argparse.FileType.__call__ returns unwrapped sys.stdin and stdout

[keviv]

BPO	22884
Nosy	@bitdancer
Files	argparse.patch

^{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 2014-11-16.14:58:06.624>
labels = ['type-bug', 'library']
title = 'argparse.FileType.__call__ returns unwrapped sys.stdin and stdout'
updated_at = <Date 2014-11-30.01:20:34.781>
user = 'https://bugs.python.org/keviv'

bugs.python.org fields:

activity = <Date 2014-11-30.01:20:34.781>
actor = 'paul.j3'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2014-11-16.14:58:06.624>
creator = 'keviv'
dependencies = []
files = ['37209']
hgrepos = []
issue_num = 22884
keywords = ['patch']
message_count = 4.0
messages = ['231249', '231251', '231254', '231874']
nosy_count = 3.0
nosy_names = ['r.david.murray', 'paul.j3', 'keviv']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22884'
versions = ['Python 3.4', 'Python 3.5']

[keviv]

When one uses a file object returned by FileType.__call__ as a context manager, sys.stdin's or sys.stdout's __exit__ will be triggered upon exit of the context, in turn calling their close method.

Perhaps the issue is that sys.stdin and sys.stdout have poor __exit__ methods, but my proposal (and it's not a particularly clean one) is to override the file object's __exit__ if it happens to be either sys.stdin or sys.stdout to simply return True when called.

[keviv]

Hmm, not sure why I thought I needed inline code formatting. It's all monospace anyway!

Anyway, I'm thinking that adding this somewhere in argparse.py:

class _WrappedIO(object):
    def __init__(self, fileobj):
        self._file = fileobj

    def __exit__(tp, val, tb):
        return True

    def __getattr__(self, name):
        return self._file.__gettattr__(name)

and then applying the attached patch *may* fix this.

[bitdancer]

Perhaps instead it could return an io object built from the file descriptor with closefd=False.

[paulj3]

Related issues are:

http://bugs.python.org/issue13824 - argparse.FileType opens a file and never closes it
http://bugs.python.org/issue14156 - argparse.FileType for '-' doesn't work for a mode of 'rb'

As discussed earlier FileType was meant as a convenience for small scripting applications, ones that don't try to close the file before exiting.

But now we are encouraged to open files in a context that guarantees closure. In 13824 I proposed a 'FileContext', and included a dummy context like this handle stdin/out.

But I think the closefd=False approach raised in 14156 and David is probably the better way to go.

I think the main thing that is lacking is a testing mechanism.

[serhiy-storchaka]

FileType has been deprecated in 3.14.