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gh-139452: Clarify redirect_stdout, stderr behavior #139490

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merged 4 commits into from
Oct 8, 2025
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gh-139452: Clarify redirect_stdout, stderr behavior #139490

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gh-139452: Clarify redirect_stdout, stderr behavior
cmaloney Oct 2, 2025
af01958
Add discard/suppress example
cmaloney Oct 3, 2025
976045e
Remove open() reference per issue comment
cmaloney Oct 6, 2025
40df344
Update per review comments
cmaloney Oct 8, 2025
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16 changes: 12 additions & 4 deletions Doc/library/contextlib.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -327,10 +327,12 @@ Functions and classes provided:
.. function:: redirect_stdout(new_target)

Context manager for temporarily redirecting :data:`sys.stdout` to
another file or file-like object.
another :term:`file object`.

This tool adds flexibility to existing functions or classes whose output
is hardwired to stdout.
is hardwired to :data:`sys.stdout`. This does not modify underlying file
objects or file descriptors. It sets the global :data:`sys.stdout` to the
provided value and at context exit sets it to the previous value.
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@rhettinger rhettinger Oct 7, 2025

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This should be left out entirely. That is already covered by the docs for sys.stdout and sys.__stdout__


For example, the output of :func:`help` normally is sent to *sys.stdout*.
You can capture that output in a string by redirecting the output to an
Expand All @@ -354,6 +356,12 @@ Functions and classes provided:
with redirect_stdout(sys.stderr):
help(pow)

To discard or suppress the output of :func:`help` without collecting the data::

with open(os.devnull, 'w') as devnull:
with redirect_stdout(devnull):
help(pow)

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@rhettinger rhettinger Oct 7, 2025

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I prefer that this be omitted as well. It stretches well beyond the original bug report. Also, it is not a practical example — the help function has no side-effects, so there is no reason to call it and ignore its output.

Note that the global side effect on :data:`sys.stdout` means that this
context manager is not suitable for use in library code and most threaded
applications. It also has no effect on the output of subprocesses.
Expand All @@ -366,8 +374,8 @@ Functions and classes provided:

.. function:: redirect_stderr(new_target)

Similar to :func:`~contextlib.redirect_stdout` but redirecting
:data:`sys.stderr` to another file or file-like object.
Similar to :func:`~contextlib.redirect_stdout` but redirecting the global
:data:`sys.stderr` to another value, typically a :term:`file object`.
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@efimov-mikhail efimov-mikhail Oct 6, 2025

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Maybe "another target" would be better here than "another value"?


This context manager is :ref:`reentrant <reentrant-cms>`.

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