Add 'context' parameter to Thread.

By default, inherit the context from the thread calling
`Thread.start()`.

Doc/library/threading.rst

@@ -334,7 +334,7 @@ since it is impossible to detect the termination of alien threads.
 
 
 .. class:: Thread(group=None, target=None, name=None, args=(), kwargs={}, *, \
-                  daemon=None)
+                  daemon=None, context="inherit")
 
    This constructor should always be called with keyword arguments.  Arguments
    are:
@@ -359,6 +359,10 @@ since it is impossible to detect the termination of alien threads.
    If ``None`` (the default), the daemonic property is inherited from the
    current thread.
 
+   *context* is the `contextvars.Context` value to use while running the thread.
+   The default is to inherit the context of the caller of :meth:`~Thread.start`.
+   If set to ``None``, the context will be empty.
+
    If the subclass overrides the constructor, it must make sure to invoke the
    base class constructor (``Thread.__init__()``) before doing anything else to
    the thread.
@@ -369,6 +373,10 @@ since it is impossible to detect the termination of alien threads.
    .. versionchanged:: 3.10
       Use the *target* name if *name* argument is omitted.
 
+   .. versionchanged:: 3.14
+      Added the *context* parameter.  Previously threads always ran with an empty
+      context.
+
    .. method:: start()
 
       Start the thread's activity.

Lib/test/test_context.py

@@ -383,6 +383,49 @@ def sub(num):
             tp.shutdown()
         self.assertEqual(results, list(range(10)))
 
+    @isolated_context
+    @threading_helper.requires_working_threading()
+    def test_context_thread_inherit(self):
+        import threading
+
+        cvar = contextvars.ContextVar('cvar')
+
+        # By default, the context of the caller is inheritied
+        def run_inherit():
+            self.assertEqual(cvar.get(), 1)
+
+        cvar.set(1)
+        thread = threading.Thread(target=run_inherit)
+        thread.start()
+        thread.join()
+
+        # If context=None is passed, the thread has an empty context
+        def run_empty():
+            with self.assertRaises(LookupError):
+                cvar.get()
+
+        thread = threading.Thread(target=run_empty, context=None)
+        thread.start()
+        thread.join()
+
+        # An explicit Context value can also be passed
+        custom_ctx = contextvars.Context()
+        custom_var = None
+
+        def setup_context():
+            nonlocal custom_var
+            custom_var = contextvars.ContextVar('custom')
+            custom_var.set(2)
+
+        custom_ctx.run(setup_context)
+
+        def run_custom():
+            self.assertEqual(custom_var.get(), 2)
+
+        thread = threading.Thread(target=run_custom, context=custom_ctx)
+        thread.start()
+        thread.join()
+
 
 # HAMT Tests
 

Lib/test/test_decimal.py

@@ -1725,8 +1725,8 @@ def test_threading(self):
         self.finish1 = threading.Event()
         self.finish2 = threading.Event()
 
-        th1 = threading.Thread(target=thfunc1, args=(self,))
-        th2 = threading.Thread(target=thfunc2, args=(self,))
+        th1 = threading.Thread(target=thfunc1, args=(self,), context=None)
+        th2 = threading.Thread(target=thfunc2, args=(self,), context=None)
 
         th1.start()
         th2.start()

Lib/threading.py

@@ -4,6 +4,8 @@
 import sys as _sys
 import _thread
 import warnings
+import contextvars as _contextvars
+
 
 from time import monotonic as _time
 from _weakrefset import WeakSet
@@ -871,7 +873,7 @@ class Thread:
     _initialized = False
 
     def __init__(self, group=None, target=None, name=None,
-                 args=(), kwargs=None, *, daemon=None):
+                 args=(), kwargs=None, *, daemon=None, context='inherit'):
         """This constructor should always be called with keyword arguments. Arguments are:
 
         *group* should be None; reserved for future extension when a ThreadGroup
@@ -888,6 +890,10 @@ class is implemented.
         *kwargs* is a dictionary of keyword arguments for the target
         invocation. Defaults to {}.
 
+        *context* is the contextvars.Context value to use for the thread.  The default
+        is to inherit the context of the caller.  Set to None to start with an empty
+        context.
+
         If a subclass overrides the constructor, it must make sure to invoke
         the base class constructor (Thread.__init__()) before doing anything
         else to the thread.
@@ -917,6 +923,7 @@ class is implemented.
             self._daemonic = daemon
         else:
             self._daemonic = current_thread().daemon
+        self._context = context
         self._ident = None
         if _HAVE_THREAD_NATIVE_ID:
             self._native_id = None
@@ -972,9 +979,15 @@ def start(self):
 
         with _active_limbo_lock:
             _limbo[self] = self
+
+        if self._context == 'inherit':
+            # No context provided, inherit the context of the caller.
+            self._context = _contextvars.copy_context()
+
         try:
             # Start joinable thread
-            _start_joinable_thread(self._bootstrap, handle=self._handle,
+            _start_joinable_thread(self._bootstrap,
+                                   handle=self._handle,
                                    daemon=self.daemon)
         except Exception:
             with _active_limbo_lock:
@@ -1050,8 +1063,17 @@ def _bootstrap_inner(self):
             if _profile_hook:
                 _sys.setprofile(_profile_hook)
 
+            if self._context is None:
+                # Run with empty context, matching behaviour of
+                # threading.local and older versions of Python.
+                run = self.run
+            else:
+                # Run with the provided or the inherited context.
+                def run():
+                    self._context.run(self.run)
+
             try:
-                self.run()
+                run()
             except:
                 self._invoke_excepthook(self)
         finally: