Python 2.x - Write binary output to stdout?

Which platform are you on?

You could try this recipe if you're on Windows (the link suggests it's Windows specific anyway).

if sys.platform == "win32":
    import os, msvcrt
    msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)

There are some references on the web that there would/should be a function in Python 3.1 to reopen sys.stdout in binary mode but I don't really know if there's a better alternative then the above for Python 2.x.

You can use unbuffered mode: python -u script.py.

-u     Force  stdin,  stdout  and stderr to be totally unbuffered.
       On systems where it matters, also put stdin, stdout and stderr
       in binary mode.

You can use argopen.argopen(), it handles dash as stdin/stdout, and fixes binary mode on Windows.

import argopen
stdout = argopen.argopen('-', 'wb')
stdout.write(some_binary_data)

In Python 2.x, all strings are binary character arrays by default, so I believe you should be able to just

>>> sys.stdout.write(data)

EDIT: I've confirmed your experience.

I created one file, gen_bytes.py

import sys
for char in range(256):
    sys.stdout.write(chr(char))

And another read_bytes.py

import subprocess
import sys

proc = subprocess.Popen([sys.executable, 'gen_bytes.py'], stdout=subprocess.PIPE)
res = proc.wait()
bytes = proc.stdout.read()
if not len(bytes) == 256:
    print 'Received incorrect number of bytes: {0}'.format(len(bytes))
    raise SystemExit(1)
if not map(ord, bytes) == range(256):
    print 'Received incorrect bytes: {0}'.format(map(ord, bytes))
    raise SystemExit(2)
print "Everything checks out"

Put them in the same directory and run read_bytes.py. Sure enough, it appears as if Python is in fact converting newlines on output. I suspect this only happens on a Windows OS.

> .\read_bytes.py
Received incorrect number of bytes: 257

Following the lead by ChristopheD, and changing gen_bytes to the following corrects the issue.

import sys

if sys.platform == "win32":
    import os, msvcrt
    msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)

for char in range(256):
    sys.stdout.write(chr(char))

I include this for completeness. ChristopheD deserves the credit.

Python 2.x - Write binary output to stdout?

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