How to Temporarily Redirect stdout/stderr in Python with a Context Manager?

Temporarily Redirecting stdout/stderr, Revisited

While it's common practice to redirect stdout/stderr to external files, it may be necessary to do so temporarily, within the scope of a specific method.

Limitations of Current Solutions

Existing solutions typically replace the entire output streams, leaving local copies of these streams unaffected. This can lead to issues when methods use local copies of the streams.

A ContextManager Solution

To address this limitation, we can implement the redirection logic using a contextmanager:

<code class="python">import os
import sys

class RedirectStdStreams(object):
    def __init__(self, stdout=None, stderr=None):
        self._stdout = stdout or sys.stdout
        self._stderr = stderr or sys.stderr

    def __enter__(self):
        self.old_stdout, self.old_stderr = sys.stdout, sys.stderr
        self.old_stdout.flush(); self.old_stderr.flush()
        sys.stdout, sys.stderr = self._stdout, self._stderr

    def __exit__(self, exc_type, exc_value, traceback):
        self._stdout.flush(); self._stderr.flush()
        sys.stdout = self.old_stdout
        sys.stderr = self.old_stderr</code>

This class allows for temporary redirection of both stdout and stderr.

Usage Example

To demonstrate usage, we can redirect the output to /dev/null and observe the behavior:

<code class="python">if __name__ == '__main__':

    devnull = open(os.devnull, 'w')
    print('Fubar')

    with RedirectStdStreams(stdout=devnull, stderr=devnull):
        print("You'll never see me")

    print("I'm back!")</code>

In this example, the message "You'll never see me" is suppressed while the redirection is active, but is visible afterwards, confirming that the redirection is temporary and affects only the scope within the context manager.

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