This patch makes it easy to pass file descriptor in using AF_UNIX.

Ruby language libraries have such methods.

see slso:
 - https://docs.ruby-lang.org/en/2.3.0/UNIXSocket.html#method-i-send_io

Hi, thanks for your contribution!

Documentation give examples implementation of your methods:

 - https://docs.python.org/3/library/socket.html#socket.socket.sendmsg 
 - https://docs.python.org/3/library/socket.html#socket.socket.recvmsg

and from here, some remarks:

 - Why allowing people to mix fds and file objects? I'm not a fan of this: I prefer the clarity of allowing only file descriptors. Have you seen such methods (allowing both fd and file obj) in the stdlib?
 - The documented implementation for recv_io checks for integer truncation, there may be a good reason, should'n you do the same?
 - The documented implementation allows to pass a message, shouldn't you at least allow for an optional message to be passed?

Adding those methods may make sense if people are copying send_fds/recv_fds in their code, but I found only two copy/paste in github, so I think the best is to put your code as a module on pypi and see, from here, if it gets popular?

You should also write a few tests, and comply to the PEP8 (your two methods should probably be separated by a newline).

Bests

FYI, multiprocessing.reduction module has send_fds and recv_fds.

Unless somebody don't think so, I think this should go as a pypi module before going to the socket module, so this issue should probably be closed.

Good idea! Initially I planned to add the functions and some other helpers around AF_UNIX to Python 3.6. You can directly copy the example functions from the documentation.

https://docs.python.org/3/library/socket.html#socket.socket.sendmsg
https://docs.python.org/3/library/socket.html#socket.socket.recvmsg

Please keep the names send_fds() and recv_fds(), and pass the message like in the examples. Some protocols use the payload to signal the count of fds. Your patch is also lacking unit tests and documentation update.

Thanks for reviews and comments. I will work on the weekend.

Take your time, feature freeze for 3.7 is in about 18 months from now.

See also bpo-37385: "test_multiprocessing fails on AMD64 FreeBSD CURRENT Shared 2.7".

New changeset 8d120f75fb8c8731464b5f7531d74cdbb897d924 by Victor Stinner (Joannah Nanjekye) in branch 'master':
bpo-28724: Add methods send_fds and recv_fds to the socket module (GH-12889)
https://github.com/python/cpython/commit/8d120f75fb8c8731464b5f7531d74cdbb897d924

Thanks Shinya Okano for the original patch.

Well done Joannah! Thanks for your tenacity :-) This PR has 100 comments and 27 commits which shows the complexity of the feature.

Honestly, I'm not 100% happy with current documentation, but I chose to merge the PR anyway. Please leave this issue open until the documentation is completed to mention corner cases like partial send (similar to sock.send vs sock.sendall).

I was looking at adding stubs for these to typeshed. Is it intentional that we ignore the flags and address arguments in the implementation?

New changeset 660592f67c0d825b06eb4a38ccc04b34a1f01f00 by Saiyang Gou in branch 'master':
bpo-28724: Doc: Move socket.send_fds and socket.recv_fds docs to right section (GH-22608)
https://github.com/python/cpython/commit/660592f67c0d825b06eb4a38ccc04b34a1f01f00

New changeset 87a392db40d52cba18ffc67730856e11cb8cf945 by Miss Islington (bot) in branch '3.9':
bpo-28724: Doc: Move socket.send_fds and socket.recv_fds docs to right section (GH-22608)
https://github.com/python/cpython/commit/87a392db40d52cba18ffc67730856e11cb8cf945