Add 'context' parameter to Thread.

* Add ``sys.flags.inherit_context``. * Add ``-X inherit_context`` and :envvar:`PYTHON_INHERIT_CONTEXT`
nascheme · Apr 9, 2025 · Dec 28, 2024 · Dec 21, 2024 · Feb 7, 2025 · Feb 7, 2025
commit 6d00c2aeec9a0e222d8df6383acde2f707f33bf6
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -535,7 +535,8 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 .. data:: flags
 
    The :term:`named tuple` *flags* exposes the status of command line
-   flags. The attributes are read only.
+   flags.  Flags should only be accessed only by name and not by index.  The
+   attributes are read only.
 
    .. list-table::
 
@@ -594,6 +595,12 @@ always available. Unless explicitly noted otherwise, all variables are read-only
       * - .. attribute:: flags.warn_default_encoding
         - :option:`-X warn_default_encoding <-X>`
 
+      * - .. attribute:: flags.gil
+        - :option:`-X gil <-X>` and :envvar:`PYTHON_GIL`
+
+      * - .. attribute:: flags.inherit_context
+        - :option:`-X inherit_context <-X>` and :envvar:`PYTHON_INHERIT_CONTEXT`
+
    .. versionchanged:: 3.2
       Added ``quiet`` attribute for the new :option:`-q` flag.
 
@@ -620,6 +627,12 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    .. versionchanged:: 3.11
       Added the ``int_max_str_digits`` attribute.
 
+   .. versionchanged:: 3.13
+      Added the ``gil`` attribute.
+
+   .. versionchanged:: 3.14
+      Added the ``inherit_context`` attribute.
+
 
 .. data:: float_info
 

diff --git a/Doc/library/threading.rst b/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=None)
 
    This constructor should always be called with keyword arguments.  Arguments
    are:
@@ -359,6 +359,17 @@ 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 :class:`~contextvars.Context` value to use when starting
+   the thread.  The default value is ``None`` which indicates that the
+   :data:`sys.flags.inherit_context` flag controls the behaviour.  If
+   the flag is true, threads will start with a copy of the context of the
+   caller of :meth:`~Thread.start`.  If false, they will start with
+   an empty context.  To explicitly start with an empty context,
+   pass a new instance of :class:`~contextvars.Context()`.  To explicitly
+   start with a copy of the current context, pass the value from
+   :func:`~contextvars.copy_context()`. The flag defaults true on
+   free-threaded builds and false otherwise.
+
    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 +380,9 @@ 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.
+
    .. method:: start()
 
       Start the thread's activity.

diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
@@ -628,6 +628,15 @@ Miscellaneous options
 
      .. versionadded:: 3.13
 
+   * :samp:`-X inherit_context={0,1}` causes :class:`~threading.Thread`
+     to, by default, use a copy of context of of the caller of
+     ``Thread.start()`` when starting.  Otherwise, threads will start
+     with an empty context.  If unset, the value of this option defaults
+     to ``1`` on free-threaded builds and to ``0`` otherwise.  See also
+     :envvar:`PYTHON_INHERIT_CONTEXT`.
+
+     .. versionadded:: 3.14
+
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
 
@@ -1221,6 +1230,16 @@ conflict.
 
    .. versionadded:: 3.13
 
+.. envvar:: PYTHON_INHERIT_CONTEXT
+
+   If this variable is set to ``1`` then :class:`~threading.Thread` will,
+   by default, use a copy of context of of the caller of ``Thread.start()``
+   when starting.  Otherwise, new threads will start with an empty context.
+   If unset, this variable defaults to ``1`` on free-threaded builds and to
+   ``0`` otherwise.  See also :option:`-X inherit_context<-X>`.
+
+   .. versionadded:: 3.14
+
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~
 

