10000 gh-99633: Add context manager support to `contextvars.Context` · python/cpython@248d2d8 · GitHub
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gh-99633: Add context manager support to contextvars.Context 
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‎Doc/library/contextvars.rst

Lines changed: 67 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -144,21 +144,79 @@ Manual Context Management
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To get a copy of the current context use the
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:func:`~contextvars.copy_context` function.
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Every thread will have a different top-level :class:`~contextvars.Context`
148-
object. This means that a :class:`ContextVar` object behaves in a similar
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fashion to :func:`threading.local` when values are assigned in different
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threads.
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Each thread has its own effective stack of :class:`!Context` objects. The
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*current context* is the :class:`!Context` object at the top of the current
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thread's stack. All :class:`!Context` objects in the stacks are considered
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to be *entered*.
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*Entering* a context, either by calling the :meth:`< 10000 span class="pl-s">~Context.run` method or
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using the context as a :term:`context manager`, pushes the context onto the
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top of the current thread's stack, making it the current context.
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*Exiting* from the current context, either by returning from the callback
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passed to :meth:`~Context.run` or by exiting the :keyword:`with` statement
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suite, pops the context off of the top of the stack, restoring the current
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context to what it was before.
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Since each thread has its own context stack, :class:`ContextVar` objects
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behave in a similar fashion to :func:`threading.local` when values are
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assigned in different threads.
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Attempting to do either of the following raises a :exc:`RuntimeError`:
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* Entering an already entered context, including contexts entered in
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other threads.
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* Exiting from a context that is not the current context.
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After exiting a context, it can later be re-entered (from any thread).
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Any changes to :class:`ContextVar` values via the :meth:`ContextVar.set`
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method are recorded in the current context. The :meth:`ContextVar.get`
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method returns the value associated with the current context. Exiting a
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context effectively reverts any changes made to context variables while the
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context was entered (if needed, the values can be restored by re-entering the
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context).
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Context implements the :class:`collections.abc.Mapping` interface.
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.. versionadded:: 3.14
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Added support for the :term:`!context management protocol` (:pep:`343`).
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The value bound to the identifier given in the :keyword:`with` statement's
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:keyword:`!as` clause (if present) is the :class:`!Context` object itself.
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Example:
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.. testcode::
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import contextvars
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var = contextvars.ContextVar("var")
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var.set("initial")
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assert var.get() == "initial"
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# Copy the current Context and enter the copy.
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with contextvars.copy_context() as ctx:
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var.set("updated")
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assert var in ctx
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assert ctx[var] == "updated"
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assert var.get() == "updated"
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# Exited ctx, so the observed value of var has reverted.
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assert var.get() == "initial"
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# But the updated value is still recorded in ctx.
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assert ctx[var] == "updated"
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# Re-entering ctx restores the updated value of var.
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with ctx:
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assert var.get() == "updated"
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154213
.. method:: run(callable, *args, **kwargs)
155214

156-
Execute ``callable(*args, **kwargs)`` code in the context object
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the *run* method is called on. Return the result of the execution
158-
or propagate an exception if one occurred.
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Enters the Context, executes ``callable(*args, **kwargs)``, then exits the
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Context. Returns *callable*'s return value, or propagates an exception if
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one occurred.
159218

160-
Any changes to any context variables that *callable* makes will
161-
be contained in the context object::
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Example::
162220

163221
var = ContextVar('var')
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var.set('spam')
@@ -186,10 +244,6 @@ Manual Context Management
186244
# However, outside of 'ctx', 'var' is still set to 'spam':
187245
# var.get() == 'spam'
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189-
The method raises a :exc:`RuntimeError` when called on the same
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context object from more than one OS thread, or when called
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recursively.
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.. method:: copy()
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Return a shallow copy of the context object.

