Should posix functions that accept fd also accept objects with .fileno()?

[larryhastings]

BPO	20431
Nosy	@vstinner, @larryhastings

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2014-01-29.03:48:20.821>
labels = ['type-feature']
title = 'Should posix functions that accept fd also accept objects with .fileno()?'
updated_at = <Date 2019-06-20.16:41:32.260>
user = 'https://github.com/larryhastings'

bugs.python.org fields:

activity = <Date 2019-06-20.16:41:32.260>
actor = 'nanjekyejoannah'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2014-01-29.03:48:20.821>
creator = 'larry'
dependencies = []
files = []
hgrepos = []
issue_num = 20431
keywords = []
message_count = 2.0
messages = ['209613', '209614']
nosy_count = 3.0
nosy_names = ['vstinner', 'larry', 'Arfrever']
pr_nums = []
priority = 'low'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20431'
versions = ['Python 3.5']

[larryhastings]

While working on posixmodule.c today I noticed that some functions that accept file descriptors call PyObject_AsFileDescriptor(). This function accepts ints. It *also* accepts objects that expose a .fileno() attribute, calling that attribute and using the integer it returns.

There are *other* functions in posixmodule.c that accept file descriptors that call _fd_converter(), which just looks for an int with a proper value.

I figure it should be one way or the other. Should everything in posix that accepts file descriptor ints accept the .fileno() convention too?

[larryhastings]

The functions that use PyObject_AsFileDescriptor() also call _PyVerify_fd(), which magically determines whether or not the file descriptor is valid. Only works on Windows though.