Fix multiprocess del guard

src/orion/executor/dask_backend.py

@@ -121,7 +121,10 @@ def submit(self, function, *args, **kwargs):
             raise
 
     def __del__(self):
-        self.client.close()
+        # This is necessary because if the factory constructor fails
+        # __del__ is executed right away but client might not be set
+        if hasattr(self, "client"):
+            self.client.close()
 
     def __enter__(self):
         return self

src/orion/executor/multiprocess_backend.py

@@ -165,18 +165,25 @@ class PoolExecutor(BaseExecutor):
         loky=Pool,  # TODO: For compatibility with joblib backend. Remove in v0.4.0.
     )
 
-    def __init__(self, n_workers, backend="multiprocess", **kwargs):
-        self.pool = PoolExecutor.BACKENDS.get(backend, ThreadPool)(n_workers)
+    def __init__(self, n_workers=-1, backend="multiprocess", **kwargs):
         super().__init__(n_workers, **kwargs)
 
+        if n_workers <= 0:
+            n_workers = multiprocessing.cpu_count()
+
+        self.pool = PoolExecutor.BACKENDS.get(backend, ThreadPool)(n_workers)
+
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
         self.pool.shutdown()
 
     def __del__(self):
-        self.pool.shutdown()
+        # This is necessary because if the factory constructor fails
+        # __del__ is executed right away but pool might not be set
+        if hasattr(self, "pool"):
+            self.pool.shutdown()
 
     def __getstate__(self):
         state = super(PoolExecutor, self).__getstate__()

src/orion/executor/single_backend.py

@@ -81,7 +81,8 @@ def __init__(self, n_workers=1, **config):
         self.nested = 0
 
     def __del__(self):
-        self.close()
+        if hasattr(self, "closed"):
+            self.close()
 
     def __enter__(self):
         self.nested += 1

tests/unittests/executor/test_executor.py

@@ -4,7 +4,7 @@
 
 import pytest
 
-from orion.executor.base import AsyncException, ExecutorClosed
+from orion.executor.base import AsyncException, ExecutorClosed, executor_factory
 from orion.executor.dask_backend import Dask
 from orion.executor.multiprocess_backend import PoolExecutor
 from orion.executor.single_backend import SingleExecutor
@@ -18,6 +18,8 @@ def thread(n):
     return PoolExecutor(n, "threading")
 
 
+executors = ["joblib", "poolexecutor", "dask", "singleexecutor"]
+
 backends = [thread, multiprocess, Dask, SingleExecutor]
 
 
@@ -188,3 +190,19 @@ def test_multisubprocess(backend):
             # access the results to make sure no exception is being
             # suppressed
             r.value
+
+
+@pytest.mark.parametrize("executor", executors)
+def test_executors_have_default_args(executor):
+
+    with executor_factory.create(executor):
+        pass
+
+
+@pytest.mark.parametrize("backend", backends)
+def test_executors_del_does_not_raise(backend):
+    # if executor init fails you can get very weird error messages,
+    # because of the deleter trying to close unallocated resources.
+
+    klass = type(backend(1))
+    klass.__del__(object())