If this is the proper way to do it, this PR should also fix the default values for poll.register and devpoll.register similarly.

Oops, I didn't notice that you proposed a PR and so I wrote PR #21067 which is the same fix :-) Copy of my comment there.

inspect.signature() doesn't see the event_mask. Is it a bug in signature which doesn't support a signature written on two lines?

cc @serhiy-storchaka @1st1 @ambv

11:47:01 vstinner@apu$ ./python 
Python 3.10.0a0 (heads/pydoc_select:486e11db95, Jun 23 2020, 11:45:39) 
>>> import select
>>> select.epoll.register.__text_signature__
'($self, /, fd,\n         eventmask=select.EPOLLIN | select.EPOLLPRI | select.EPOLLOUT)'

>>> import inspect
>>> inspect.signature(select.epoll.register)
<Signature (self, /, fd)>

So pydoc doesn't show the whole signature:

./python -m pydoc select
(...)
     |  register(self, /, fd)
     |      Registers a new fd or raises an OSError if the fd is already registered.
     |      
     |        fd
     |          the target file descriptor of the operation
     |        eventmask
     |          a bit set composed of the various EPOLL constants
     |      
     |      The epoll interface supports all file descriptors that support poll.

If this is the proper way to do it, this PR should also fix the default values for poll.register and devpoll.register similarly.

IMO the fix is correct, so I concur thta the two other functions signature should also be fixed.

By the way, I suggest to rephase the NEWS entry to explain that the change fix the "signature" rather than fixing "pydoc".

I fixed a few others as well -- good catch

Thanks @asottile for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9.
🐍🍒⛏🤖

Sorry, @asottile and @vstinner, I could not cleanly backport this to 3.9 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker d051801052211b533c46a593b1c1bccf649a171c 3.9

Sorry @asottile and @vstinner, I had trouble checking out the 3.8 backport branch.
Please backport using cherry_picker on command line.
cherry_picker d051801052211b533c46a593b1c1bccf649a171c 3.8

@asottile: Automated backport failed. Can you try to backport it manually (use cherry_picker or git cherry-pick -x)?

GH-21097 is a backport of this pull request to the 3.9 branch.

GH-21098 is a backport of this pull request to the 3.8 branch.

pydoc doesn't show the eventmask parameter whereas it is in select.epoll.register.text_signature. I created bpo-41095: "inspect.signature() doesn't parse text_signature containing a newline character".