How to write binary data to stdout in python 3?

import os
os.write(1, a.tostring())

or, os.write(sys.stdout.fileno(), …) if that's more readable than 1 for you.

A better way:

import sys
sys.stdout.buffer.write(b"some binary data")

An idiomatic way of doing so, which is only available for Python 3, is:

with os.fdopen(sys.stdout.fileno(), "wb", closefd=False) as stdout:
    stdout.write(b"my bytes object")
    stdout.flush()

The good part is that it uses the normal file object interface, which everybody is used to in Python.

Notice that I'm setting closefd=False to avoid closing sys.stdout when exiting the with block. Otherwise, your program wouldn't be able to print to stdout anymore. However, for other kind of file descriptors, you may want to skip that part.