3.14rc2 segfaults when running MkDocs

[pawamoy]

Crash report

What happened?

mkdir /tmp/test-mkdocs
cd /tmp/test-mkdocs
uv venv --python 3.14.0rc2
uv pip install mkdocs
. .venv/bin/activate
mkdocs new .  # crashes

If we were to run mkdocs new . from another Python version, then following commands like mkdocs serve or mkdocs build would crash too.

Note that this isn't new with 3.14.0rc2, previous 3.14 versions would segfault too. I'm just reporting now, sorry 😄

Very happy to provide more information if you let me know how to do that 😄

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0rc2 (main, Aug 14 2025, 16:22:12) [Clang 20.1.4 ]

[StanFromIreland]

What is the traceback? How do you know this is not an issue with MkDocs, has it been reported to them?

[pawamoy]

There's no traceback, it just segfaults. But maybe I'm missing something. I don't know that it's not an issue with MkDocs. I suspect that the Python interpreter segfaulting is generally a CPython issue, not a tool/library one given this tool/library doesn't mess with CPython's internals (which MkDocs doesn't indeed). But again, maybe I'm missing something 🙂 I can report it there too if you want.

[pawamoy]

For some reason I was suspecting that the segfault comes from PyYaml, and it seems I was right:

% python
Python 3.14.0rc2 (main, Aug 14 2025, 16:22:12) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yaml
[1]    25288 segmentation fault (core dumped)  python

(MkDocs does depend on and import PyYaml)

So maybe it's just an issue on PyYaml's side.

[StanFromIreland]

PyYaml does not support 3.14, per PyPI, possibly the source of the issue?

[pawamoy]

Do you infer that from the classifiers? Package managers seem OK with resolving and installing it on 3.14, so I thought it was deliberate from PyYaml's devs (it's easy to forget to update classifiers 😄), but maybe not.

[pawamoy]

Oh, I see they don't provide wheels for 3.14 indeed. I suppose uv builds it from source then 🤔? Feel free to close if that's enough hint that the issue lies in PyYaml and not CPython 🙂 (and thank you so much for the extremely fast responses!)

[johnslavik]

I can't reproduce the provided minimal repro with mkdocs.

I can't reproduce the segfault upon import yaml in GCC build:

Details

❯ uv pip install pyyaml
Resolved 1 package in 1ms
Installed 1 package in 2ms
 + pyyaml==6.0.2

❯ .venv/bin/python -c 'import yaml'

❯ .venv/bin/python -VV
Python 3.14.0rc2+ (heads/3.14:78ee4868617, Sep  7 2025, 15:28:24) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)]

❯ uv pip install mkdocs
Resolved 16 packages in 319ms
Prepared 1 package in 43ms
Installed 15 packages in 14ms
 + click==8.2.1
 + ghp-import==2.1.0
 + jinja2==3.1.6
 + markdown==3.9
 + markupsafe==3.0.2
 + mergedeep==1.3.4
 + mkdocs==1.6.1
 + mkdocs-get-deps==0.2.0
 + packaging==25.0
 + pathspec==0.12.1
 + platformdirs==4.4.0
 + python-dateutil==2.9.0.post0
 + pyyaml-env-tag==1.1
 + six==1.17.0
 + watchdog==6.0.0

❯ .venv/bin/python
Python 3.14.0rc2+ (heads/3.14:78ee4868617, Sep  7 2025, 15:28:24) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yaml
>>> exit

I rebuilt with Clang 20.1.8, can't reproduce as well (pyyaml 6.0.2 as well)

Details

❯ uv pip install mkdocs
Resolved 16 packages in 3ms
Installed 15 packages in 11ms
 + click==8.2.1
 + ghp-import==2.1.0
 + jinja2==3.1.6
 + markdown==3.9
 + markupsafe==3.0.2
 + mergedeep==1.3.4
 + mkdocs==1.6.1
 + mkdocs-get-deps==0.2.0
 + packaging==25.0
 + pathspec==0.12.1
 + platformdirs==4.4.0
 + python-dateutil==2.9.0.post0
 + pyyaml-env-tag==1.1
 + six==1.17.0
 + watchdog==6.0.0

❯ .venv/bin/python
Python 3.14.0rc2+ (heads/3.14:78ee4868617, Sep  7 2025, 15:34:51) [Clang 20.1.8 (Fedora 20.1.8-4.fc42)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yaml
>>> exit

I'm sure an sdist was used in still successful path:

❯ .venv/bin/python -VV
Python 3.14.0rc2+ (heads/3.14:78ee4868617, Sep  7 2025, 15:34:51) [Clang 20.1.8 (Fedora 20.1.8-4.fc42)]

