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Clarify that every thread has its own default context in contextvars #99246
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Doc/library/contextvars.rst
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:class:`~contextvars.Context`. This means that a | ||
:class:`~contextvars.ContextVar` can generally be used as a drop-in | ||
replacement for a :func:`threading.local` variable. |
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I think that saying that ContextVar
can be used as a drop-in for threading.local
might bring people to blindly replace threading.local
with ContextVar
, but care is needed. Some things (mostly C stuff) are just bound to OS threads, so threading.local
is what you want for them, while with ContextVar
there can be many contexts per OS thread (e.g. asyncio tasks).
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Unfortunately I don't think I follow your argument, Can you suggest a better wording for this?
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I think the first sentence provides good information, although a couple notes on the wording:
-
"By default" -- is there a way for a thread to not have a unique context? Maybe with the C API? PEP-567 says "Context.run() raises a RuntimeError when called on the same context object from more than one OS thread" so it shouldn't be possible to "share" a
Context
. If so, I'd remove "By default". -
"default Context" -- maybe instead say "root Context" or "initial Context" or "top-level Context"?
Regarding the second sentence, what is the motivation for having a note about it? Is the idea to encourage people to switch to ContextVar
when it's the better choice?
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Regarding the second sentence, what is the motivation for having a note about it?
The motivation is to clarify the following sentence:
Context managers that have state should use Context Variables
instead of :func:threading.local()
to prevent their state from
bleeding to other code unexpectedly, when used in concurrent code.
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My suggestion is to replacie the note+paragraph with the following:
Every thread has a unique current :class:`~contextvars.Context`.
If you're currently using :func:`threading.local()` to store contextual information,
such as context manager state or per-request state in web frameworks,
consider using Context Variables instead. This will prevent the state from
bleeding to other contexts unexpectedly, when used in concurrent code
that is running in the same thread.
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I don't agree. I want to highlight more that every thread has is own context.
Thanks for the feedback I will take it into account into the final version
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There is a later line "Context
objects hold strong references to context variables which prevents context variables from being properly garbage collected." Is this also true for threading.local()
? And if not, is that potentially a problem for using "drop-in replacement"?
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That's also true for thread locals, they also hold strong references to what they contain. Is just that they are less tricky to leak because they don't have the layers separating the key and the container.
Going from thread local to context variable (as long as you place them at top level as the doc say) will not create ownership problems in general
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In any case I will use a different wording as I just want to focus on the per thread default guarantee here, not on the equivalence between this and thread locals.
Signed-off-by: Pablo Galindo <pablogsal@gmail.com>
Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>
Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10, 3.11. |
Sorry @pablogsal, I had trouble checking out the |
GH-100365 is a backport of this pull request to the 3.10 branch. |
GH-100366 is a backport of this pull request to the 3.9 branch. |
…ythonGH-99246) (cherry picked from commit cb60b61) Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
…ythonGH-99246) (cherry picked from commit cb60b61) Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
Thanks @pablogsal for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11. |
GH-100367 is a backport of this pull request to the 3.11 branch. |
…ythonGH-99246) (cherry picked from commit cb60b61) Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
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I think this is very surprising behaviour and it seems better to fail outright. Third, adding this new failure broke a couple tests. To fix them (and to e.g. allow `inspect.signature(select.epoll.register)` as in python#85267), I add constant folding of a couple binary operations to RewriteSymbolics. (There's some discussion of making signature expression evaluation arbitrary powerful in python#68155. I think that's out of scope. The additional constant folding here is pretty straightforward, useful, and not much of a slippery slope) Fourth, while python#85267 is incorrect about the cause of the issue, it turns out if you had consecutive newlines in __text_signature__, you'd get `tokenize.TokenError`. 