make loky a default on Windows and MacOS but not on Linux

Author: basnijholt

adaptive/runner.py

@@ -5,11 +5,14 @@
 import inspect
 import itertools
 import pickle
+import platform
 import time
 import traceback
 import warnings
 from contextlib import suppress
 
+import loky
+
 from adaptive.notebook_integration import in_ipynb, live_info, live_plot
 
 try:
@@ -33,22 +36,23 @@
 except ModuleNotFoundError:
     with_mpi4py = False
 
-try:
-    import loky
-
-    with_loky = True
-except ModuleNotFoundError:
-    with_loky = False
 
 with suppress(ModuleNotFoundError):
     import uvloop
 
     asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
 
 
-_default_executor = (
-    loky.get_reusable_executor if with_loky else concurrent.ProcessPoolExecutor
-)
+if platform.system() == "Linux":
+    _default_executor = concurrent.ProcessPoolExecutor
+else:
+    # On Windows and MacOS functions, the __main__ module must be
+    # importable by worker subprocesses. This means that
+    # ProcessPoolExecutor will not work in the interactive interpreter.
+    # On Linux the whole environment is forked, so the issue does
+    # not appear.
+    # See https://docs.python.org/3/library/concurrent.futures.html#processpoolexecutor
+    _default_executor = loky.get_reusable_executor
 
 
 class BaseRunner(metaclass=abc.ABCMeta):
@@ -814,7 +818,7 @@ def _get_ncores(ex):
         ex, (concurrent.ProcessPoolExecutor, concurrent.ThreadPoolExecutor)
     ):
         return ex._max_workers  # not public API!
-    elif with_loky and isinstance(ex, loky.reusable_executor._ReusablePoolExecutor):
+    elif isinstance(ex, loky.reusable_executor._ReusablePoolExecutor):
         return ex._max_workers  # not public API!
     elif isinstance(ex, SequentialExecutor):
         return 1

adaptive/tests/test_runner.py

@@ -15,7 +15,6 @@
     stop_after,
     with_distributed,
     with_ipyparallel,
-    with_loky,
 )
 
 
@@ -139,7 +138,6 @@ def test_distributed_executor():
     assert learner.npoints > 0
 
 
-@pytest.mark.skipif(not with_loky, reason="loky not installed")
 def test_loky_executor(loky_executor):
     learner = Learner1D(lambda x: x, (-1, 1))
     BlockingRunner(

setup.py

@@ -30,6 +30,7 @@ def get_version_and_cmdclass(package_name):
     "sortedcontainers >= 2.0",
     "atomicwrites",
     "cloudpickle",
+    "loky >= 2.9",
 ]
 
 extras_require = {
@@ -55,7 +56,6 @@ def get_version_and_cmdclass(package_name):
         "dill",
         "distributed",
         "ipyparallel>=6.2.5",  # because of https://github.com/ipython/ipyparallel/issues/404
-        "loky",
         "scikit-optimize>=0.8.1",  # because of https://github.com/scikit-optimize/scikit-optimize/issues/931
         "wexpect" if os.name == "nt" else "pexpect",
     ],