gh-138813: Fix mutable default kwargs={} in multiprocessing BaseProcess and DummyProc...

* gh-138813: Default `BaseProcess` `kwargs` to `None` (#138814)

Set `BaseProcess.__init__(..., kwargs=None)` and initialize `kwargs` with
`dict(kwargs) if kwargs else {}`. This avoids a shared mutable default and
matches threading.Thread behavior.

Co-authored-by: Dmitrii Chuprov <cheese@altlinux.org>
* DummyProcess kwargs=None (which threading.Thread accepts properly)

Co-authored-by: Gregory P. Smith <greg@krypto.org>

diff --git a/Lib/multiprocessing/dummy/__init__.py b/Lib/multiprocessing/dummy/__init__.py
index 6a1468609e347b3a0b9281e5c9e6ec311fcb37e5..7dc5d1c8dde8484e59728eb6dfa7a36c158dd2ef 100644 (file)
--- a/Lib/multiprocessing/dummy/__init__.py
+++ b/Lib/multiprocessing/dummy/__init__.py
@@ -33,7 +33,7 @@ from queue import Queue
 
 class DummyProcess(threading.Thread):
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None):
         threading.Thread.__init__(self, group, target, name, args, kwargs)
         self._pid = None
         self._children = weakref.WeakKeyDictionary()

diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index 9db322be1aa6d6346fde20878d60889bd9c4fd90..262513f295fde56f2a0ca763e15ab4fb43d68424 100644 (file)
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -77,7 +77,7 @@ class BaseProcess(object):
     def _Popen(self):
         raise NotImplementedError
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={},
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None,
                  *, daemon=None):
         assert group is None, 'group argument must be None for now'
         count = next(_process_counter)
@@ -89,7 +89,7 @@ class BaseProcess(object):
         self._closed = False
         self._target = target
         self._args = tuple(args)
-        self._kwargs = dict(kwargs)
+        self._kwargs = dict(kwargs) if kwargs else {}
         self._name = name or type(self).__name__ + '-' + \
                      ':'.join(str(i) for i in self._identity)
         if daemon is not None:

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index d9e572961152b342ca437304d6efdbac02acedf8..850744e47d0e0b8378917acf055b0b67ad5ab98a 100644 (file)
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -5278,6 +5278,23 @@ class TestInvalidHandle(unittest.TestCase):
                           multiprocessing.connection.Connection, -1)
 
 
+#
+# Regression tests for BaseProcess kwargs handling
+#
+
+class TestBaseProcessKwargs(unittest.TestCase):
+    def test_default_kwargs_not_shared_between_instances(self):
+        # Creating multiple Process instances without passing kwargs
+        # must create independent empty dicts (no shared state).
+        p1 = multiprocessing.Process(target=lambda: None)
+        p2 = multiprocessing.Process(target=lambda: None)
+        self.assertIsInstance(p1._kwargs, dict)
+        self.assertIsInstance(p2._kwargs, dict)
+        self.assertIsNot(p1._kwargs, p2._kwargs)
+        # Mutating one should not affect the other
+        p1._kwargs['x'] = 1
+        self.assertNotIn('x', p2._kwargs)
+
 
 @hashlib_helper.requires_hashdigest('sha256')
 class OtherTest(unittest.TestCase):

diff --git a/Misc/NEWS.d/next/Library/2025-09-17-08-32-43.gh-issue-138813.LHkHjX.rst b/Misc/NEWS.d/next/Library/2025-09-17-08-32-43.gh-issue-138813.LHkHjX.rst
new file mode 100644 (file)
index 0000000..97f4d76
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-09-17-08-32-43.gh-issue-138813.LHkHjX.rst
@@ -0,0 +1 @@
+:class:`!multiprocessing.BaseProcess` defaults ``kwargs`` to ``None`` instead of a shared dictionary.