
-try:
-    from qiskit.providers.ibmq import IBMQ
-except ImportError:
-    suppress_warnings = os.environ.get('QISKIT_SUPPRESS_PACKAGING_WARNINGS', '')
-    if suppress_warnings.upper() != 'Y':
-        if not _config.get('suppress_packaging_warnings') or suppress_warnings.upper() == 'N':
-            warnings.warn('Could not import the IBMQ provider from the '
-                          'qiskit-ibmq-provider package. Install '
-                          'qiskit-ibmq-provider or check your installation.',
-                          RuntimeWarning)
-
 # Moved to after IBMQ and Aer imports due to import issues
 # with other modules that check for IBMQ (tools)
 from qiskit.execute import execute  # noqa
@@ -88,3 +65,61 @@
         "Using Qiskit with Python 3.6 is deprecated as of the 0.17.0 release. "
         "Support for running Qiskit with Python 3.6 will be removed in a "
         "future release.", DeprecationWarning)
+
+
+class AerWrapper:
+
+    def __init__(self):
+        self.aer = None
+
+    def __bool__(self):
+        if self.aer is None:
+            try:
+                from qiskit.providers.aer import Aer
+                self.aer = Aer
+            except ImportError:
+                return False
+        return True
+
+    def __getattr__(self, attr):
+        if not self.aer:
+            try:
+                from qiskit.providers.aer import Aer
+                self.aer = Aer
+            except ImportError:
+                raise ImportError('Could not import the Aer provider from the '
+                                  'qiskit-aer package. Install qiskit-aer or '
+                                  'check your installation.')
+        return getattr(self.aer, attr)
+
+
+
+class IBMQWrapper:
+
+    def __init__(self):
+        self.ibmq = None
+
+    def __bool__(self):
+        if self.ibmq is None:
+            try:
+                from qiskit.providers.ibmq import IBMQ
+                self.ibmq = IBMQ
+            except ImportError:
+                return False
+        return True
+
+    def __getattr__(self, attr):
+        if not self.ibmq:
+            try:
+                from qiskit.providers.ibmq import IBMQ
+                self.ibmq = IBMQ
+            except ImportError:
+                raise ImportError('Could not import the IBMQ provider from the '
+                                  'qiskit-ibmq-provider package. Install '
+                                  'qiskit-ibmq-provider or check your  '
+                                  'installation.')
+        return getattr(self.ibmq, attr)
+
+
+Aer = AerWrapper()
+IBMQ = IBMQWrapper()

qiskit/test/mock/fake_backend.py

@@ -23,7 +23,7 @@
 from qiskit.exceptions import QiskitError
 
 try:
-    from qiskit import Aer
+    from qiskit.providers.aer import Aer
     HAS_AER = True
 except ImportError:
     HAS_AER = False

qiskit/user_config.py

@@ -41,7 +41,6 @@ class UserConfig:
     circuit_mpl_style = default
     circuit_mpl_style_path = ~/.qiskit:<default location>
     transpile_optimization_level = 1
-    suppress_packaging_warnings = False
     parallel = False
     num_processes = 4
 
@@ -116,12 +115,6 @@ def read_config_file(self):
                 self.settings['transpile_optimization_level'] = (
                     transpile_optimization_level)
 
-            # Parse package warnings
-            package_warnings = self.config_parser.getboolean(
-                'default', 'suppress_packaging_warnings', fallback=False)
-            if package_warnings:
-                self.settings['suppress_packaging_warnings'] = package_warnings
-
             # Parse parallel
             parallel_enabled = self.config_parser.getboolean(
                 'default', 'parallel', fallback=PARALLEL_DEFAULT)

releasenotes/notes/lazy-load-provider-factory-bdfb3925a2514ef7.yaml

@@ -0,0 +1,22 @@
+---
+upgrade:
+  - |
+    The ``qiskit.Aer`` and ``qiskit.IBMQ`` top level attributes are now lazy
+    loaded. This means that the objects will always now exist and warnings will
+    no longer be raised on import if ``qiskit-aer`` or ``qiskit-ibmq-provider``
+    are not installed (or can't be found by Python). If you were checking for
+    the presence of ``qiskit-aer`` or ``qiskit-ibmq-provider`` using these
+    module attributes and explicitly comparing to ``None`` or looking for the
+    absence of the attribute this no longer will work because they are always
+    defined as an object now.  You can either explicitly use
+    ``qiskit.providers.aer.Aer`` and ``qiskit.providers.ibmq.IBMQ`` or
+    check ``bool(qiskit.Aer)`` and ``bool(qiskit.IBMQ)`` instead to determine
+    if those providers are present. This change was necessary to avoid potential
+    import cycle issues between the qiskit packages and also to improve the
+    import time when Aer or IBMQ is not being used.
+  - |
+    The user config file option ``suppress_packaging_warnings`` option in the
+    user config file and the ``QISKIT_SUPPRESS_PACKAGING_WARNINGS`` environment
+    variable no longer has any effect and will be silently ignored. These
+    warnings have been removed and will no longer be emitted at import time
+    from the qiskit module.

