The tp_clear handler of the module object calls a custom clear function if the PyModuleDef.m_clear field is set. This function can set an exception which will be leaked to the garbage collector. An exception in tp_clear is not expected and caused a crash in the garbage collector. In the master branch it will cause just writing a traceback to stderr (see issue33622), but in any case it would be better to handle the failure locally in the module's tp_clear.

I'm not sure what tp_clear should do in this situation.
Other than propagating the exception to the GC, the most reasonable behavior seems to be to write the exception to stderr and ignore it -- but I think having the GC do that would be more robust.

IOW, I think raising an exception from tp_clear is reasonable, and if that caused (causes?) a crash, it's a bug.

I've attached a reproduction case. Here's the setup.py that I used:

```
from distutils.core import Extension, setup

extension = Extension("breaky", ["breaky.c"])

setup(
    name="tp_clear",
    version="0.1.0",
    ext_modules=[extension],
)
```

which I built and ran with:
```
python setup.py build_ext --inplace
python -c "import breaky, gc; a = breaky.Breaky(); a = 1"
```

I've just realised that this issue was specific to tp_clear on a module and not on objects. tp_clear on modules has already been fixed in https://bugs.python.org/issue33622.
I think this is closable.

New changeset d7c387384a27f37e4e3fa7890c859212c56b45b2 by Miss Islington (bot) (Serhiy Storchaka) in branch 'master':
bpo-33714: Output an exception raised in module's m_clear(). (GH-16592)
https://github.com/python/cpython/commit/d7c387384a27f37e4e3fa7890c859212c56b45b2

Indeed, it can be closed.
Thanks Serhiy for the better error message!

stage: patch review -> resolved