@@ -179,6 +179,7 @@ typedef struct PyConfig {
     int use_frozen_modules;
     int safe_path;
     int int_max_str_digits;
+    int inherit_context;
 #ifdef __APPLE__
     int use_system_logger;
 #endif

diff --git a/Lib/test/test_capi/test_config.py b/Lib/test/test_capi/test_config.py
@@ -55,6 +55,7 @@ def test_config_get(self):
             ("filesystem_errors", str, None),
             ("hash_seed", int, None),
             ("home", str | None, None),
+            ("inherit_context", int, None),
             ("import_time", bool, None),
             ("inspect", bool, None),
             ("install_signal_handlers", bool, None),
@@ -98,7 +99,7 @@ def test_config_get(self):
         ]
         if support.Py_DEBUG:
             options.append(("run_presite", str | None, None))
-        if sysconfig.get_config_var('Py_GIL_DISABLED'):
+        if support.Py_GIL_DISABLED:
             options.append(("enable_gil", int, None))
             options.append(("tlbc_enabled", int, None))
         if support.MS_WINDOWS:
@@ -170,7 +171,7 @@ def test_config_get_sys_flags(self):
             ("warn_default_encoding", "warn_default_encoding", False),
             ("safe_path", "safe_path", False),
             ("int_max_str_digits", "int_max_str_digits", False),
-            # "gil" is tested below
+            # "gil" and "inherit_context" are tested below
         ):
             with self.subTest(flag=flag, name=name, negate=negate):
                 value = config_get(name)
@@ -182,11 +183,14 @@ def test_config_get_sys_flags(self):
                          config_get('use_hash_seed') == 0
                          or config_get('hash_seed') != 0)
 
-        if sysconfig.get_config_var('Py_GIL_DISABLED'):
+        if support.Py_GIL_DISABLED:
             value = config_get('enable_gil')
             expected = (value if value != -1 else None)
             self.assertEqual(sys.flags.gil, expected)
 
+        expected_inherit_context = 1 if support.Py_GIL_DISABLED else 0
+        self.assertEqual(sys.flags.inherit_context, expected_inherit_context)
+
     def test_config_get_non_existent(self):
         # Test PyConfig_Get() on non-existent option name
         config_get = _testcapi.config_get

@@ -1,3 +1,4 @@
+import sys
 import collections.abc
 import concurrent.futures
 import contextvars
@@ -383,6 +384,60 @@ 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')
+
+        def run_context_none():
+            if sys.flags.inherit_context:
+                expected = 1
+            else:
+                expected = None
+            self.assertEqual(cvar.get(None), expected)
+
+        # By default, context is inherited based on the
+        # sys.flags.inherit_context option.
+        cvar.set(1)
+        thread = threading.Thread(target=run_context_none)
+        thread.start()
+        thread.join()
+
+        # Passing 'None' explicitly should have same behaviour as not
+        # passing parameter.
+        thread = threading.Thread(target=run_context_none, 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()
+
+        # You can also pass a new Context() object to start with an empty context
+        def run_empty():
+            with self.assertRaises(LookupError):
+                cvar.get()
+
+        thread = threading.Thread(target=run_empty, context=contextvars.Context())
+        thread.start()
+        thread.join()
+
 
 # HAMT Tests
 

diff --git a/Lib/test/test_decimal.py b/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,))
-        th2 = threading.Thread(target=thfunc2, args=(self,))
+        th1 = threading.Thread(target=thfunc1, args=(self,),
+                               context=contextvars.Context())
+        th2 = threading.Thread(target=thfunc2, args=(self,),
+                               context=contextvars.Context())
 
         th1.start()
         th2.start()

diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
@@ -50,7 +50,7 @@
 INIT_LOOPS = 4
 MAX_HASH_SEED = 4294967295
 
-ABI_THREAD = 't' if sysconfig.get_config_var('Py_GIL_DISABLED') else ''
+ABI_THREAD = 't' if support.Py_GIL_DISABLED else ''
 # PLATSTDLIB_LANDMARK copied from Modules/getpath.py
 if os.name == 'nt':
     PLATSTDLIB_LANDMARK = f'{sys.platlibdir}'
@@ -60,6 +60,10 @@
     PLATSTDLIB_LANDMARK = (f'{sys.platlibdir}/python{VERSION_MAJOR}.'
                            f'{VERSION_MINOR}{ABI_THREAD}/lib-dynload')
 
