How to duplicate sys.stdout to a log file?

Question

Edit: Since it appears that there\'s either no solution, or I\'m doing something so non-standard that nobody knows - I\'ll revise my question to also ask: What is the best w

Answer 1

Here is another solution, which is more general than the others -- it supports splitting output (written to sys.stdout) to any number of file-like objects. There's no requirement that __stdout__ itself is included.

import sys

class multifile(object):
    def __init__(self, files):
        self._files = files
    def __getattr__(self, attr, *args):
        return self._wrap(attr, *args)
    def _wrap(self, attr, *args):
        def g(*a, **kw):
            for f in self._files:
                res = getattr(f, attr, *args)(*a, **kw)
            return res
        return g

# for a tee-like behavior, use like this:
sys.stdout = multifile([ sys.stdout, open('myfile.txt', 'w') ])

# all these forms work:
print 'abc'
print >>sys.stdout, 'line2'
sys.stdout.write('line3\n')

NOTE: This is a proof-of-concept. The implementation here is not complete, as it only wraps methods of the file-like objects (e.g. write), leaving out members/properties/setattr, etc. However, it is probably good enough for most people as it currently stands.

What I like about it, other than its generality, is that it is clean in the sense it doesn't make any direct calls to write, flush, os.dup2, etc.

Answer 2

I wrote a tee() implementation in Python that should work for most cases, and it works on Windows also.

https://github.com/pycontribs/tendo

Also, you can use it in combination with logging module from Python if you want.

Answer 3

What you really want is logging module from standard library. Create a logger and attach two handlers, one would be writing to a file and the other to stdout or stderr.

See Logging to multiple destinations for details

Answer 4

You can also add stderr as well, based on shx2's answer above using class multifile :

class Log(object):

    def __init__(self, path_log, mode="w", encoding="utf-8"):
        h = open(path_log, mode, encoding=encoding)
        sys.stdout = multifile([ sys.stdout, h ])
        sys.stderr = multifile([ sys.stderr, h ])

    def __enter__(self):
        """ Necessary if called by with (or with... as) """
        return self     # only necessary if "as"

    def __exit__(self, type, value, tb):
        """ Necessary if call by with """
        pass

    def __del__(self):
        if sys is not None:
            # restoring
            sys.stdout = sys.__stdout__
            sys.stderr = sys.__stderr__

log = Log("test.txt")
print("line 1")
print("line 2", file=sys.stderr)
del log
print("line 3 only on screen")

Answer 5

Since you're comfortable spawning external processes from your code, you could use tee itself. I don't know of any Unix system calls that do exactly what tee does.

# Note this version was written circa Python 2.6, see below for
# an updated 3.3+-compatible version.
import subprocess, os, sys

# Unbuffer output (this ensures the output is in the correct order)
sys.stdout = os.fdopen(sys.stdout.fileno(), 'w', 0)

tee = subprocess.Popen(["tee", "log.txt"], stdin=subprocess.PIPE)
os.dup2(tee.stdin.fileno(), sys.stdout.fileno())
os.dup2(tee.stdin.fileno(), sys.stderr.fileno())

print "\nstdout"
print >>sys.stderr, "stderr"
os.spawnve("P_WAIT", "/bin/ls", ["/bin/ls"], {})
os.execve("/bin/ls", ["/bin/ls"], os.environ)

You could also emulate tee using the multiprocessing package (or use processing if you're using Python 2.5 or earlier).

Update

Here is a Python 3.3+-compatible version:

import subprocess, os, sys

tee = subprocess.Popen(["tee", "log.txt"], stdin=subprocess.PIPE)
# Cause tee's stdin to get a copy of our stdin/stdout (as well as that
# of any child processes we spawn)
os.dup2(tee.stdin.fileno(), sys.stdout.fileno())
os.dup2(tee.stdin.fileno(), sys.stderr.fileno())

# The flush flag is needed to guarantee these lines are written before
# the two spawned /bin/ls processes emit any output
print("\nstdout", flush=True)
print("stderr", file=sys.stderr, flush=True)

# These child processes' stdin/stdout are 
os.spawnve("P_WAIT", "/bin/ls", ["/bin/ls"], {})
os.execve("/bin/ls", ["/bin/ls"], os.environ)

Answer 6

If you wish to log all output to a file AND output it to a text file then you can do the following. It's a bit hacky but it works:

import logging
debug = input("Debug or not")
if debug == "1":
    logging.basicConfig(level=logging.DEBUG, filename='./OUT.txt')
    old_print = print
    def print(string):
        old_print(string)
        logging.info(string)
print("OMG it works!")

EDIT: Note that this does not log errors unless you redirect sys.stderr to sys.stdout

EDIT2: A second issue is that you have to pass 1 argument unlike with the builtin function.

EDIT3: See the code before to write stdin and stdout to console and file with stderr only going to file

import logging, sys
debug = input("Debug or not")
if debug == "1":
    old_input = input
    sys.stderr.write = logging.info
    def input(string=""):
        string_in = old_input(string)
        logging.info("STRING IN " + string_in)
        return string_in
    logging.basicConfig(level=logging.DEBUG, filename='./OUT.txt')
    old_print = print
    def print(string="", string2=""):
        old_print(string, string2)
        logging.info(string)
        logging.info(string2)
print("OMG")
b = input()
print(a) ## Deliberate error for testing