Merge branch 'thread_inherit_context' into gh-128384-warnings-contextvar

Author: nascheme

Doc/library/decimal.rst

@@ -1884,13 +1884,20 @@ the current thread.
 
 If :func:`setcontext` has not been called before :func:`getcontext`, then
 :func:`getcontext` will automatically create a new context for use in the
-current thread.
-
-The new context is copied from a prototype context called *DefaultContext*. To
-control the defaults so that each thread will use the same values throughout the
-application, directly modify the *DefaultContext* object. This should be done
-*before* any threads are started so that there won't be a race condition between
-threads calling :func:`getcontext`. For example::
+current thread.  New context objects have default values set from the
+:data:`decimal.DefaultContext` object.
+
+The :data:`sys.flags.thread_inherit_context` flag affects the context for
+new threads.  If the flag is false, new threads will start with an empty
+context.  In this case, :func:`getcontext` will create a new context object
+when called and use the default values from *DefaultContext*.  If the flag
+is true, new threads will start with a copy of context from the caller of
+:meth:`Thread.start`.
+
+To control the defaults so that each thread will use the same values throughout
+the application, directly modify the *DefaultContext* object. This should be
+done *before* any threads are started so that there won't be a race condition
+between threads calling :func:`getcontext`. For example::
 
    # Set applicationwide defaults for all threads about to be launched
    DefaultContext.prec = 12

Lib/test/test_decimal.py

@@ -1726,6 +1726,9 @@ def test_threading(self):
         self.finish1 = threading.Event()
         self.finish2 = threading.Event()
 
+        # This test wants to start threads with an empty context, no matter
+        # the setting of sys.flags.thread_inherit_context.  We pass the
+        # 'context' argument explicitly with an empty context instance.
         th1 = threading.Thread(target=thfunc1, args=(self,),
                                context=contextvars.Context())
         th2 = threading.Thread(target=thfunc2, args=(self,),