Issue 22389: Add contextlib.redirect_stderr() - Python tracker

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)

Why not adding a new redirect_stderr() function?

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.

+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.

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?

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...)

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

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

Good point. Will update the patch. Thanks!

@Berker Peksag: Also, please update the docs.

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

Here's an updated patch.

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".

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

Good point. Patch updated. Thanks for the reviews!

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

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

messages: + msg229062

messages: + msg228839

nosy: + berker.peksag
messages: + msg226825

keywords: + patch
stage: patch review

nosy: + ncoghlan