./python -m test test_interpreters test_struct fails with: KeyError: concurrent.interpreters._queues.UNBOUND

Bug report

Bug description:

I routinely use on my Ubuntu machines pyenv and a script to install python versions:

Click to view

 python_ver=3.14.2

# setup pyenv and install latest python
# https://github.com/pyenv/pyenv
# https://github.com/python/pyperformance

if ! command -v pyenv &>/dev/null; then
  # dependencies according to pyenv
  #sudo apt update; sudo apt install -y build-essential libssl-dev zlib1g-dev libbz2-dev libreadline-dev libsqlite3-dev libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev

  # Workaround missing libmpdec-dev on ubuntu 24.04 (it should be re introduced in ubuntu 26.04):
  # https://launchpad.net/~ondrej/+archive/ubuntu/php
  # https://deb.sury.org/
  sudo add-apt-repository -y ppa:ondrej/php

  # dependencies according to python build doc dropping packages used in CI (ccache, cmake, gdb, inetutils-inetd, lcov, lzma, strace, xvfb)
  # https://devguide.python.org/getting-started/setup-building/index.html#build-dependencies
  # https://github.com/python/cpython/blob/main/.github/workflows/posix-deps-apt.sh
  sudo apt update; sudo apt install -y build-essential pkg-config libb2-dev libbz2-dev libffi-dev libgdbm-dev libgdbm-compat-dev liblzma-dev libncurses5-dev libreadline6-dev libsqlite3-dev libssl-dev lzma-dev tk-dev uuid-dev zlib1g-dev libzstd-dev libmpdec-dev

  pyenv_dir="$HOME/.pyenv"
  git clone https://github.com/pyenv/pyenv.git "$pyenv_dir"

  cat << 'EOF' >> "$HOME/.profile"

# pyenv: keep at the end of the file
export PYENV_ROOT="$HOME/.pyenv"
command -v pyenv >/dev/null || export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init -)"
EOF

  cat << 'EOF' >> "$HOME/.bashrc"

# pyenv: keep at the end of the file
export PYENV_ROOT="$HOME/.pyenv"
command -v pyenv >/dev/null || export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init -)"
EOF

  . "$HOME/.profile"
  pushd "$pyenv_dir" && src/configure && make -C src && popd
  #CC="clang" CXX="clang++" LDFLAGS="${LDFLAGS:+$LDFLAGS }-fuse-ld=lld" PYTHON_CONFIGURE_OPTS="--enable-optimizations --with-lto --with-tail-call-interp --enable-bolt" PYTHON_CFLAGS="-march=native -mtune=native" pyenv install "$python_ver"
  CC="clang" CXX="clang++" LDFLAGS="${LDFLAGS:+$LDFLAGS }-fuse-ld=lld" PYTHON_CONFIGURE_OPTS="--enable-optimizations --with-lto" PYTHON_CFLAGS="-march=native -mtune=native" pyenv install "$python_ver"
  #PYTHON_CONFIGURE_OPTS="--enable-optimizations --with-lto" PYTHON_CFLAGS="-march=native -mtune=native" pyenv install "$python_ver"
  pyenv global "$python_ver"
fi

test_struct fails in python3 -m test with python 3.14.2 (and 3.14.1) with this traceback:

Click to view

0:18:03 load avg: 0.84 [387/491] test_struct
Exception in initializer:
Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
test test_struct failed -- Traceback (most recent call last):
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/test/test_struct.py", line 818, in test_endian_table_init_subinterpreters
    self.assertListEqual(list(results), [None] * 5)
                         ~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/_base.py", line 639, in result_iterator
    yield _result_or_cancel(fs.pop())
          ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/_base.py", line 311, in _result_or_cancel
    return fut.result(timeout)
           ~~~~~~~~~~^^^^^^^^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/_base.py", line 450, in result
    return self.__get_result()
           ~~~~~~~~~~~~~~~~~^^
  File "/home/usr00/.pyenv/versions/3.14.2/lib/python3.14/concurrent/futures/_base.py", line 395, in __get_result
    raise self._exception
concurrent.futures.interpreter.BrokenInterpreterPool: A thread initializer failed, the thread pool is not usable anymore

0:18:04 load avg: 0.84 [387/491/1] test_struct failed (1 error)