❯ uv pip install --python .venv pyyaml --force-reinstall --no-binary pyyaml
Resolved 1 package in 124ms
Prepared 1 package in 0.39ms
Uninstalled 1 package in 1ms
Installed 1 package in 3ms
 ~ pyyaml==6.0.2

❯ uv pip show pyyaml
Name: pyyaml
Version: 6.0.2
Location: /home/bswck/Python/cpython/.venv/lib/python3.14/site-packages
Requires:
Required-by: mkdocs, mkdocs-get-deps, pyyaml-env-tag

❯ .venv/bin/python
Python 3.14.0rc2+ (heads/3.14:78ee4868617, Sep  7 2025, 15:34:51) [Clang 20.1.8 (Fedora 20.1.8-4.fc42)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import yaml
>>> exit

[johnslavik]

@pawamoy can you show the output of python -X faulthandler -c 'import yaml'? Maybe that helps just a little bit. For which version of pyyaml and mkdocs does this crash happen for, exactly?

[pawamoy]

Sure, thanks so much @bswck 😄

% uv run python -X faulthandler -c 'import yaml'
Fatal Python error: Segmentation fault

Current thread 0x00007fc488f69780 [python3] (most recent call first):
  File "<frozen importlib._bootstrap>", line 491 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 1064 in exec_module
  File "<frozen importlib._bootstrap>", line 938 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1342 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1371 in _find_and_load
  File "/tmp/test-yaml/.venv/lib/python3.14/site-packages/yaml/cyaml.py", line 7 in <module>
  File "<frozen importlib._bootstrap>", line 491 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 762 in exec_module
  File "<frozen importlib._bootstrap>", line 938 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1342 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1371 in _find_and_load
  File "/tmp/test-yaml/.venv/lib/python3.14/site-packages/yaml/__init__.py", line 13 in <module>
  File "<frozen importlib._bootstrap>", line 491 in _call_with_frames_removed
  File "<frozen importlib._bootstrap_external>", line 762 in exec_module
  File "<frozen importlib._bootstrap>", line 938 in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1342 in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 1371 in _find_and_load
  File "<string>", line 1 in <module>

Current thread's C stack trace (most recent call first):
  Binary file "/tmp/test-yaml/.venv/bin/python3", at _Py_DumpStack+0x27 [0x4c1a47]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x54e296]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x54e19a]
  Binary file "/usr/lib/libc.so.6", at +0x3e540 [0x7fc488c3e540]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyUnstable_Code_NewWithPosOnlyArgs+0x224 [0x1c9e2a4]
  Binary file "/tmp/test-yaml/.venv/lib/python3.14/site-packages/yaml/_yaml.cpython-314-x86_64-linux-gnu.so", at +0x7d42 [0x7fc488693d42]
  Binary file "/tmp/test-yaml/.venv/lib/python3.14/site-packages/yaml/_yaml.cpython-314-x86_64-linux-gnu.so", at +0x8028 [0x7fc488694028]
  Binary file "/tmp/test-yaml/.venv/lib/python3.14/site-packages/yaml/_yaml.cpython-314-x86_64-linux-gnu.so", at +0x97cc [0x7fc4886957cc]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyModule_ExecDef+0x8a [0x1b39a90]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x4a1ec9]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a61e7e]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a15299]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a43c06]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a43b46]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyObject_CallMethodObjArgs+0xa6 [0x1a43a26]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyImport_ImportModuleLevelObject+0x5fa [0x1a2163a]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a1b048]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a80361]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyEval_EvalCode+0xa7 [0x1a8006f]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1acc047]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a61e7e]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a15299]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a43c06]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a43b46]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyObject_CallMethodObjArgs+0xa6 [0x1a43a26]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyImport_ImportModuleLevelObject+0x5fa [0x1a2163a]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a1b048]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a80361]
  Binary file "/tmp/test-yaml/.venv/bin/python3", at PyEval_EvalCode+0xa7 [0x1a8006f]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1acc047]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a61e7e]
  Binary file "/tmp/test-yaml/.venv/bin/python3" [0x1a15299]
  <truncated rest of calls>

Extension modules: yaml._yaml (total: 1)

Note that my local version of Python 3.14.0rc2 is the one provided by uv (originally the indygreg's standalone builds), see https://github.com/astral-sh/python-build-standalone.

[johnslavik]

I see. I've used the standalone build but I'm not getting yaml._yaml (the C extension), and you are. I'll try to force yaml._yaml to be installed. That's why if I'm getting False on python -c 'import yaml; print(yaml.__with_libyaml__)' (and you get a segfault) and you should get a sensible import spec from python -c 'import importlib; print(importlib.find_spec("yaml._yaml"))'.

[ZeroIntensity]

Let's shift discussion to the PyYAML issue yaml/pyyaml#884