How do I prevent a C shared library to print on stdout in python?

I work with a python lib that imports a C shared library that prints on stdout. I want a clean output in order to use it with pipes or to redirect in files. The prints are done outside of python, in the shared library.

At the beginning, my approach was:

# file: test.py
import os
from ctypes import *
from tempfile import mktemp

libc = CDLL("libc.so.6")

print # That's here on purpose, otherwise hello word is always printed

tempfile = open(mktemp(),'w')
savestdout = os.dup(1)
os.close(1)
if os.dup(tempfile.fileno()) != 1:
    assert False, "couldn't redirect stdout - dup() error"

# let's pretend this is a call to my library
libc.printf("hello world\n")

os.close(1)
os.dup(savestdout)
os.close(savestdout)

This first approach is half working:
- For some reason, it needs a "print" statement just before moving stdout, otherwise hello word is always printed. As a result it will print an empty line instead of all the fuzz the library usually outputs.
- More annoying, it fails when redirecting to a file:

$python test.py > foo && cat foo

hello world

My second python attempt was inspired from another similar thread given in the comments:

import os
import sys
from ctypes import *
libc = CDLL("libc.so.6")

devnull = open('/dev/null', 'w')
oldstdout = os.dup(sys.stdout.fileno())
os.dup2(devnull.fileno(), 1)

# We still pretend this is a call to my library
libc.printf("hello\n")

os.dup2(oldstdout, 1)

This one also fails to prevent "hello" from printing.

Since I felt this was a bit low level, I then decided to go completely with ctypes. I took inspiration from this C program, which does not print anything:

#include <stdio.h>

int main(int argc, const char *argv[]) {
    char buf[20];
    int saved_stdout = dup(1);
    freopen("/dev/null", "w", stdout);

    printf("hello\n"); // not printed

    sprintf(buf, "/dev/f