Can't pickle `sre_constants`

[charmoniumQ]

Bug report

In [30]: import pickle, sre_constants

In [31]: a = pickle.dumps(sre_constants.SUCCESS)

In [32]: pickle.loads(a)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-32-7038544c8e94> in <cell line: 1>()
----> 1 pickle.loads(a)

TypeError: __new__() missing 1 required positional argument: 'name'

Even if we decide sre_constants.* are not picklable, the TypeError should happen when trying to dump, not when trying to load. This could cause a user to write pickle files they can't ever read.

Your environment

• CPython versions tested on: 3.9, 3.10
• Operating system and architecture: Ubuntu 20.04 x86_64

[serhiy-storchaka]

It is equally easy to fix pickling these constants, and to make them explicitly non-pickleable.

They were pickleable before 3.5, but it was broken so long ago, that it does not make sense to fix pickleability in 3.10 and 3.9. So I am going to make them explicitly non-pickleable in 3.10 and 3.9 and fix pickling in 3.11.

[serhiy-storchaka]

On other hand, there is no reason of making them pickleable. It is an implementation detail, and they are defined in a private module.