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gh-109070: multiprocessing.get_context will not set the start method globally #135678

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gh-109070: multiprocessing.get_context will not set the start method globally #135678

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4 changes: 1 addition & 3 deletions Lib/multiprocessing/context.py
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Original file line number Diff line number Diff line change
Expand Up @@ -236,9 +236,7 @@ def __init__(self, context):

def get_context(self, method=None):
if method is None:
if self._actual_context is None:
self._actual_context = self._default_context
return self._actual_context
return self._actual_context or self._default_context
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I wish I could say "or perhaps even just return self._default_context?" given the documentation for .get_context() today. BUT code relies on .get_context() to establish the actual set context and thus prevent it from being changed without force=True so that different contexts are not used between API surfaces at the default module level (Lib/multiprocessing/__init__.py sets module level attributes to be the multiprocessing.context._default_context instance attributes)

So I think we really are best off just documenting the existing behavior. As is already done for get_start_method() in the docs right below get_context() - https://docs.python.org/3/library/multiprocessing.html
Examples: All of the things defined in BaseContext that when used have a ctx=self.get_context() call in them. Once something is used with the default context we don't want to allow a set_start_method call to change it.

else:
return super().get_context(method)

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6 changes: 6 additions & 0 deletions Lib/test/_test_multiprocessing.py
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Expand Up @@ -5811,6 +5811,12 @@ def check_context(self, ctx):
p.join()
self.assertEqual(child_method, ctx.get_start_method())

def test_default_context(self):
# Get_context should not have side effect, see gh-109070.
multiprocessing.set_start_method(None, force=True)
multiprocessing.get_context()
self.assertIsNone(multiprocessing.context._default_context._actual_context)

def test_context(self):
for method in ('fork', 'spawn', 'forkserver'):
try:
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2025-06-18-23-59-40.gh-issue-109070.2XNJ7X.rst
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@@ -0,0 +1 @@
:func:`multiprocessing.get_context` will not set the start method globally.
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