Redirect stdout to a file in Python?

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

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

It is similar to:

import sysfrom contextlib import contextmanager@contextmanagerdef 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 osfrom contextlib import redirect_stdoutstdout_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 osimport sysfrom contextlib import contextmanagerdef 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@contextmanagerdef 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 osimport sysstdout_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 flushC 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_libraryexcept ImportError:    libc = Noneelse:    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_functionimport syswith 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 sysclass 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