+if support.Py_GIL_DISABLED:
+    DEFAULT_INHERIT_CONTEXT = 1
+else:
+    DEFAULT_INHERIT_CONTEXT = 0
 
 # If we are running from a build dir, but the stdlib has been installed,
 # some tests need to expect different results.
@@ -586,6 +590,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'tracemalloc': 0,
         'perf_profiling': 0,
         'import_time': False,
+        'inherit_context': DEFAULT_INHERIT_CONTEXT,
         'code_debug_ranges': True,
         'show_ref_count': False,
         'dump_refs': False,

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -1845,8 +1845,9 @@ def test_pythontypes(self):
         # symtable entry
         # XXX
         # sys.flags
-        # FIXME: The +1 will not be necessary once gh-122575 is fixed
-        check(sys.flags, vsize('') + self.P * (1 + len(sys.flags)))
+        # FIXME: The +2 is for the 'gil' and 'inherit_context' flags and
+        # will not be necessary once gh-122575 is fixed
+        check(sys.flags, vsize('') + self.P * (2 + len(sys.flags)))
 
     def test_asyncgen_hooks(self):
         old = sys.get_asyncgen_hooks()

diff --git a/Lib/threading.py b/Lib/threading.py
@@ -3,6 +3,7 @@
 import os as _os
 import sys as _sys
 import _thread
+import _contextvars
 
 from time import monotonic as _time
 from _weakrefset import WeakSet
@@ -871,7 +872,7 @@ class Thread:
     _initialized = False
 
     def __init__(self, group=None, target=None, name=None,
-                 args=(), kwargs=None, *, daemon=None):
+                 args=(), kwargs=None, *, daemon=None, context=None):
         """This constructor should always be called with keyword arguments. Arguments are:
 
         *group* should be None; reserved for future extension when a ThreadGroup
@@ -888,6 +889,14 @@ 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 value is None, which means to check
+        sys.flags.inherit_context.  If that flag is true, use a copy of
+        the context of the caller.  If false, use an empty context.  To
+        explicitly start with an empty context, pass a new instance of
+        contextvars.Context().  To explicitly start with a copy of the
+        current context, pass the value from contextvars.copy_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 +926,7 @@ 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
@@ -972,6 +982,16 @@ def start(self):
 
         with _active_limbo_lock:
             _limbo[self] = self
+
+        if self._context is None:
+            # No context provided
+            if _sys.flags.inherit_context:
+                # start with a copy of the context of the caller
+                self._context = _contextvars.copy_context()
+            else:
+                # start with an empty context
+                self._context = _contextvars.Context()
+
         try:
             # Start joinable thread
             _start_joinable_thread(self._bootstrap, handle=self._handle,
@@ -1051,7 +1071,7 @@ def _bootstrap_inner(self):
                 _sys.setprofile(_profile_hook)
 
             try:
-                self.run()
+                self._context.run(self.run)
             except:
                 self._invoke_excepthook(self)
         finally:

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-01-06-10-55-41.gh-issue-128555.tAK_AY.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-01-06-10-55-41.gh-issue-128555.tAK_AY.rst
@@ -0,0 +1,16 @@
+Add the :data:`sys.flags.inherit_context` flag.
+
+* This flag is set to true by default on the free-threaded build
+  and false otherwise.  If the flag is true, starting a new thread using
+  :class:`threading.Thread` will, by default, use a copy of the
+  :class:`contextvars.Context` from the caller of
+  :meth:`threading.Thread.start` rather than using an empty context.
+
+* Add the :option:`-X inherit_context <-X>` command-line option and
+  :envvar:`PYTHON_INHERIT_CONTEXT` environment variable, which set the
+  :data:`~sys.flags.inherit_context` flag.
+
+* Add the ``context`` keyword parameter to :class:`~threading.Thread`.  It can
+  be used to explicitly pass a context value to be used by a new thread.
+
+* Make the :mod:`_contextvars` module built-in.