[issue22995] Restrict default pickleability

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

Your explanation still doesn't entirely make sense to me yet because the crash is happening in what seems to be a Cython implementation class, not a generated class based on a .pyx in some other project using Cython.

I tried to search the Cython Trac but either my Trac-fu sucks or nobody has yet reported compatibility problems with Python 3.5.1 to upstream.  I don't see any other relevant bug reports, and Cython's git head passes its test suite with Python 3.5.1 (other than the asyncio_generators test which is probably unrelated and has already been reported to the Cython mailing list).

It's always possible of course that the Python change is triggering a bug not caught by the Cython test suite, but something as fundamental as pickling would be surprising if not tested.

Turning the error into a warning does have a lot of appeal for 3.5 maintenance.  It could be kept as an error in 3.6.  At the least, can the exception text include more information about *why* the pickling failure occurs. (Note that the exact error message can be raised in multiple places, so I had to build a custom 3.5.1 to distinguish between them; I don't have gdb set up in the build environment yet.)

It's still worth better understanding why this change is causing the bugs in projects using Cython, and reviewing what happens with Python 3.5.0.  For completeness, I also want to verify that there isn't some problem specifically related to Ubuntu's version of Python 3.5.1, although the tests against Cython's git head use the same version.

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<http://bugs.python.org/issue22995>
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