Use context=None to mean inherit.

This matches what asyncio-task does.

Author: nascheme

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, context="inherit")
+                  daemon=None, context=None)
 
    This constructor should always be called with keyword arguments.  Arguments
    are:
@@ -361,7 +361,8 @@ since it is impossible to detect the termination of alien threads.
 
    *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.
+   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

Lib/test/test_decimal.py

@@ -44,6 +44,7 @@
 import random
 import inspect
 import threading
+import contextvars
 
 
 if sys.platform == 'darwin':
@@ -1725,8 +1726,10 @@ def test_threading(self):
         self.finish1 = threading.Event()
         self.finish2 = threading.Event()
 
-        th1 = threading.Thread(target=thfunc1, args=(self,), context=None)
-        th2 = threading.Thread(target=thfunc2, args=(self,), context=None)
+        th1 = threading.Thread(target=thfunc1, args=(self,),
+                               context=contextvars.Context())
+        th2 = threading.Thread(target=thfunc2, args=(self,),
+                               context=contextvars.Context())
 
         th1.start()
         th2.start()

Lib/threading.py

@@ -873,7 +873,7 @@ class Thread:
     _initialized = False
 
     def __init__(self, group=None, target=None, name=None,
-                 args=(), kwargs=None, *, daemon=None, context='inherit'):
+                 args=(), kwargs=None, *, daemon=None, context=None):
         """This constructor should always be called with keyword arguments. Arguments are:
 
         *group* should be None; reserved for future extension when a ThreadGroup
@@ -891,8 +891,8 @@ class is implemented.
         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.
+        is to inherit 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
@@ -980,7 +980,7 @@ def start(self):
         with _active_limbo_lock:
             _limbo[self] = self
 
-        if self._context == 'inherit':
+        if self._context is None:
             # No context provided, inherit the context of the caller.
             self._context = _contextvars.copy_context()