Redirecting the output of a python function from STDOUT to variable in Python

Question

This is what I am trying to achieve


def fun():
    runner = InteractiveConsole()
    while(True):
        code = raw_input()
        code.rstrip('\n')
        # I want to achieve the following
        # By default the output and error of the 'code' is sent to STDOUT and STDERR
        # I want to obtain the output in two variables out and err
        out,err = runner.push(code)

All the solution that I have looked at till now, use either pipes to issue separate script execution command (which is not possible in my case). Any other way I can achieve this?

AFAIK the output can only be sent to an object which has a `write()` method on it, so you can create a class which contains a `write` method and make `out` an instance of it. — Ashwini Chaudhary, Nov 06 '12 at 11:31
@AshwiniChaudhary write() is just for writing data to stderr stream. What about the data sent to stdout. — gibraltar, Nov 06 '12 at 11:33
Possible duplicate: [Can I redirect the stdout in python into some sort of string buffer?](http://stackoverflow.com/q/1218933/222914) — Janne Karila, Nov 06 '12 at 11:37
@AshwiniChaudhary: OP is using python 2.x, so `print >> out, "something"` instead — Eric, Nov 06 '12 at 11:45

Eric · Answer 1 · 2019-01-09T15:56:11.330

8

import StringIO, sys
from contextlib import contextmanager

@contextmanager
def redirected(out=sys.stdout, err=sys.stderr):
    saved = sys.stdout, sys.stderr
    sys.stdout, sys.stderr = out, err
    try:
        yield
    finally:
        sys.stdout, sys.stderr = saved


def fun():
    runner = InteractiveConsole()
    while True:
        out = StringIO.StringIO()
        err = StringIO.StringIO()
        with redirected(out=out, err=err):
            out.flush()
            err.flush()
            code = raw_input()
            code.rstrip('\n')
            # I want to achieve the following
            # By default the output and error of the 'code' is sent to STDOUT and STDERR
            # I want to obtain the output in two variables out and err
            runner.push(code)
            output = out.getvalue()
        print output

In newer versions of python, this contezt manager is built in:

with contextlib.redirect_stdout(out), contextlib.redirect_stderr(err):
    ...

edited Jan 09 '19 at 15:56

answered Nov 06 '12 at 11:38

Eric

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You can also recover them from `sys.__stdout__` and `sys.__stderr__`, so no need of `saved`. – Ashwini Chaudhary Nov 06 '12 at 11:49
@AshwiniChaudhary: That fails if I nest two redirects – Eric Nov 06 '12 at 11:49
@Eric I have a doubt. In your approach, the vaiable output records the output received till now. Is there a way I can obtain the output just from the current function call. – gibraltar Nov 06 '12 at 13:00
`sys.stdout` also has `.encoding` but `StringIO` does not have that (and other attributes. If the code inside `redirected` relies on those attributes it will fail. Otherwise nice solution – dashesy Feb 26 '18 at 22:29
1

It should be `import io` instead of `import StringIO` I guess (see [here](https://docs.python.org/3.10/library/io.html#io.StringIO) - even for 2.7 code). Then `StringIO.StringIO()` becomes `io.StringIO()`. – Pascal Mar 26 '22 at 22:08

score 1 · Answer 2 · edited May 23 '17 at 12:04

InteractiveConsole doesn't expose any API for setting a file like object for output or errors, you'll need to monkey patch sys.stdout and sys.stderr. As always with monkey patching, be mindful of what the side effects might be. In this case, you'd be replacing the global stdin and stdout file objects with your own implementation, which might swallow up unintended output as well (especially if you're using any threads).

It would be slightly safer to 'tee' the output with something like:

import sys
import StringIO


class TeeBuffer(object):

    def __init__(self, real):
        self.real = real
        self.buf = StringIO.StringIO()

    def write(self, val):
        self.real.write(val)
        self.buf.write(val)


def fun():
    runner = InteractiveConsole()

    out = TeeBuffer(sys.stdout)
    err = TeeBuffer(sys.stderr)

    sys.stdout = out
    sys.stderr = err

    while(True):
        code = raw_input()
        code.rstrip('\n')

        out, err = runner.push(code)
        outstr = out.buf.getvalue()
        errstr = err.buf.getvalue()

    sys.stdout = out.real
    sys.stderr = err.real

Then your user still sees the output, without you having to worry about printing it back out to the correct place on each run.

That's not really monkeypatching... Monkeypatching would be modifying the write method of the builtin stdout. — Eric, Nov 06 '12 at 11:53
It's monkey patching the `stdout` field of the `sys` object with a different representation. — jbowes, Nov 06 '12 at 11:59

score 0 · Answer 3 · answered Nov 06 '12 at 11:38

You can use a context manager to redirect stdout temporarily:

@contextmanager
def stdout_redirected(new_stdout):
    save_stdout = sys.stdout
    sys.stdout = new_stdout
    try:
        yield None
    finally:
        sys.stdout = save_stdout
Used as follows:
with opened(filename, "w") as f:
    with stdout_redirected(f):
        print "Hello world"
This isn't thread-safe, of course, but neither is doing this same dance manually. In single-threaded programs (for example, in scripts) it is a popular way of doing things.

It's easy to tweak this to redirect both stdout and stderr to cStringIOs:

@contextmanager
def out_redirected():
    save_stdout = sys.stdout
    save_stderr = sys.stderr
    sys.stdout = cStringIO.String()
    sys.stderr = cStringIO.String()
    try:
        yield sys.stdout, sys.stderr
    finally:
        sys.stdout = save_stdout
        sys.stderr = save_stderr

You'd use this as

with out_redirected() as out, err:
    runner.push(code)
    out.seek(0)
    print out.read()

Redirecting the output of a python function from STDOUT to variable in Python

3 Answers3

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