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Add docstring. Factor-out common code. (pythonGH-100480) * pythongh-94808: improve test coverage of number formatting (python#99472) * pythongh-100454: Start running SSL tests with OpenSSL 3.1.0-beta1 (python#100456) * pythongh-100268: Add is_integer method to int (python#100439) This improves the lives of type annotation users of `float` - which type checkers implicitly treat as `int|float` because that is what most code actually wants. Before this change a `.is_integer()` method could not be assumed to exist on things annotated as `: float` due to the method not existing on both types. * pythongh-77771: Add enterabs example in sched (python#92716) Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com> * pythonGH-91166: Implement zero copy writes for `SelectorSocketTransport` in asyncio (python#31871) Co-authored-by: Guido van Rossum <gvanrossum@gmail.com> * pythonGH-91166: Implement zero copy writes for `SelectorSocketTransport` in asyncio (python#31871) Co-authored-by: Guido van Rossum <gvanrossum@gmail.com> * Misc Itertools recipe tweaks (pythonGH-100493) * pythongh-100357: Convert several functions in `bltinsmodule` to AC (python#100358) * Remove wrong comment about `repr` in `test_unicode` (python#100495) * pythongh-99908: Tutorial: Modernize the 'data-record class' example (python#100499) Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com> * pythongh-100474: Fix handling of dirs named index.html in http.server (pythonGH-100475) If you had a directory called index.html or index.htm within a directory, it would cause http.server to return a 404 Not Found error instead of the directory listing. This came about due to not checking that the index was a regular file. I have also added a test case for this situation. Automerge-Triggered-By: GH:merwok * pythongh-100287: Fix unittest.mock.seal with AsyncMock (python#100496) * pythongh-99535: Add test for inheritance of annotations and update documentation (python#99990) * pythongh-100428: Make float documentation more accurate (python#100437) Previously, the grammar did not accept `float("10")`. 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Co-authored-by: hauntsaninja <hauntsaninja@gmail.com> * pythongh-99308: Clarify re docs for byte pattern group names (python#99311) * pythongh-92446: Improve argparse choices docs; revert bad change to lzma docs (python#94627) Based on the definition of the collections.abc classes, it is more accurate to use "sequence" instead of "container" when describing argparse choices. A previous attempt at fixing this in python#92450 was mistaken; this PR reverts that change. Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com> * Fix name of removed `inspect.Signature.from_builtin` method in 3.11.0a2 changelog (python#100525) * pythongh-100520: Fix `rst` markup in `configparser` docstrings (python#100524) * pythongh-99509: Add `__class_getitem__` to `multiprocessing.queues.Queue` (python#99511) * pythongh-94603: micro optimize list.pop (pythongh-94604) * Remove `NoneType` redefinition from `clinic.py` (python#100551) * pythongh-100553: Improve accuracy of sqlite3.Row iter test (python#100555) * pythonGH-98831: Modernize a ton of simpler instructions (python#100545) * load_const and load_fast aren't families for now * Don't decref unmoved names * Modernize GET_ANEXT * Modernize GET_AWAITABLE * Modernize ASYNC_GEN_WRAP * Modernize YIELD_VALUE * Modernize POP_EXCEPT (in more than one way) * Modernize PREP_RERAISE_STAR * Modernize LOAD_ASSERTION_ERROR * Modernize LOAD_BUILD_CLASS * Modernize STORE_NAME * Modernize LOAD_NAME * Modernize LOAD_CLASSDEREF * Modernize LOAD_DEREF * Modernize STORE_DEREF * Modernize COPY_FREE_VARS (mark it as done) * Modernize LIST_TO_TUPLE * Modernize LIST_EXTEND * Modernize SET_UPDATE * Modernize SETUP_ANNOTATIONS * Modernize DICT_UPDATE * Modernize DICT_MERGE * Modernize MAP_ADD * Modernize IS_OP * Modernize CONTAINS_OP * Modernize CHECK_EXC_MATCH * Modernize IMPORT_NAME * Modernize IMPORT_STAR * Modernize IMPORT_FROM * Modernize JUMP_FORWARD (mark it as done) * Modernize JUMP_BACKWARD (mark it as done) Signed-off-by: Filipe Laíns <lains@archlinux.org> Signed-off-by: Filipe Laíns <lains@riseup.net> Co-authored-by: Jeremy Paige <ucodery@gmail.com> Co-authored-by: Carlton Gibson <carlton@noumenal.es> Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com> Co-authored-by: Jon Burdo <jon@jonburdo.com> Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com> Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com> Co-authored-by: Thomas Grainger <tagrain@gmail.com> Co-authored-by: Brad Wolfe <brad.wolfe@gmail.com> Co-authored-by: Richard Kojedzinszky <rkojedzinszky@users.noreply.github.com> Co-authored-by: František Nesveda <fnesveda@users.noreply.github.com> Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com> Co-authored-by: Nikita Sobolev <mail@sobolevn.me> Co-authored-by: Łukasz Langa <lukasz@langa.pl> Co-authored-by: Dennis Sweeney <36520290+sweeneyde@users.noreply.github.com> Co-authored-by: Bisola Olasehinde <horlasehinde@gmail.com> Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM> Co-authored-by: Pieter Eendebak <P.T.eendebak@tudelft.nl> Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com> Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com> Co-authored-by: Dominic Socular <BBH@awsl.rip> Co-authored-by: Serhiy Storchaka <storchaka@gmail.com> Co-authored-by: Hai Shi <shihai1992@gmail.com> Co-authored-by: amaajemyfren <32741226+amaajemyfren@users.noreply.github.com> Co-authored-by: Brandt Bucher <brandtbucher@microsoft.com> Co-authored-by: david-why <david_why@outlook.com> Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com> Co-authored-by: penguin_wwy <940375606@qq.com> Co-authored-by: Eli Schwartz <eschwartz93@gmail.com> Co-authored-by: Itamar Ostricher <itamarost@gmail.com> Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com> Co-authored-by: Eric Wieser <wieser.eric@gmail.com> Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com> Co-authored-by: Bill Fisher <william.w.fisher@gmail.com> Co-authored-by: Petr Viktorin <encukou@gmail.com> Co-authored-by: Ken Jin <kenjin@python.org> Co-authored-by: Carl Meyer <carl@oddbird.net> Co-authored-by: Filipe Laíns <lains@riseup.net> Co-authored-by: Raymond Hettinger <rhettinger@users.noreply.github.com> Co-authored-by: Barney Gale <barney.gale@gmail.com> Co-authored-by: Brett Cannon <brett@python.org> Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com> Co-authored-by: Eryk Sun <eryksun@gmail.com> Co-authored-by: Sebastian Berg <sebastianb@nvidia.com> Co-authored-by: Illia Volochii <illia.volochii@gmail.com> Co-authored-by: JosephSBoyle <48555120+JosephSBoyle@users.noreply.github.com> Co-authored-by: James Frost <git@frost.cx> Co-authored-by: MonadChains <monadchains@gmail.com> Co-authored-by: Bart Broere <mail@bartbroere.eu> Co-authored-by: Glyph <code@glyph.im> Co-authored-by: hauntsaninja <hauntsaninja@gmail.com> Co-authored-by: Ilya Kulakov <kulakov.ilya@gmail.com> Co-authored-by: Guy Yagev <yourlefthandman8@gmail.com> Co-authored-by: Jakub Kuczys <me@jacken.men>
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