gh-128555: Add 'context' keyword parameter to Thread.

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 +359,11 @@ 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 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
    the thread.
@@ -369,6 +374,12 @@ 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
+      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 :class:`~contextvars.Context()` if your thread
+      requires an empty context.
+
    .. method:: start()
 
       Start the thread's activity.

Lib/test/test_context.py

@@ -383,6 +383,54 @@ 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, behaviour is to inherit
+        thread = threading.Thread(target=run_inherit, 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
 

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()

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=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 +890,11 @@ 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 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
         else to the thread.
@@ -917,6 +924,7 @@ 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 +980,15 @@ def start(self):
 
         with _active_limbo_lock:
             _limbo[self] = self
+
+        if self._context is None:
+            # No context provided, inherit a copy of 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:
@@ -1051,7 +1065,13 @@ def _bootstrap_inner(self):
                 _sys.setprofile(_profile_hook)
 
             try:
-                self.run()
+                if self._context is None:
+                    # Run with empty context, matching behaviour of
+                    # threading.local and older versions of Python.
+                    self.run()
+                else:
+                    # Run with the provided or the inherited context.
+                    self._context.run(self.run)
             except:
                 self._invoke_excepthook(self)
         finally:

Makefile.pre.in

@@ -420,6 +420,7 @@ PARSER_HEADERS= \
 # Python
 
 PYTHON_OBJS=	\
+		Python/_contextvars.o \
 		Python/_warnings.o \
 		Python/Python-ast.o \
 		Python/Python-tokenize.o \

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:`threading.Thread.start` rather than using an empty context. The
+``_contextvars`` module is now built-in.

Modules/Setup

@@ -132,7 +132,6 @@ PYTHONPATH=$(COREPYTHONPATH)
 
 #_asyncio _asynciomodule.c
 #_bisect _bisectmodule.c
-#_contextvars _contextvarsmodule.c
 #_csv _csv.c
 #_datetime _datetimemodule.c
 #_decimal _decimal/_decimal.c

Modules/Setup.stdlib.in

@@ -31,7 +31,6 @@
 @MODULE_ARRAY_TRUE@array arraymodule.c
 @MODULE__ASYNCIO_TRUE@_asyncio _asynciomodule.c
 @MODULE__BISECT_TRUE@_bisect _bisectmodule.c
-@MODULE__CONTEXTVARS_TRUE@_contextvars _contextvarsmodule.c
 @MODULE__CSV_TRUE@_csv _csv.c
 @MODULE__HEAPQ_TRUE@_heapq _heapqmodule.c
 @MODULE__JSON_TRUE@_json _json.c

Modules/config.c.in

@@ -29,6 +29,7 @@ extern PyObject* PyInit__imp(void);
 extern PyObject* PyInit_gc(void);
 extern PyObject* PyInit__ast(void);
 extern PyObject* PyInit__tokenize(void);
+extern PyObject* PyInit__contextvars(void);
 extern PyObject* _PyWarnings_Init(void);
 extern PyObject* PyInit__string(void);
 
@@ -55,6 +56,9 @@ struct _inittab _PyImport_Inittab[] = {
     /* This lives in gcmodule.c */
     {"gc", PyInit_gc},
 
+    /* This lives in Python/_contextvars.c */
+    {"_contextvars", PyInit__contextvars},
+
     /* This lives in _warnings.c */
     {"_warnings", _PyWarnings_Init},
 

PCbuild/pythoncore.vcxproj

@@ -423,7 +423,6 @@
     </ClCompile>
     <ClCompile Include="..\Modules\_codecsmodule.c" />
     <ClCompile Include="..\Modules\_collectionsmodule.c" />
-    <ClCompile Include="..\Modules\_contextvarsmodule.c" />
     <ClCompile Include="..\Modules\_csv.c" />
     <ClCompile Include="..\Modules\_functoolsmodule.c" />
     <ClCompile Include="..\Modules\_hacl\Hacl_Hash_MD5.c" />
@@ -570,6 +569,7 @@
     <ClCompile Include="..\PC\config.c" />
     <ClCompile Include="..\PC\msvcrtmodule.c" />
     <ClCompile Include="..\Python\pyhash.c" />
+    <ClCompile Include="..\Python\_contextvars.c" />
     <ClCompile Include="..\Python\_warnings.c" />
     <ClCompile Include="..\Python\asdl.c" />
     <ClCompile Include="..\Python\assemble.c" />

PCbuild/pythoncore.vcxproj.filters

@@ -1262,6 +1262,9 @@
     <ClCompile Include="..\PC\msvcrtmodule.c">
       <Filter>PC</Filter>
     </ClCompile>
+    <ClCompile Include="..\Python\_contextvars.c">
+      <Filter>Python</Filter>
+    </ClCompile>
     <ClCompile Include="..\Python\_warnings.c">
       <Filter>Python</Filter>
     </ClCompile>
@@ -1526,9 +1529,6 @@
     <ClCompile Include="..\Objects\odictobject.c">
       <Filter>Objects</Filter>
     </ClCompile>
-    <ClCompile Include="..\Modules\_contextvarsmodule.c">
-      <Filter>Modules</Filter>
-    </ClCompile>
     <ClCompile Include="$(zlibDir)\adler32.c">
       <Filter>Modules\zlib</Filter>
     </ClCompile>

Modules/_contextvarsmodule.c → Python/_contextvars.c

@@ -1,6 +1,6 @@
 #include "Python.h"
 
-#include "clinic/_contextvarsmodule.c.h"
+#include "clinic/_contextvars.c.h"
 
 /*[clinic input]
 module _contextvars

configure.ac

@@ -7709,7 +7709,6 @@ dnl always enabled extension modules
 PY_STDLIB_MOD_SIMPLE([array])
 PY_STDLIB_MOD_SIMPLE([_asyncio])
 PY_STDLIB_MOD_SIMPLE([_bisect])
-PY_STDLIB_MOD_SIMPLE([_contextvars])
 PY_STDLIB_MOD_SIMPLE([_csv])
 PY_STDLIB_MOD_SIMPLE([_heapq])
 PY_STDLIB_MOD_SIMPLE([_json])