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 <[email protected]>

Author: zeff-ir

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 @@ def __init__(self, group=None, target=None, name=None, args=(), kwargs={},
         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: