Redirect stdout to a file in Python?

Question

If you want to do the redirection within the Python script, setting sys.stdout to a file object does the trick:

import sys
sys.stdout = open('file', 'w')
print('test')
sys.stdout.close()

A far more common method is to use shell redirection when executing (same on Windows and Linux):

$ python foo.py > file

There is contextlib.redirect_stdout() function in Python 3.4+:

from contextlib import redirect_stdout

with open('help.txt', 'w') as f:
    with redirect_stdout(f):
        print('it now prints to `help.text`')

It is similar to:

import sys
from contextlib import contextmanager

@contextmanager
def redirect_stdout(new_target):
    old_target, sys.stdout = sys.stdout, new_target # replace sys.stdout
    try:
        yield new_target # run some code with the replaced stdout
    finally:
        sys.stdout = old_target # restore to the previous value

that can be used on earlier Python versions. The latter version is not reusable. It can be made one if desired.

It doesn't redirect the stdout at the file descriptors level e.g.:

import os
from contextlib import redirect_stdout

stdout_fd = sys.stdout.fileno()
with open('output.txt', 'w') as f, redirect_stdout(f):
    print('redirected to a file')
    os.write(stdout_fd, b'not redirected')
    os.system('echo this also is not redirected')

b'not redirected' and 'echo this also is not redirected' are not redirected to the output.txt file.

To redirect at the file descriptor level, os.dup2() could be used:

import os
import sys
from contextlib import contextmanager

def fileno(file_or_fd):
    fd = getattr(file_or_fd, 'fileno', lambda: file_or_fd)()
    if not isinstance(fd, int):
        raise ValueError("Expected a file (`.fileno()`) or a file descriptor")
    return fd

@contextmanager
def stdout_redirected(to=os.devnull, stdout=None):
    if stdout is None:
       stdout = sys.stdout

    stdout_fd = fileno(stdout)
    # copy stdout_fd before it is overwritten
    #NOTE: `copied` is inheritable on Windows when duplicating a standard stream
    with os.fdopen(os.dup(stdout_fd), 'wb') as copied: 
        stdout.flush()  # flush library buffers that dup2 knows nothing about
        try:
            os.dup2(fileno(to), stdout_fd)  # $ exec >&to
        except ValueError:  # filename
            with open(to, 'wb') as to_file:
                os.dup2(to_file.fileno(), stdout_fd)  # $ exec > to
        try:
            yield stdout # allow code to be run with the redirected stdout
        finally:
            # restore stdout to its previous value
            #NOTE: dup2 makes stdout_fd inheritable unconditionally
            stdout.flush()
            os.dup2(copied.fileno(), stdout_fd)  # $ exec >&copied

The same example works now if stdout_redirected() is used instead of redirect_stdout():

import os
import sys

stdout_fd = sys.stdout.fileno()
with open('output.txt', 'w') as f, stdout_redirected(f):
    print('redirected to a file')
    os.write(stdout_fd, b'it is redirected now\n')
    os.system('echo this is also redirected')
print('this is goes back to stdout')

The output that previously was printed on stdout now goes to output.txt as long as stdout_redirected() context manager is active.

Note: stdout.flush() does not flush C stdio buffers on Python 3 where I/O is implemented directly on read()/write() system calls. To flush all open C stdio output streams, you could call libc.fflush(None) explicitly if some C extension uses stdio-based I/O:

try:
    import ctypes
    from ctypes.util import find_library
except ImportError:
    libc = None
else:
    try:
        libc = ctypes.cdll.msvcrt # Windows
    except OSError:
        libc = ctypes.cdll.LoadLibrary(find_library('c'))

def flush(stream):
    try:
        libc.fflush(None)
        stream.flush()
    except (AttributeError, ValueError, IOError):
        pass # unsupported

You could use stdout parameter to redirect other streams, not only sys.stdout e.g., to merge sys.stderr and sys.stdout:

def merged_stderr_stdout():  # $ exec 2>&1
    return stdout_redirected(to=sys.stdout, stdout=sys.stderr)

Example:

from __future__ import print_function
import sys

with merged_stderr_stdout():
     print('this is printed on stdout')
     print('this is also printed on stdout', file=sys.stderr)

Note: stdout_redirected() mixes buffered I/O (sys.stdout usually) and unbuffered I/O (operations on file descriptors directly). Beware, there could be buffering issues.

To answer, your edit: you could use python-daemon to daemonize your script and use logging module (as @erikb85 suggested) instead of print statements and merely redirecting stdout for your long-running Python script that you run using nohup now.

you can try this too much better

import sys

class Logger(object):
    def __init__(self, filename="Default.log"):
        self.terminal = sys.stdout
        self.log = open(filename, "a")

    def write(self, message):
        self.terminal.write(message)
        self.log.write(message)

sys.stdout = Logger("yourlogfilename.txt")
print "Hello world !" # this is should be saved in yourlogfilename.txt

The other answers didn't cover the case where you want forked processes to share your new stdout.

To do that:

from os import open, close, dup, O_WRONLY

old = dup(1)
close(1)
open("file", O_WRONLY) # should open on 1

..... do stuff and then restore

close(1)
dup(old) # should dup to 1
close(old) # get rid of left overs