gh-159964: Propagate contextvars to ThreadPoolExecutor tasks

Doc/library/concurrent.futures.rst

@@ -190,6 +190,11 @@
    pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
    as well as any attempt to submit more jobs to the pool.
 
+   Each callable submitted with :meth:`submit` runs in a copy of the
+   :mod:`contextvars` context of the thread that submits it. Changes to
+   context-variable bindings made by a task do not affect the submitting
+   thread's context or the context used by later tasks.
+
    .. versionchanged:: 3.5
       If *max_workers* is ``None`` or
       not given, it will default to the number of processors on the machine,
@@ -219,6 +224,9 @@
       Default value of *max_workers* is changed to
       ``min(32, (os.process_cpu_count() or 1) + 4)``.
 
+   .. versionchanged:: 3.16
+      Tasks run in a copy of the submitting thread's context.
+
 
 .. _threadpoolexecutor-example:
 

Lib/concurrent/futures/thread.py

@@ -6,6 +6,7 @@
 __author__ = 'Brian Quinlan (brian@sweetapp.com)'
 
 from concurrent.futures import _base
+import contextvars
 import itertools
 import queue
 import threading
@@ -54,7 +55,7 @@ def prepare(cls, initializer, initargs):
         def create_context():
             return cls(initializer, initargs)
         def resolve_task(fn, args, kwargs):
-            return (fn, args, kwargs)
+            return (contextvars.copy_context(), fn, args, kwargs)
         return create_context, resolve_task
 
     def __init__(self, initializer, initargs):
@@ -69,8 +70,8 @@ def finalize(self):
         pass
 
     def run(self, task):
-        fn, args, kwargs = task
-        return fn(*args, **kwargs)
+        context, fn, args, kwargs = task
+        return context.run(fn, *args, **kwargs)
 
 
 class _WorkItem:

Lib/test/test_concurrent_futures/test_thread_pool.py

@@ -1,4 +1,5 @@
 import contextlib
+import contextvars
 import multiprocessing as mp
 import multiprocessing.process
 import multiprocessing.util
@@ -14,6 +15,34 @@
 
 
 class ThreadPoolExecutorTest(ThreadPoolMixin, ExecutorTest, BaseTestCase):
+    def test_contextvars_are_copied_for_each_task(self):
+        value = contextvars.ContextVar("value", default="default")
+
+        def read_value():
+            return value.get()
+
+        def change_value():
+            value.set("worker")
+
+        with self.executor_type(max_workers=1) as executor:
+            token = value.set("first")
+            try:
+                self.assertEqual(executor.submit(read_value).result(), "first")
+            finally:
+                value.reset(token)
+
+            self.assertEqual(executor.submit(read_value).result(), "default")
+
+            token = value.set("third")
+            try:
+                self.assertEqual(executor.submit(read_value).result(), "third")
+            finally:
+                value.reset(token)
+
+            # Changes made by one task must not leak into the next task.
+            executor.submit(change_value).result()
+            self.assertEqual(executor.submit(read_value).result(), "default")
+
     def test_map_submits_without_iteration(self):
         """Tests verifying issue 11777."""
         finished = []

Misc/NEWS.d/next/Library/2026-10-21-07-31-00.gh-issue-159964.K7mP2x.rst

@@ -0,0 +1 @@
+Fix :class:`concurrent.futures.ThreadPoolExecutor` to run each submitted callable in a copy of the submitting thread's :mod:`contextvars` context.