[New-bugs-announce] [issue28134] socket.socket(fileno=fd) does not work as documented

Christian Heimes report at bugs.python.org
Tue Sep 13 15:13:31 EDT 2016

New submission from Christian Heimes:

Documentation of socket.socket(fileno) https://docs.python.org/3/library/socket.html#socket.socket says:

> If fileno is specified, the other arguments are ignored, causing the socket with the specified file descriptor to return.

The feature does not work. fileno does not infer the values for family, type and proto from the fd. Instead if uses the other arguments. I don't see how this feature should have ever worked on POSIX. There are no calls to getsockopt() with SO_DOMAIN, SO_TYPE and SO_PROTOCOL.

$ ./python 
Python 3.7.0a0 (default:6bcedf96d25f, Sep 13 2016, 20:48:50) 
[GCC 6.1.1 20160621 (Red Hat 6.1.1-3)] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> import socket
>>> uds = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
>>> uds
<socket.socket fd=3, family=AddressFamily.AF_UNIX, type=SocketKind.SOCK_STREAM, proto=0>
>>> s = socket.socket(fileno=uds.fileno())
>>> s
<socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0>
>>> s.family == uds.family
>>> s2 = socket.socket(type=socket.SOCK_DGRAM, fileno=uds.fileno())
>>> s2
<socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketKind.SOCK_DGRAM, proto=0>

messages: 276327
nosy: christian.heimes
priority: normal
severity: normal
status: open
title: socket.socket(fileno=fd) does not work as documented
versions: Python 3.5, Python 3.6, Python 3.7

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