Add blurb, update docs/comments.

Author: nascheme

Doc/library/threading.rst

@@ -359,10 +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`.
-   To start with an empty context, pass a new instance of
-   :class:`contextvars.Context()`
+   *context* is the ``contextvars.Context`` value to use while running
+   the thread.  The default value is ``None`` which means to use a copy
+   of the context of the caller of :meth:`~Thread.start`.  To start with
+   an empty context, pass a new instance of :class:`contextvars.Context()`
 
    If the subclass overrides the constructor, it must make sure to invoke the
    base class constructor (``Thread.__init__()``) before doing anything else to
@@ -375,8 +375,10 @@ since it is impossible to detect the termination of alien threads.
       Use the *target* name if *name* argument is omitted.
 
    .. versionchanged:: 3.14
-      Added the *context* parameter.  Previously threads always ran with an empty
-      context.
+      Threads now inherit the context of the caller of :meth:`Thread.start`
+      instead of starting with an empty context.  The ``context`` parameter
+      was added.  Pass a new ``contextvars.Context()`` if your thread
+      requires an empty context.
 
    .. method:: start()
 

Lib/threading.py

@@ -890,9 +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.  To start with an empty context,
-        pass a new instance of contextvars.Context().
+        *context* is the contextvars.Context value to use for the thread.
+        The default value is None, which means to use a copy of the context
+        of the caller.  To start with an empty context, pass a new instance
+        of contextvars.Context().
 
         If a subclass overrides the constructor, it must make sure to invoke
         the base class constructor (Thread.__init__()) before doing anything
@@ -981,7 +982,7 @@ def start(self):
             _limbo[self] = self
 
         if self._context is None:
-            # No context provided, inherit the context of the caller.
+            # No context provided, inherit a copy of the context of the caller.
             self._context = _contextvars.copy_context()
 
         try:

Misc/NEWS.d/next/Core_and_Builtins/2025-01-06-10-55-41.gh-issue-128555.tAK_AY.rst

@@ -0,0 +1,4 @@
+Starting a new thread using :class:`threading.Thread` will now, by default,
+use a copy of the :class:`contextvars.Context` from the caller of
+:meth:`Thread.start` rather than using an empty context. The `_contextvars`
+module is now built-in.