gh-135726: multiprocessing.freeze_support no longer sets the start method globally.

Lib/multiprocessing/context.py

@@ -145,9 +145,18 @@ def freeze_support(self):
         '''Check whether this is a fake forked process in a frozen executable.
         If so then run code specified by commandline and exit.
         '''
-        if self.get_start_method() == 'spawn' and getattr(sys, 'frozen', False):
-            from .spawn import freeze_support
-            freeze_support()
+        if not getattr(sys, 'frozen', False):
+            return
+
+        # GH-135726: allow_none=True prevents this "get" call from setting the
+        # default context as the selected one as a side-effect.
+        method = (self.get_start_method(allow_none=True)
+                  or _default_context.get_start_method(allow_none=True))
+        if method != "spawn":
+            return
+
+        from .spawn import freeze_support
+        freeze_support()
 
     def get_logger(self):
         '''Return package logger -- if it does not already exist then

Lib/test/_test_multiprocessing.py

@@ -5932,6 +5932,13 @@ def test_context(self):
             self.assertRaises(ValueError, ctx.set_start_method, None)
             self.check_context(ctx)
 
+    def test_freeze_support_side_effect(self):
+        # GH-135726: freeze_support() should not set the start method
+        # as a side-feect,
+        multiprocessing.set_start_method(None, force=True)
+        multiprocessing.freeze_support()
+        self.assertIsNone(multiprocessing.get_start_method(allow_none=True))
+
     def test_context_check_module_types(self):
         try:
             ctx = multiprocessing.get_context('forkserver')

Misc/NEWS.d/next/Library/2025-06-22-15-42-10.gh-issue-135726._oO5IN.rst

@@ -0,0 +1,2 @@
+:func:`multiprocessing.freeze_support` no longer sets the start method
+globally.