Tested on Ubuntu 24.04 (gcc 14.2.0 - clang 21.1.8) and Ubuntu 20.04 (gcc 11.4.0), several CPUs.

python 3.14.0 and 3.13.x, bult in the same way, pass smoothly python3 -m test

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

In my tests this is sufficient to trigger the bug.
python3 -m test test_interpreters test_struct

This command is enough to trigger the issue:

$ cat bisect 
test.test_struct.StructTest.test_endian_table_init_subinterpreters
test.test_interpreters.test_queues.LowLevelTests.test_highlevel_reloaded

$ ./python -m test test_interpreters test_struct --matchfile=bisect

Short reproducer:

import importlib
import unittest
from concurrent.interpreters import _queues as queues

class Tests(unittest.TestCase):
    def test_step1(self):
        importlib.reload(queues)

    def test_step2(self):
        from concurrent.futures import InterpreterPoolExecutor

        code = "import struct"
        with InterpreterPoolExecutor(max_workers=1) as executor:
            results = executor.map(exec, [code] * 1)
            self.assertListEqual(list(results), [None] * 1)

if __name__ == "__main__":
    unittest.main()

where step 1 comes from test_highlevel_reloaded() which is related to the issue #115490.

@ericsnowcurrently

cc @ericsnowcurrently

I mark this issue as a release blocker since the release script runs tests sequentially.

Hitting this too while runing ./python Lib/test/test_runpy.py. (Thanks for OP to narrow down this to ./python -m test test_interpreters test_struct, I had a hard time to repro this in my end...)

I made a PR to fix this, not sure if the right fix though.

I think the root of the problem lies in the fact that we're doing some hacky things to implement a singleton for UNBOUND:

cpython/Lib/concurrent/interpreters/_crossinterp.py

Lines 41 to 59 in 5730fd5

    
           @classonly 
        
           def singleton(cls, kind, module, name='UNBOUND'): 
        
               doc = cls.__doc__ 
        
               if doc: 
        
                   doc = doc.replace( 
        
                       'cross-interpreter container', kind, 
        
                   ).replace( 
        
                       'cross-interpreter', kind, 
        
                   ) 
        
               subclass = type( 
        
                   f'Unbound{kind.capitalize()}Item', 
        
                   (cls,), 
        
                   { 
        
                       "_MODULE": module, 
        
                       "_NAME": name, 
        
                       "__doc__": doc, 
        
                   }, 
        
               ) 
        
               return object.__new__(subclass)

As this issue shows, these shenanigans are fragile. Ned has a great article on why singletons aren't a great idea in Python: Singleton is a bad idea. That article seems to apply here, so I'm in favor of eliminating singleton enforcement entirely (and just defining UNBOUND as an object(), which would fix the problem), but I'm not one to make Eric's decisions for him.

I also made one more PR based on the idea to remove singleton. I am worried that this one may change the original code's intention, thus I made 2 PR instead of 1. Please pick up the preferred one to continue the discussion, thanks!

Merge branch 'main' into pythongh-142414

I just found another case that stops working (running forever) after the attempted fixes:

./python -m test test_interpreters test_concurrent_futures.test_interpreter_pool

[Update] I managed to find a solution by just fixing the test + making the singleton object picklable.

I mark this issue as a release blocker since the release script runs tests sequentially.

Not any more, since python/release-tools#304.

Hey, folks. Thank you for troubleshooting the problem and narrowing it down. Since it's been a while and I am hitting the same issue, I took a crack at looking at the code. The singleton is a red herring. It's actually not even a singleton. 🙂

The issue is with reloading the single module in tests while other modules, that have already imported the variables, keeping the old references. This breaks identity checks, dictionary key lookups, and exception handling. Luckily isolating importlib.reload inside a subinterpreter seems to work, so I hope this means we can resolve this.

test_struct still fails with the same error in the new python 3.14.3 for all my builds (Ubuntu 22.04 and 24.04, gcc and clang).