test/python/algorithms/test_vqe.py

@@ -167,7 +167,7 @@ def test_with_aer_statevector(self):
         """Test VQE with Aer's statevector_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -190,7 +190,7 @@ def test_with_aer_qasm(self):
         """Test VQE with Aer's qasm_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -215,7 +215,7 @@ def test_with_aer_qasm_snapshot_mode(self):
         """Test the VQE using Aer's qasm_simulator snapshot mode."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -308,7 +308,7 @@ def test_vqe_expectation_select(self):
         """Test expectation selection with Aer's qasm_simulator."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return

test/python/opflow/legacy/test_weighted_pauli_operator.py

@@ -498,7 +498,7 @@ def test_evaluate_with_aer_mode(self):
         """ evaluate with aer mode test """
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return

test/python/opflow/test_state_op_meas_evals.py

@@ -63,7 +63,7 @@ def test_coefficients_correctly_propagated(self):
         """Test that the coefficients in SummedOp and states are correctly used."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -89,7 +89,7 @@ def test_is_measurement_correctly_propagated(self):
         """Test if is_measurement property of StateFn is propagated to converted StateFn."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return
@@ -103,7 +103,7 @@ def test_parameter_binding_on_listop(self):
         """Test passing a ListOp with differing parameters works with the circuit sampler."""
         try:
             # pylint: disable=import-outside-toplevel
-            from qiskit import Aer
+            from qiskit.providers.aer import Aer
         except Exception as ex:  # pylint: disable=broad-except
             self.skipTest("Aer doesn't appear to be installed. Error: '{}'".format(str(ex)))
             return

test/python/test_user_config.py

@@ -31,18 +31,6 @@ def test_empty_file_read(self):
         self.assertEqual({},
                          config.settings)
 
-    def test_invalid_suppress_packaging_warnings(self):
-        test_config = """
-        [default]
-        suppress_packaging_warnings = 76
-        """
-        self.addCleanup(os.remove, self.file_path)
-        with open(self.file_path, 'w') as file:
-            file.write(test_config)
-            file.flush()
-            config = user_config.UserConfig(self.file_path)
-            self.assertRaises(ValueError, config.read_config_file)
-
     def test_invalid_optimization_level(self):
         test_config = """
         [default]
@@ -100,37 +88,6 @@ def test_optimization_level_valid(self):
                  'parallel_enabled': user_config.PARALLEL_DEFAULT},
                 config.settings)
 
-    def test_valid_suppress_packaging_warnings_false(self):
-        test_config = """
-        [default]
-        suppress_packaging_warnings = false
-        """
-        self.addCleanup(os.remove, self.file_path)
-        with open(self.file_path, 'w') as file:
-            file.write(test_config)
-            file.flush()
-            config = user_config.UserConfig(self.file_path)
-            config.read_config_file()
-            self.assertEqual(
-                {'parallel_enabled': user_config.PARALLEL_DEFAULT},
-                config.settings)
-
-    def test_valid_suppress_packaging_warnings_true(self):
-        test_config = """
-        [default]
-        suppress_packaging_warnings = true
-        """
-        self.addCleanup(os.remove, self.file_path)
-        with open(self.file_path, 'w') as file:
-            file.write(test_config)
-            file.flush()
-            config = user_config.UserConfig(self.file_path)
-            config.read_config_file()
-            self.assertEqual(
-                {'parallel_enabled': user_config.PARALLEL_DEFAULT,
-                 'suppress_packaging_warnings': True},
-                config.settings)
-
     def test_invalid_num_processes(self):
         test_config = """
         [default]
@@ -197,6 +154,5 @@ def test_all_options_valid(self):
                               'circuit_mpl_style_path': ['~', '~/.qiskit'],
                               'transpile_optimization_level': 3,
                               'num_processes': 15,
-                              'parallel_enabled': False,
-                              'suppress_packaging_warnings': True},
+                              'parallel_enabled': False},
                              config.settings)