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gh-157216: Clarify that set_forkserver_preload() does not cover lazy imports #157229

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gh-157216: Clarify that set_forkserver_preload() does not cover lazy imports #157229

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8 changes: 8 additions & 0 deletions Doc/library/multiprocessing.rst
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Expand Up @@ -1245,6 +1245,14 @@ Miscellaneous
For this to work, it must be called before the forkserver process has been
launched (before creating a :class:`Pool` or starting a :class:`Process`).

Only modules directly imported at top level by *module_names* are preloaded.
If a preloaded module lazily imports dependencies inside functions or methods
(for example, :meth:`datetime.datetime.strptime` importing ``_strptime`` on
its first invocation), those dependencies are not loaded into the forkserver
process and will instead be imported separately in each child process upon
first use. To inherit those as well, call such functions at module level
within a preloaded module.

The *on_error* parameter controls how :exc:`ImportError` exceptions during
module preloading are handled: ``"ignore"`` (default) silently ignores
failures, ``"warn"`` causes the forkserver subprocess to emit an
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