Qiskit · mergify · Mar 4, 2021 · Nov 11, 2020 · Jan 12, 2021 · Jan 12, 2021
diff --git a/qiskit/tools/parallel.py b/qiskit/tools/parallel.py
@@ -50,21 +50,39 @@
 
 import os
 from concurrent.futures import ProcessPoolExecutor
+import sys
+
 from qiskit.exceptions import QiskitError
 from qiskit.utils.multiprocessing import local_hardware_info
 from qiskit.tools.events.pubsub import Publisher
 from qiskit import user_config
 
 CONFIG = user_config.get_config()
 
+if os.getenv('QISKIT_PARALLEL', None) is not None:
+    PARALLEL_DEFAULT = os.getenv('QISKIT_PARALLEL', None).lower() == 'true'
+else:
+    # Default False on Windows
+    if sys.platform == 'win32':
+        PARALLEL_DEFAULT = False
+    # On macOS default false on Python >=3.8
+    elif sys.platform == 'darwin':
+        if sys.version_info[0] == 3 and sys.version_info[1] >= 8:
+            PARALLEL_DEFAULT = False
+        else:
+            PARALLEL_DEFAULT = True
+    # On linux (and other OSes) default to True
+    else:
+        PARALLEL_DEFAULT = True
+
 # Set parallel flag
 if os.getenv('QISKIT_IN_PARALLEL') is None:
     os.environ['QISKIT_IN_PARALLEL'] = 'FALSE'
 
 if os.getenv("QISKIT_NUM_PROCS") is not None:
     CPU_COUNT = int(os.getenv('QISKIT_NUM_PROCS'))
 else:
-    CPU_COUNT = CONFIG.get('num_processes') or local_hardware_info()['cpus']
+    CPU_COUNT = CONFIG.get('num_process', local_hardware_info()['cpus'])
 
 
 def _task_wrapper(param):
@@ -118,7 +136,7 @@ def _callback(_):
 
     # Run in parallel if not Win and not in parallel already
     if num_processes > 1 and os.getenv('QISKIT_IN_PARALLEL') == 'FALSE' \
-            and CONFIG.get('parallel_enabled', user_config.PARALLEL_DEFAULT):
+            and CONFIG.get('parallel_enabled', PARALLEL_DEFAULT):
         os.environ['QISKIT_IN_PARALLEL'] = 'TRUE'
         try:
             results = []

diff --git a/qiskit/user_config.py b/qiskit/user_config.py
@@ -14,22 +14,13 @@
 
 import configparser
 import os
-import sys
 from warnings import warn
 
 from qiskit import exceptions
 
 DEFAULT_FILENAME = os.path.join(os.path.expanduser("~"),
                                 '.qiskit', 'settings.conf')
 
-if os.getenv('QISKIT_PARALLEL', None) is not None:
-    PARALLEL_DEFAULT = os.getenv('QISKIT_PARALLEL', None).lower() == 'true'
-else:
-    if sys.platform in {'darwin', 'win32'}:
-        PARALLEL_DEFAULT = False
-    else:
-        PARALLEL_DEFAULT = True
-
 
 class UserConfig:
     """Class representing a user config file
@@ -132,8 +123,9 @@ def read_config_file(self):
 
             # Parse parallel
             parallel_enabled = self.config_parser.getboolean(
-                'default', 'parallel', fallback=PARALLEL_DEFAULT)
-            self.settings['parallel_enabled'] = parallel_enabled
+                'default', 'parallel', fallback=None)
+            if parallel_enabled is not None:
+                self.settings['parallel_enabled'] = parallel_enabled
 
             # Parse num_processes
             num_processes = self.config_parser.getint(
@@ -158,7 +150,7 @@ def get_config():
     """
     filename = os.getenv('QISKIT_SETTINGS', DEFAULT_FILENAME)
     if not os.path.isfile(filename):
-        return {'parallel_enabled': PARALLEL_DEFAULT}
+        return {}
     user_config = UserConfig(filename)
     user_config.read_config_file()
     return user_config.settings
diff --git a/releasenotes/notes/paralle-config-options-97c1ad44ab76e052.yaml b/releasenotes/notes/paralle-config-options-97c1ad44ab76e052.yaml
@@ -24,12 +24,12 @@ features:
     config file.
 upgrade:
   - |
-    By default on macOS :func:`~qiskit.tools.parallel_map` will no longer run
-    in multiple processes. This is a change from previous releases where the
-    default behavior was that :func:`~qiskit.tools.parallel_map` would launch
-    multiple processes. This change was made because with newer versions of
-    macOS (especially with Python 3.8) multiprocessing is either unreliable
-    or adds significant overhead because of the change in Python 3.8 to launch
-    new processes with ``spawn`` instead of ``fork``. To re-enable parallel
-    execution on macOS you can use the user config file ``parallel` option or
+    By default on macOS with Python >=3.8 :func:`~qiskit.tools.parallel_map` will
+    no longer run in multiple processes. This is a change from previous releases
+    where the default behavior was that :func:`~qiskit.tools.parallel_map` would
+    launch multiple processes. This change was made because with newer versions of
+    macOS with Python 3.8 and 3.9 multiprocessing is either unreliable or adds
+    significant overhead because of the change in Python 3.8 to launch new
+    processes with ``spawn`` instead of ``fork``. To re-enable parallel execution on
+    macOS with Python >= 3.8 you can use the user config file ``parallel` option or
     set the environment variable ``QISKIT_PARALLEL`` to ``True``.
diff --git a/test/python/test_user_config.py b/test/python/test_user_config.py
@@ -68,8 +68,7 @@ def test_circuit_drawer_valid(self):
             file.flush()
             config = user_config.UserConfig(self.file_path)
             config.read_config_file()
-            self.assertEqual({'circuit_drawer': 'latex',
-                              'parallel_enabled': user_config.PARALLEL_DEFAULT},
+            self.assertEqual({'circuit_drawer': 'latex'},
                              config.settings)
 
     def test_optimization_level_valid(self):
@@ -84,8 +83,7 @@ def test_optimization_level_valid(self):
             config = user_config.UserConfig(self.file_path)
             config.read_config_file()
             self.assertEqual(
-                {'transpile_optimization_level': 1,
-                 'parallel_enabled': user_config.PARALLEL_DEFAULT},
+                {'transpile_optimization_level': 1},
                 config.settings)
 
     def test_invalid_num_processes(self):
@@ -113,8 +111,7 @@ def test_valid_num_processes(self):
             config = user_config.UserConfig(self.file_path)
             config.read_config_file()
             self.assertEqual(
-                {'parallel_enabled': user_config.PARALLEL_DEFAULT,
-                 'num_processes': 31},
+                {'num_processes': 31},
                 config.settings)
 
     def test_valid_parallel(self):