‎Doc/whatsnew/3.14.rst

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Original file line numberDiff line numberDiff line change
@@ -112,6 +112,13 @@ ast
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(Contributed by Bénédikt Tran in :gh:`121141`.)
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contextvars
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-----------
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* Added support for the :term:`!context management protocol` (:pep:`343`) to
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:class:`contextvars.Context`. (Contributed by Richard Hansen in :gh:`99634`.)
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121+
115122
ctypes
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------
117124

‎Lib/test/test_context.py

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Original file line numberDiff line numberDiff line change
@@ -3,6 +3,7 @@
33
import functools
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import gc
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import random
6+
import threading
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import time
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import unittest
89
import weakref
@@ -361,6 +362,79 @@ def sub(num):
361362
tp.shutdown()
362363
self.assertEqual(results, list(range(10)))
363364

365+
@isolated_context
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def test_context_manager(self):
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cvar = contextvars.ContextVar('cvar', default='initial')
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self.assertEqual(cvar.get(), 'initial')
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with contextvars.copy_context():
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self.assertEqual(cvar.get(), 'initial')
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cvar.set('updated')
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self.assertEqual(cvar.get(), 'updated')
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self.assertEqual(cvar.get(), 'initial')
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def test_context_manager_as_binding(self):
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ctx = contextvars.copy_context()
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with ctx as ctx_as_binding:
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self.assertIs(ctx_as_binding, ctx)
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380+
def test_context_manager_nested(self):
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with contextvars.copy_context() as outer_ctx:
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with contextvars.copy_context() as inner_ctx:
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self.assertIsNot(outer_ctx, inner_ctx)
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@isolated_context
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def test_context_manager_enter_again_after_exit(self):
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cvar = contextvars.ContextVar('cvar', default='initial')
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self.assertEqual(cvar.get(), 'initial')
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with contextvars.copy_context() as ctx:
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cvar.set('updated')
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self.assertEqual(cvar.get(), 'updated')
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self.assertEqual(cvar.get(), 'initial')
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with ctx:
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self.assertEqual(cvar.get(), 'updated')
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self.assertEqual(cvar.get(), 'initial')
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@threading_helper.requires_working_threading()
398+
def test_context_manager_rejects_exit_from_different_thread(self):
399+
ctx = contextvars.copy_context()
400+
thread = threading.Thread(target=ctx.__enter__)
401+
thread.start()
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thread.join()
403+
with self.assertRaises(RuntimeError):
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ctx.__exit__(None, None, None)
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def test_context_manager_is_not_reentrant(self):
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with self.subTest('context manager then context manager'):
408+
with contextvars.copy_context() as ctx:
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with self.assertRaises(RuntimeError):
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with ctx:
411+
pass
412+
with self.subTest('context manager then run method'):
413+
with contextvars.copy_context() as ctx:
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with self.assertRaises(RuntimeError):
415+
ctx.run(lambda: None)
416+
with self.subTest('run method then context manager'):
417+
ctx = contextvars.copy_context()
418+
419+
def fn():
420+
with self.assertRaises(RuntimeError):
421+
with ctx:
422+
pass
423+
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ctx.run(fn)
425+
426+
def test_context_manager_rejects_noncurrent_exit(self):
427+
with contextvars.copy_context() as outer_ctx:
428+
with contextvars.copy_context() as inner_ctx:
429+
self.assertIsNot(outer_ctx, inner_ctx)
430+
with self.assertRaises(RuntimeError):
431+
outer_ctx.__exit__(None, None, None)
432+
433+
def test_context_manager_rejects_nonentered_exit(self):
434+
ctx = contextvars.copy_context()
435+
with self.assertRaises(RuntimeError):
436+
ctx.__exit__(None, None, None)
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364438

365439
# HAMT Tests
366440

‎Misc/ACKS

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Original file line numberDiff line numberDiff line change
@@ -716,6 +716,7 @@ Michael Handler
716716
Andreas Hangauer
717717
Milton L. Hankins
718718
Carl Bordum Hansen
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Richard Hansen
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Stephen Hansen
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Barry Hantman
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Lynda Hardman

‎Misc/NEWS.d/next/Library/2022-11-21-01-24-46.gh-issue-99633.vhrNRe.rst

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
1+
Added support for the :term:`!context management protocol` (:pep:`343`) to
2+
:class:`contextvars.Context`. Patch by Richard Hansen.

