bpo-38501: Add a warning section to multiprocessing.Pool docs about resource managing (GH-19466)
diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 492f94c..ec9521f 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -439,7 +439,8 @@
>>> def f(x):
... return x*x
...
- >>> p.map(f, [1,2,3])
+ >>> with p:
+ ... p.map(f, [1,2,3])
Process PoolWorker-1:
Process PoolWorker-2:
Process PoolWorker-3:
@@ -2127,6 +2128,16 @@
Note that the methods of the pool object should only be called by
the process which created the pool.
+ .. warning::
+ :class:`multiprocessing.pool` objects have internal resources that need to be
+ properly managed (like any other resource) by using the pool as a context manager
+ or by calling :meth:`close` and :meth:`terminate` manually. Failure to do this
+ can lead to the process hanging on finalization.
+
+ Note that is **not correct** to rely on the garbage colletor to destroy the pool
+ as CPython does not assure that the finalizer of the pool will be called
+ (see :meth:`object.__del__` for more information).
+
.. versionadded:: 3.2
*maxtasksperchild*