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gh-129889: Support context manager protocol by contextvars.Token (#129888)
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Lines changed: 223 additions & 2 deletions

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‎Doc/library/contextvars.rst‎

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@@ -101,6 +101,21 @@ Context Variables
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the value of the variable to what it was before the corresponding
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*set*.
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The token supports :ref:`context manager protocol <context-managers>`
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to restore the corresponding context variable value at the exit from
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:keyword:`with` block::
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var = ContextVar('var', default='default value')
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with var.set('new value'):
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assert var.get() == 'new value'
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assert var.get() == 'default value'
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.. versionadded:: next
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Added support for usage as a context manager.
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.. attribute:: Token.var
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A read-only property. Points to the :class:`ContextVar` object

‎Doc/whatsnew/3.14.rst‎

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@@ -1,4 +1,3 @@
1-
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****************************
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What's new in Python 3.14
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****************************
@@ -362,6 +361,13 @@ concurrent.futures
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supplying a *mp_context* to :class:`concurrent.futures.ProcessPoolExecutor`.
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(Contributed by Gregory P. Smith in :gh:`84559`.)
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contextvars
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-----------
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* Support context manager protocol by :class:`contextvars.Token`.
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(Contributed by Andrew Svetlov in :gh:`129889`.)
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ctypes
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------
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‎Lib/test/test_context.py‎

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@@ -383,6 +383,115 @@ def sub(num):
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tp.shutdown()
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self.assertEqual(results, list(range(10)))
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def test_token_contextmanager_with_default(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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with c.set(36):
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self.assertEqual(c.get(), 36)
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self.assertEqual(c.get(), 42)
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ctx.run(fun)
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def test_token_contextmanager_without_default(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c')
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def fun():
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with c.set(36):
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self.assertEqual(c.get(), 36)
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with self.assertRaisesRegex(LookupError, "<ContextVar name='c'"):
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c.get()
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ctx.run(fun)
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def test_token_contextmanager_on_exception(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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with c.set(36):
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self.assertEqual(c.get(), 36)
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raise ValueError("custom exception")
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self.assertEqual(c.get(), 42)
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with self.assertRaisesRegex(ValueError, "custom exception"):
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ctx.run(fun)
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def test_token_contextmanager_reentrant(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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token = c.set(36)
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with self.assertRaisesRegex(
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RuntimeError,
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"<Token .+ has already been used once"
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):
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with token:
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with token:
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self.assertEqual(c.get(), 36)
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self.assertEqual(c.get(), 42)
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ctx.run(fun)
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def test_token_contextmanager_multiple_c_set(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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with c.set(36):
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self.assertEqual(c.get(), 36)
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c.set(24)
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self.assertEqual(c.get(), 24)
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c.set(12)
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self.assertEqual(c.get(), 12)
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self.assertEqual(c.get(), 42)
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ctx.run(fun)
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def test_token_contextmanager_with_explicit_reset_the_same_token(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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with self.assertRaisesRegex(
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RuntimeError,
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"<Token .+ has already been used once"
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):
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with c.set(36) as token:
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self.assertEqual(c.get(), 36)
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c.reset(token)
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self.assertEqual(c.get(), 42)
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self.assertEqual(c.get(), 42)
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ctx.run(fun)
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def test_token_contextmanager_with_explicit_reset_another_token(self):
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ctx = contextvars.Context()
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c = contextvars.ContextVar('c', default=42)
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def fun():
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with c.set(36):
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self.assertEqual(c.get(), 36)
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token = c.set(24)
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self.assertEqual(c.get(), 24)
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c.reset(token)
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self.assertEqual(c.get(), 36)
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self.assertEqual(c.get(), 42)
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ctx.run(fun)
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386495

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# HAMT Tests
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‎Misc/NEWS.d/next/Library/2025-02-10-09-45-49.gh-issue-129889.PBHXU5.rst‎

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@@ -0,0 +1,2 @@
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Support context manager protocol by :class:`contextvars.Token`. Patch by
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Andrew Svetlov.

‎Python/clinic/context.c.h‎

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‎Python/context.c‎

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@@ -1231,9 +1231,47 @@ static PyGetSetDef PyContextTokenType_getsetlist[] = {
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{NULL}
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};
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/*[clinic input]
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_contextvars.Token.__enter__ as token_enter
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Enter into Token context manager.
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[clinic start generated code]*/
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static PyObject *
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token_enter_impl(PyContextToken *self)
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/*[clinic end generated code: output=9af4d2054e93fb75 input=41a3d6c4195fd47a]*/
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{
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return Py_NewRef(self);
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}
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/*[clinic input]
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_contextvars.Token.__exit__ as token_exit
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type: object
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val: object
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tb: object
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/
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Exit from Token context manager, restore the linked ContextVar.
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[clinic start generated code]*/
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static PyObject *
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token_exit_impl(PyContextToken *self, PyObject *type, PyObject *val,
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PyObject *tb)
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/*[clinic end generated code: output=3e6a1c95d3da703a input=7f117445f0ccd92e]*/
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{
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int ret = PyContextVar_Reset((PyObject *)self->tok_var, (PyObject *)self);
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if (ret < 0) {
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return NULL;
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}
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Py_RETURN_NONE;
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}
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static PyMethodDef PyContextTokenType_methods[] = {
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{"__class_getitem__", Py_GenericAlias,
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METH_O|METH_CLASS, PyDoc_STR("See PEP 585")},
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TOKEN_ENTER_METHODDEF
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TOKEN_EXIT_METHODDEF
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{NULL}
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};
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