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Add --without-decimal-contextvar option to use just threads in decimal #83975

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Add --without-decimal-contextvar option to use just threads in decimal#83975
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skrah
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3.7 (EOL)end of life3.8 (EOL)end of life3.9 (EOL)end of lifeextension-modulesC modules in the Modules dirtype-featureA feature request or enhancement

Description

@skrah
skrah
mannequin
opened on Feb 29, 2020
BPO 39794
Nosy @rhettinger, @gpshead, @mdickinson, @skrah
PRs
  • bpo-39794: Add --without-decimal-contextvar #18702
  • [3.8] bpo-39794: Add --without-decimal-contextvar (GH-18702) #18713
  • [3.7] bpo-39794: Add --without-decimal-contextvar (GH-18702) #18714
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = 'https://github.com/skrah'
closed_at = None
created_at = <Date 2020-02-29.11:24:13.696>
labels = ['extension-modules', '3.8', 'type-bug', '3.7', '3.9']
title = 'Add --without-decimal-contextvar option to use just threads in decimal'
updated_at = <Date 2020-03-03.15:31:20.696>
user = 'https://github.com/skrah'

    bugs.python.org fields:

    activity = <Date 2020-03-03.15:31:20.696>
actor = 'skrah'
assignee = 'skrah'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2020-02-29.11:24:13.696>
creator = 'skrah'
dependencies = []
files = []
hgrepos = []
issue_num = 39794
keywords = ['patch']
message_count = 11.0
messages = ['362971', '362972', '363003', '363016', '363019', '363092', '363111', '363150', '363154', '363163', '363268']
nosy_count = 5.0
nosy_names = ['rhettinger', 'gregory.p.smith', 'mark.dickinson', 'skrah', 'boytsovea']
pr_nums = ['18702', '18713', '18714']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39794'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

    Activity

    1. skrah commented on Feb 29, 2020

      @skrah
      skrahmannequin
      on Feb 29, 2020
      MannequinAuthor

      bpo-39776 has shown that it is hard to understand the interaction between
      ContextVars and threading in embedded scenarios.

      I want to understand the code again, so I'm adding back a compile time
      option to enable the thread local context that was present prior to
      f13f12d.

    2. skrah
      added
      3.7 (EOL)end of life
      3.8 (EOL)end of life
      3.9 (EOL)end of life
      extension-modulesC modules in the Modules dir
      on Feb 29, 2020
    3. skrah
      self-assigned this
      on Feb 29, 2020
    4. skrah
      added
      type-bugAn unexpected behavior, bug, or error
      3.7 (EOL)end of life
      3.8 (EOL)end of life
      3.9 (EOL)end of life
      extension-modulesC modules in the Modules dir
      on Feb 29, 2020
    5. skrah
      self-assigned this
      on Feb 29, 2020
    6. skrah
      added
      type-bugAn unexpected behavior, bug, or error
      on Feb 29, 2020

    7. skrah commented on Feb 29, 2020

      @skrah
      skrahmannequin
      on Feb 29, 2020
      MannequinAuthor

      Also, when I'm debugging things like bpo-39776, I don't want to
      switch between Python versions.

    8. skrah commented on Feb 29, 2020

      @skrah
      skrahmannequin
      on Feb 29, 2020
      MannequinAuthor

      New changeset 815280e by Stefan Krah in branch 'master':
      bpo-39794: Add --without-decimal-contextvar (bpo-18702)
      815280e

    9. skrah commented on Feb 29, 2020

      @skrah
      skrahmannequin
      on Feb 29, 2020
      MannequinAuthor

      New changeset 4d70124 by Stefan Krah in branch '3.8':
      [3.8] bpo-39794: Add --without-decimal-contextvar (GH-18702)
      4d70124

    10. skrah commented on Feb 29, 2020

      @skrah
      skrahmannequin
      on Feb 29, 2020
      MannequinAuthor

      New changeset c4ca1f8 by Stefan Krah in branch '3.7':
      [3.7] bpo-39794: Add --without-decimal-contextvar (GH-18702)
      c4ca1f8

    11. gpshead commented on Mar 1, 2020

      @gpshead
      gpshead
      on Mar 1, 2020
      Member

      While i don't think this was needed in 3.7 and 3.8, you kept the default behavior the same so I don't think it is a problem.

      If someone builds an interpreter with this configure flag, does it break compatibility with anything that code may have started to expect as of 3.7?

    12. rhettinger commented on Mar 2, 2020

      @rhettinger
      rhettinger
      on Mar 2, 2020
      Contributor

      +1 for the backport. A build option isn't new feature, but it does provide a debugging tool and work-around for people who are having problems.

    13. skrah commented on Mar 2, 2020

      @skrah
      skrahmannequin
      on Mar 2, 2020
      MannequinAuthor

      For the people who find this issue via a search engine:

      1. The defaults in 3.7, 3.8 and 3.9 are unchanged (you get
        the ContextVar).

      2. ./configure --without-decimal-contextvar enables the exact
        TLS context code from 3.3-3.6. There is no new code.

      3. On Windows you need to edit PC\pyconfig.h to enable the TLS
        context.

      4. There are extensive tests for the flag in:

        Modules/_decimal/tests/runall-memorydebugger.sh

    14. skrah commented on Mar 2, 2020

      @skrah
      skrahmannequin
      on Mar 2, 2020
      MannequinAuthor

      If someone builds an interpreter with this configure flag, does it break
      compatibility with anything that code may have started to expect as of 3.7?

      Yes, anything that relies on the implicit context being coroutine-safe
      does not work. The only test that expectedly breaks in the stdlib is:

      Lib/test/test_asyncio/test_context.py

      Basically you can't use operators inside coroutines in the same thread
      if both coroutines use a different implicit context.

      You can of course replace all operators inside coroutines by the
      corresponding context methods:

         # Unsafe with TLS context:
   async def foo():
       with localcontext(Context(prec=10)):
           x = Decimal(1) / 9

   # Safe, just avoid the TLS context:
   async def foo():
       c = Context(prec=10)
       x = c.divide(Decimal(1), 9)

    15. skrah commented on Mar 2, 2020

      @skrah
      skrahmannequin
      on Mar 2, 2020
      MannequinAuthor

      To make things more clear (it is good that Gregory asked):

      ONLY use this flag if you don't use coroutines at all
      OR control ALL async libraries in your application.

      Random async libraries from PyPI may rely on the
      ContextVar and silently give incorrect results.

    16. skrah commented on Mar 3, 2020

      @skrah
      skrahmannequin
      on Mar 3, 2020
      MannequinAuthor

      bpo-39776 is fixed, but _pydecimal was also affected, see msg363266.

    17. ezio-melotti
      transferred this issue fromon Apr 10, 2022
    18. serhiy-storchaka
      added this to Threading issues 🧵on May 20, 2022
    19. serhiy-storchaka
      added
      type-featureA feature request or enhancement
      and removed
      type-bugAn unexpected behavior, bug, or error
      on Jul 3, 2026

    20. serhiy-storchaka commented on Jul 3, 2026

      @serhiy-storchaka
      serhiy-storchaka
      on Jul 3, 2026
      Member

      The work was completed and backported in 2020, and the option was later removed in 3.13. Closing as completed.

    21. serhiy-storchaka
      closed this as completedon Jul 3, 2026
    22. github-project-automation
      moved this to Done in Threading issues 🧵on Jul 3, 2026
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