Temporarily Redirect stdout/stderr

Is it possible to temporarily redirect stdout/stderr in Python (i.e. for the duration of a method)?

Edit:

The problem with the current solutions (which I at first remembered but then forgot) is that they don't redirect; rather, they just replace the streams in their entirety. Hence, if a method has a local copy of one the variable for any reason (e.g. because the stream was passed as a parameter to something), it won't work.

Any solutions?

Solution 1:

You can also put the redirection logic in a contextmanager.

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

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!")

Solution 2:

To solve the issue that some function might have cached sys.stdout stream as a local variable and therefore replacing the global sys.stdout won't work inside that function, you could redirect at a file descriptor level (sys.stdout.fileno()) e.g.:

from __future__ import print_function
import os
import sys

def some_function_with_cached_sys_stdout(stdout=sys.stdout):
    print('cached stdout', file=stdout)

with stdout_redirected(to=os.devnull), merged_stderr_stdout():
    print('stdout goes to devnull')
    some_function_with_cached_sys_stdout()
    print('stderr also goes to stdout that goes to devnull', file=sys.stderr)
print('stdout is back')
some_function_with_cached_sys_stdout()
print('stderr is back', file=sys.stderr)

stdout_redirected() redirects all output for sys.stdout.fileno() to a given filename, file object, or file descriptor (os.devnull in the example).

stdout_redirected() and merged_stderr_stdout() are defined here.

Solution 3:

I am not sure what temporary redirection means. But, you can reassign streams like this and reset it back.

temp = sys.stdout
sys.stdout = sys.stderr
sys.stderr = temp

Also to write to sys.stderr within print stmts like this.

 print >> sys.stderr, "Error in atexit._run_exitfuncs:"

Regular print will to stdout.

Solution 4:

starting from python 3.4 there is the context manager contextlib.redirect_stdout:

from contextlib import redirect_stdout

with open('yourfile.txt', 'w') as f:
    with redirect_stdout(f):
        # do stuff...

to completely silence stdout this works:

from contextlib import redirect_stdout

with redirect_stdout(None):
    # do stuff...

Solution 5:

It's possible with a decorator such as the following:

import sys

def redirect_stderr_stdout(stderr=sys.stderr, stdout=sys.stdout):
    def wrap(f):
        def newf(*args, **kwargs):
            old_stderr, old_stdout = sys.stderr, sys.stdout
            sys.stderr = stderr
            sys.stdout = stdout
            try:
                return f(*args, **kwargs)
            finally:
                sys.stderr, sys.stdout = old_stderr, old_stdout

        return newf
    return wrap

Use as:

@redirect_stderr_stdout(some_logging_stream, the_console):
def fun(...):
    # whatever

or, if you don't want to modify the source for fun, call it directly as

redirect_stderr_stdout(some_logging_stream, the_console)(fun)

But note that this is not thread-safe.