Click to view

0:15:32 load avg: 0.80 [386/492] test_strtod
0:15:33 load avg: 0.80 [386/492] test_strtod passed
0:15:33 load avg: 0.80 [387/492] test_struct
Exception in initializer:
Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
flag = _UNBOUND_CONSTANT_TO_FLAG[op]
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
ctx.initialize()
~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
self.results = interpreters.create_queue(maxsize)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
unbound = _serialize_unbound(unbounditems)
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
return _crossinterp.serialize_unbound(unbound)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
flag = _UNBOUND_CONSTANT_TO_FLAG[op]
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
ctx.initialize()
~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
self.results = interpreters.create_queue(maxsize)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
unbound = _serialize_unbound(unbounditems)
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
return _crossinterp.serialize_unbound(unbound)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
flag = _UNBOUND_CONSTANT_TO_FLAG[op]
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
ctx.initialize()
~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
self.results = interpreters.create_queue(maxsize)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
unbound = _serialize_unbound(unbounditems)
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
return _crossinterp.serialize_unbound(unbound)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
flag = _UNBOUND_CONSTANT_TO_FLAG[op]
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
ctx.initialize()
~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
self.results = interpreters.create_queue(maxsize)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
unbound = _serialize_unbound(unbounditems)
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
return _crossinterp.serialize_unbound(unbound)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
flag = _UNBOUND_CONSTANT_TO_FLAG[op]
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/thread.py", line 99, in _worker
ctx.initialize()
~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/interpreter.py", line 64, in initialize
self.results = interpreters.create_queue(maxsize)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 75, in create
unbound = _serialize_unbound(unbounditems)
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_queues.py", line 56, in _serialize_unbound
return _crossinterp.serialize_unbound(unbound)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
test test_struct failed -- Traceback (most recent call last):
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/test/test_struct.py", line 824, in test_endian_table_init_subinterpreters
self.assertListEqual(list(results), [None] * 5)
~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/_base.py", line 639, in result_iterator
yield _result_or_cancel(fs.pop())
~~~~~~~~~~~~~~~~~^^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/_base.py", line 311, in _result_or_cancel
return fut.result(timeout)
~~~~~~~~~~^^^^^^^^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/_base.py", line 450, in result
return self.__get_result()
~~~~~~~~~~~~~~~~~^^
File "/home/usr00/.pyenv/versions/3.14.3/lib/python3.14/concurrent/futures/_base.py", line 395, in __get_result
raise self._exception
concurrent.futures.interpreter.BrokenInterpreterPool: A thread initializer failed, the thread pool is not usable anymore

0:15:33 load avg: 0.80 [387/492/1] test_struct failed (1 error)
0:15:33 load avg: 0.80 [388/492/1] test_structseq
0:15:34 load avg: 0.80 [388/492/1] test_structseq passed

@ppigazzini

Hey, @ppigazzini, while Core team is looking for the fix, I think the approach I suggested in gh-144096 could be used as a quick relief. You can temporarily hot patch with:
git fetch origin --depth 2 9208c1d95b194f3cea595251ca1b2b9d267b09cf && git show 9208c1d95b194f3cea595251ca1b2b9d267b09cf | git apply

Or just ignore while running tests: ./python -m test test_interpreters test_struct -i test.test_interpreters.test_queues.LowLevelTests.test_highlevel_reloaded -i test.test_interpreters.test_channels.LowLevelTests.test_highlevel_reloaded

@ericsnowcurrently

Gentle ping for review on the linked PR #144096: it isolates the module-reload side-effect causing the test_struct flake in this issue.

Change is minimal/safe. No runtime impact, just ensures that importlib.reload in tests doesn't pollute the rest of the suite. It preserves the original test intent from #115493.

This helps test-suite reliability (including PGO builds). CI green, merges cleanly.

@ericsnowcurrently (and anyone else interested), would appreciate a look when you can. Thanks!

pythongh-142414: Make value, not identity, matter for UNBOUND* objects

I'm having the same problem in the main branch. I built it like this:
CC=gcc CXX=g++ ./configure

boss@oops-lab ~/projects/oss/cpython/main (main %2⚑1)
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/15.2.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust,cobol --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues --with-build-config=bootstrap-lto --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-libstdcxx-backtrace --enable-link-serialization=1 --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 15.2.1 20260209 (GCC)

logs

0:30:25 load avg: 5.09 [397/501/4] test_struct
Exception in initializer:
Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 74, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 55, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 74, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 55, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 74, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 55, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 74, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 55, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
Exception in initializer:
Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 88, in serialize_unbound
    flag = _UNBOUND_CONSTANT_TO_FLAG[op]
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
KeyError: concurrent.interpreters._queues.UNBOUND

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/thread.py", line 99, in _worker
    ctx.initialize()
    ~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/interpreter.py", line 64, in initialize
    self.results = interpreters.create_queue(maxsize)
                   ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 74, in create
    unbound = _serialize_unbound(unbounditems)
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_queues.py", line 55, in _serialize_unbound
    return _crossinterp.serialize_unbound(unbound)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/interpreters/_crossinterp.py", line 90, in serialize_unbound
    raise NotImplementedError(f'unsupported unbound replacement op {op!r}')
NotImplementedError: unsupported unbound replacement op concurrent.interpreters._queues.UNBOUND
test test_struct failed -- Traceback (most recent call last):
  File "/home/boss/projects/oss/cpython/main/Lib/test/test_struct.py", line 1003, in test_endian_table_init_subinterpreters
    self.assertListEqual(list(results), [None] * 5)
                         ~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/_base.py", line 666, in result_iterator
    yield _result_or_cancel(fs.pop())
          ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/_base.py", line 311, in _result_or_cancel
    return fut.result(timeout)
           ~~~~~~~~~~^^^^^^^^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/_base.py", line 450, in result
    return self.__get_result()
           ~~~~~~~~~~~~~~~~~^^
  File "/home/boss/projects/oss/cpython/main/Lib/concurrent/futures/_base.py", line 395, in __get_result
    raise self._exception
concurrent.futures.interpreter.BrokenInterpreterPool: A thread initializer failed, the thread pool is not usable anymore

0:30:27 load avg: 5.09 [397/501/5] test_struct failed (1 error)

gh-142414: Isolate reloading modules in test_interpreters (#144096)

[3.14] gh-142414: Isolate reloading modules in test_interpreters (GH-…

FWIW I was able to produce the same issue on macOS with /python.exe -m test.
But changes from aa24c20 from seems to fixed it.

$ gcc -v
Apple clang version 21.0.0 (clang-2100.1.1.101)
Target: arm64-apple-darwin25.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

I merged the #144096 change to avoid side effects from the test cases that check that the _interpreters extension module tolerates the pure Python modules being reloaded.

Beyond that, our policy on module reloading is generally "If you hot reload a module without cleaning up everything that references it, you get to keep all the shiny pieces when it inevitably breaks". On those grounds, #142515 and #142577 would be closed as "don't do that" (where "that" is reloading concurrent.interpreters submodules without also reloading concurrent.interpreters itself to update the exported API objects).

However, having the two different UNBOUND singletons seems genuinely odd, so there may still be some genuine code cleanup worth implementing here.

Cleanup proposals can go in a separate issue that explains the benefits separately from this test failure (since the modified testing means we won't be hitting that anymore)

pythongh-142414: Isolate reloading modules in test_interpreters (pyth…

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`./python -m test test_interpreters test_struct` fails with: KeyError: concurrent.interpreters._queues.UNBOUND #142414

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	@classonly
	def singleton(cls, kind, module, name='UNBOUND'):
	doc = cls.__doc__
	if doc:
	doc = doc.replace(
	'cross-interpreter container', kind,
	).replace(
	'cross-interpreter', kind,
	)
	subclass = type(
	f'Unbound{kind.capitalize()}Item',
	(cls,),
	{
	"_MODULE": module,
	"_NAME": name,
	"__doc__": doc,
	},
	)
	return object.__new__(subclass)

./python -m test test_interpreters test_struct fails with: KeyError: concurrent.interpreters._queues.UNBOUND #142414

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Bug description:

CPython versions tested on:

Operating systems tested on:

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ppigazzini commented on Dec 10, 2025

vstinner commented on Dec 10, 2025

vstinner commented on Dec 10, 2025

vstinner commented on Dec 10, 2025

gkirchou commented on Dec 10, 2025

ZeroIntensity commented on Dec 10, 2025

note35 commented on Dec 11, 2025

note35 commented on Dec 11, 2025

hugovk commented on Dec 13, 2025

Allactaga commented on Jan 21, 2026

ppigazzini commented on Feb 3, 2026

Allactaga commented on Feb 4, 2026

Allactaga commented on Feb 21, 2026

ashm-dev commented on Mar 18, 2026

kesara commented on Aug 30, 2026

ncoghlan commented on Aug 30, 2026

ncoghlan commented on Aug 30, 2026

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`./python -m test test_interpreters test_struct` fails with: KeyError: concurrent.interpreters._queues.UNBOUND #142414