Add contextlib.redirect_stderr()

[warsaw]

BPO	22389
Nosy	@warsaw, @rhettinger, @ncoghlan, @vstinner, @berkerpeksag, @1st1
Files	issue22389.diff issue22389_v2.diff issue22389_v3.diff

^{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 = 'https://github.com/berkerpeksag'
closed_at = <Date 2014-11-28.21:29:02.364>
created_at = <Date 2014-09-11.14:25:13.633>
labels = ['type-feature', 'library']
title = 'Add contextlib.redirect_stderr()'
updated_at = <Date 2014-11-28.21:29:02.363>
user = 'https://github.com/warsaw'

bugs.python.org fields:

activity = <Date 2014-11-28.21:29:02.363>
actor = 'berker.peksag'
assignee = 'berker.peksag'
closed = True
closed_date = <Date 2014-11-28.21:29:02.364>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2014-09-11.14:25:13.633>
creator = 'barry'
dependencies = []
files = ['36608', '36844', '36874']
hgrepos = []
issue_num = 22389
keywords = ['patch']
message_count = 17.0
messages = ['226774', '226775', '226776', '226810', '226811', '226814', '226825', '227656', '227657', '227658', '227792', '228839', '228840', '228878', '229062', '229109', '231830']
nosy_count = 8.0
nosy_names = ['barry', 'rhettinger', 'ncoghlan', 'vstinner', 'python-dev', 'berker.peksag', 'yselivanov', 'John Isidore']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22389'
versions = ['Python 3.5']

[warsaw]

redirect_stdout is almost exactly what I want, except I want to redirect stderr! redirect_stdout.__init__() should take a 'stream_name' argument (possibly keyword-only) which could be set to 'stderr'. I propose it's implemented as setattr(sys, stream_name, new_target)

[vstinner]

Why not adding a new redirect_stderr() function?

[warsaw]

On Sep 11, 2014, at 02:25 PM, STINNER Victor wrote:

Why not adding a new redirect_stderr() function?

With a little refactoring redirect_stdout into a subclass, that would work
too.

[rhettinger]

+1 on Victor's suggestion. I don't think hypergeneralizing it is the way to go. That adds too much complexity for too little benefit.

[vstinner]

redirect_stdout("stderr", stream) looks wrong to be: you want to redirect "stdout" or "stderr"?

If you want to redirect something else (ex: stdin), you can still implement the very simple pattern:

old_stdin = sys.stdin
try:
  sys.stdin = mock_input
  ...
finally:
  sys.stdin = old_stdin

By the way, I'm not convinced that we should add redirect_stderr.

@barry: How many usage of this new functions do you see in the standard library?

[ncoghlan]

I'm fine with adding "redirect_stderr" - better to have the obvious
counterpart, rather than hypergeneralising, or having to explain why it's
missing. It's *currently* missing largely on a "wait for someone to ask"
basis, and Barry asked.

(Tangentially related, I should do a contextlib2 release at some point, but
my previous CI provider shut down...)

[berkerpeksag]

Here's a simple implementation. I will add tests and update the documentation.

[1st1]

@berker Peksag: The patch looks fine, although I would rename 'redirect_stream' -> '_redirect_stream' or '_RedirectStream'

[berkerpeksag]

Good point. Will update the patch. Thanks!

[1st1]

@berker Peksag: Also, please update the docs.

[JohnIsidore]

There is stdout_redirected() function [1] that allows to redirect a file object given as stdout patameter including sys.stderr. It works at a file descriptor level i.e. it supports redirecting subprocess' output too but it doesn't work for StringIO (no fd).

[1] http://stackoverflow.com/questions/4675728/redirect-stdout-to-a-file-in-python/22434262#22434262

[berkerpeksag]

Here's an updated patch.

[vstinner]

issue22389_v2.diff:

.. function:: redirect_stdout(new_target)
+ redirect_stderr(new_target)

I would prefer to have two distinct entries in the documentation. The redirect_stderr() doc can be after redirect_stdout() and just say "Similar to redirect_stdout() but redirects sys.stderr".

[1st1]

I think that Victor is right, it would be better to have two distinct entries in the docs. Besides that - LGTM.

[berkerpeksag]

Good point. Patch updated. Thanks for the reviews!

[ncoghlan]

I just pushed the docs fix for issue bpo-21061, so the docs part of the patch may need tweaking in order to apply automatically.

[python-dev]

New changeset 7f12c9c09fb6 by Berker Peksag in branch 'default':
Issue bpo-22389: Add contextlib.redirect_stderr().
https://hg.python.org/cpython/rev/7f12c9c09fb6