Redirect file descriptors

This is a Python trick with supressing of console output. Actually implementation does not supresses anything, but maps (dups) "source" file descriptors to "destination" file descriptors, so you can continue to use already existing descriptor, but it will point out to another one. "Another one" are got from list, but if dstfd list in lesser than srcfd list, then missing will be filled with "devnull", which allows us to suppress console output - this is the one way to use this class.

It is OS independent and was tested in Linux, Windows. Such "magia" happens only in context, after exit from context old values of descriptors will be restored. Note, that this works for children process too, sure. So, if you start some child process, you can "mask" its descriptors (redirect them somewhere).

import os, sys
from itertools import zip_longest

class FdRedir:
    def __init__(self, srcfd, dstfd):
        self.src = srcfd
        self.dst = dstfd
        self.null = os.open(os.devnull, os.O_RDWR)
        self.dups = [os.dup(f) for f in self.src]
    def __enter__(self):
        homo = zip_longest(self.src, self.dst, fillvalue=self.null)
        for t, f in homo:
            os.dup2(f, t)
        return self
    def __exit__(self, xtyp, xval, xtb):
        iso = zip(self.dups, self.src)
        for f, t in iso:
            os.dup2(f, t)
        for f in self.dups:
            os.close(f)
        os.close(self.null)

# Example of usage

with FdRedir((1, 2), ()) as fr:
    print('hidden')
    print('hidden', file=sys.stderr)
    with FdRedir([1], [fr.dups[0]]):
        print('visible tmp')
        print('hidden', file=sys.stderr)
    print('hidden')
print('visible end')

will print

visible tmp
visible end

which is expected: we suppress output to stdout, stderr descriptors in first context, then temporary allow stdout (by map it to fr.dups[0]).