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The captured_stderr and captured_stdin context managers aren't documented, and should be.

Added a patch

Sorry for noise, missed changes in Doc/library/test.rst. Updated patch added

Print to stdin?

Yes, I agree that this is not an obvious sample, but:
$ ./python -m test.test_support | grep test_captured_stdin
test_captured_stdin (__main__.TestSupport) ... ok
What's about that?
with captured_stdin() as s:
 s.write('hello\n')
 s.seek(0)
 captured = input()
assert captured == 'hello'

I believe it is a mechanical mistake in test_captured_stdin().

`captured_stdin` test changed, docs updated.

I think it will be better to describe all three functions together (as sys.stdin/stdout/stderr).
.. function:: captured_stdin()
 captured_stdout()
 captured_stderr()
... description ...
 Example use::
 ... example for captured_stdin() ...
 ... example for captured_stdout() ...

Joining the documentation for captured_stderr and captured_stdout makes
sense, as they can really use a single example, and the usage is
completely parallel.
I'd rather see captured_stdin handled separately, perhaps with some
additional comments in the example, to emphasize the intended usage
pattern:
 with support.captured_stdin() as s:
 # Prepare simulated input:
 s.write('hello\n')
 s.seek(0)
 # Call test code that consumes from stdin:
 captured = input()
 self.assertEqual(captured, "hello")

Amended, as discussed. Also changed documentation from "how to use a context managers with that" to "how it works".

+1 for issue17987_4.patch
Thanks, Dmi!

> I'd rather see captured_stdin handled separately, perhaps with some
> additional comments in the example, to emphasize the intended usage
> pattern:
However all three functions share a description.
sys.stdin also has totally different usage pattern than sys.stdout/stderr, but it's documentation joined with documentation of all sys.std* streams.
I think functions should be mentioned in order of it's fd numbers (0 - stdin, 1
- stdout, 2 - stderr). And example for capture_stdout() looks more appropriate then for capture_stderr().

Thanks for review, Serhiy - updated.

Fred or Serhiy - any news here? I'm signed Contributor Agreement few days ago, just waiting a change in my profile here, please don't worry about copyright :-)

From v5 of the patch:
+ A context managers that temporarily replaces the :data:`sys.stdin` /
+ :data:`sys.stdout` / :data:`sys.stderr` stream with :class:`io.StringIO`
+ object.
I'd go with singular nouns instead of trying to map across them with plurals:
 Context manager that temporarily replaces the named stream with an
 :class:`io.StringIO` object.

LGTM with Fred's suggestion.

Amended

New changeset af2416c2e27c by R David Murray in branch '3.3':
#17987: properly document support.captured_xxx.
http://hg.python.org/cpython/rev/af2416c2e27c
New changeset d0f7f1996001 by R David Murray in branch 'default':
Merge #17987: properly document support.captured_xxx.
http://hg.python.org/cpython/rev/d0f7f1996001 

Thanks, Dmi.

Yeah, my first patch applied :) thanks, David