‎Python/clinic/context.c.h

Lines changed: 69 additions & 1 deletion
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‎Python/context.c

Lines changed: 63 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -144,7 +144,6 @@ _PyContext_Exit(PyThreadState *ts, PyObject *octx)
144144
}
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146146
if (ts->context != (PyObject *)ctx) {
147-
/* Can only happen if someone misuses the C API */
148147
PyErr_SetString(PyExc_RuntimeError,
149148
"cannot exit context: thread state references "
150149
"a different context object");
@@ -521,6 +520,67 @@ context_tp_contains(PyContext *self, PyObject *key)
521520
}
522521

523522

523+
/*[clinic input]
524+
_contextvars.Context.__enter__
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526+
Context manager enter.
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528+
Automatically called by the 'with' statement. Using the Context object as a
529+
context manager is an alternative to calling the Context.run() method.
530+
531+
Example:
532+
533+
var = contextvars.ContextVar('var')
534+
var.set('initial')
535+
536+
with contextvars.copy_context():
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# Changes to context variables will be rolled back upon exiting the
538+
# `with` statement.
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var.set('updated')
540+
print(var.get()) # 'updated'
541+
542+
# The context variable value has been rolled back.
543+
print(var.get()) # 'initial'
544+
[clinic start generated code]*/
545+
546+
static PyObject *
547+
_contextvars_Context___enter___impl(PyContext *self)
548+
/*[clinic end generated code: output=7374aea8983b777a input=fffe71e56ca17ee4]*/
549+
{
550+
// The new ref added here is for the `with` statement's `as` binding. It is
551+
// decremented when the variable goes out of scope, which can be before or
552+
// after `PyContext_Exit` is called. (The binding can go out of scope
553+
// immediately -- before the `with` suite even runs -- if there is no `as`
554+
// clause. Or it can go out of scope long after the `with` suite completes
555+
// because `with` does not have its own scope.) Because of this timing, two
556+
// references are needed: the one added in `PyContext_Enter` and the one
557+
// added here.
558+
return PyContext_Enter((PyObject *)self) < 0 ? NULL : Py_NewRef(self);
559+
}
560+
561+
562+
/*[clinic input]
563+
_contextvars.Context.__exit__
564+
exc_type: object
565+
exc_val: object
566+
exc_tb: object
567+
/
568+
569+
Context manager exit.
570+
571+
Automatically called at the conclusion of a 'with' statement when the Context is
572+
used as a context manager. See the Context.__enter__() method for more details.
573+
[clinic start generated code]*/
574+
575+
static PyObject *
576+
_contextvars_Context___exit___impl(PyContext *self, PyObject *exc_type,
577+
PyObject *exc_val, PyObject *exc_tb)
578+
/*[clinic end generated code: output=4608fa9151f968f1 input=ff70cbbf6a112b1d]*/
579+
{
580+
return PyContext_Exit((PyObject *)self) < 0 ? NULL : Py_None;
581+
}
582+
583+
524584
/*[clinic input]
525585
_contextvars.Context.get
526586
key: object
@@ -641,6 +701,8 @@ context_run(PyContext *self, PyObject *const *args,
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642702

643703
static PyMethodDef PyContext_methods[] = {
704+
_CONTEXTVARS_CONTEXT___ENTER___METHODDEF
705+
_CONTEXTVARS_CONTEXT___EXIT___METHODDEF
644706
_CONTEXTVARS_CONTEXT_GET_METHODDEF
645707
_CONTEXTVARS_CONTEXT_ITEMS_METHODDEF
646708
_CONTEXTVARS_CONTEXT_KEYS_METHODDEF

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