multiprocessing.get_context will not set the start method globally

Author: aisk

Lib/multiprocessing/context.py

@@ -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
         else:
             return super().get_context(method)
 

Lib/test/_test_multiprocessing.py

@@ -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:

Misc/NEWS.d/next/Library/2025-06-18-23-59-40.gh-issue-109070.2XNJ7X.rst

@@ -0,0 +1 @@
+:func:`multiprocessing.get_context` will not set the start method globally.