Python version of freopen()

Question

Is there anything in python that can replicate the functionality of freopen() in C or C++? To be precise, I want to replicate the functionality of:

freopen("input.txt","r",stdin);

and

freopen("output.txt","w",stdout);

And then use the same (standard) functions for console I/O for file I/O. Any ideas?

related: [Redirect stdout to a file in Python?](http://stackoverflow.com/q/4675728/4279) — jfs, Dec 28 '15 at 23:46

ForEveR · Answer 1 · 2013-05-16T08:32:07.987

10

sys.stdout is simply file object, so, you can reopen it to another destination

out = sys.stdout
sys.stdout = open('output.txt', 'w')
// do some work
sys.stdout = out

out is only for recovering sys.stdout destination to default after work (as suggested Martijn Pieters - you can recover it by using sys.__stdout__, or not recover at all, if you don't need it).

edited May 16 '13 at 08:32

answered May 16 '13 at 08:16

ForEveR

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@Quixotic yes, sys.stdin and 'r' – ForEveR May 16 '13 at 08:23
Also a good idea to have `old = sys.stdout` in case you need it again – jamylak May 16 '13 at 08:25
1

@jamylak: Note that there is also `sys.__stdout__`, holding the default, so you can restore from that. – Martijn Pieters May 16 '13 at 08:27
@MartijnPieters much better idea, unless `sys.stdout` has already been changed but that's unlikely – jamylak May 16 '13 at 08:29

score 8 · Answer 2 · edited Jan 20 '18 at 14:22

8

Try this:

import sys
sys.stdin = open('input.txt', 'r') 
sys.stdout = open('output.txt', 'w')

Text files are self explanatory. You can now run this code on Sublime Text or any other text editor.

edited Jan 20 '18 at 14:22

Vasilis G.

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answered Jan 20 '18 at 13:58

Kushagra Kukreja

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@Vasilis does't works for input with spaces in between. Eg 4 6. Help – Optider Apr 03 '20 at 07:03
@Anuj hello there, I think you need to ping Kushagra Kukreja who posted the answer above on this one. – Vasilis G. Apr 04 '20 at 15:31

score 6 · Answer 3 · answered May 16 '13 at 08:26

6

If you're working on *nix platform, you can write your own freopen.

def freopen(f,option,stream):
    import os
    oldf = open(f,option)
    oldfd = oldf.fileno()
    newfd = stream.fileno()
    os.close(newfd)
    os.dup2(oldfd, newfd)

import sys
freopen("hello","w",sys.stdout)

print "world"

answered May 16 '13 at 08:26

Marcus

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As specified in POSIX, [`dup2` will close `newfd` if it is already open](https://pubs.opengroup.org/onlinepubs/9699919799/functions/dup.html). In fact, it's better not to close `newfd` explicitly first, since `dup2` guarantees that the close-and-reuse happens atomically. – 200_success Dec 10 '20 at 06:07

score 4 · Answer 4 · edited Dec 10 '20 at 06:37

You may also want to look at the contextmanager decorator in contextlib for temporary redirection:

from contextlib import contextmanager 
import sys 

@contextmanager
def stdout_redirected(new_stdout):
    save_stdout = sys.stdout
    sys.stdout  = new_stdout
    try:
        yield
    finally:
        sys.stdout = save_stdout

Example:

 with open(filename, "w") as f:
     with stdout_redirected(f):
         print "Hello"

score 0 · Answer 5 · answered Feb 08 '18 at 08:10

This should help:

import sys

def freopen(filename, mode):
    if mode == "r":
        sys.stdin = open(filename, mode)

    elif mode == "w":
        sys.stdout = open(filename, mode)

# ---- MAIN ----

freopen("input.txt", "r")
freopen("output.txt", "w")

Python version of freopen()

5 Answers5

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