python - prevent IOError: [Errno 5] Input/output error when running without stdout

Question

I have a script that runs automatically on server through cronjob and it import and run several other scripts.

Some of them use prints, which naturally creates IOError: [Errno 5] Input/output error because the script runs without any SSH / terminal connected, so there's no proper stdout setup.

There are lots of questions about this subject but I couldn't find anyone that actually solve it, assuming I can't remove the print or change the executed scripts.

I tried several things, including:

class StdOut(object):
    def __init__(self):
        pass
    def write(self, string):
        pass
sys.stdout = StdOut()
sys.stderr = StdOut()

and

from __future__ import print_function
import __builtin__

def print(*args, **kwargs):
        pass
    __builtin__.print = print

But none of it works. I assume it only affect the module itself and not the modules I import / run later.

So how can I create a stub stdout that will affect all modules in the process? Like I said, I don't want to change the scripts that are executed from the main module, but I can change everything inside the importing module. And just to clearify - everything is imported, no new processes are spawned etc.

Thanks,

Answer 1

Modifying the builtin or changing sys.stdout should work (except for subprocesses—but you ruled those out) as long as you do it early enough. If not, though, there's a lower level trick that's much easier:

• run your python scripts with I/O redirection that discards output:

  python foo.py >/dev/null 2>&1
  

  (assuming Unix-y scripts, as implied by "cron" in the question)

• or, redirect file descriptors 1 and 2 (same idea as above, but done within your Python startup rather than as part of the cron-invoked command):

  import os
  fd = os.open(os.devnull, os.O_RDWR)
  # NB: even if stdin is closed, fd >= 0
  os.dup2(fd, 1)
  os.dup2(fd, 2)
  if fd > 2:
      os.close(fd)
  

  (this particular bit of code has the side effect of making /dev/null act as stdin, if all descriptors were closed). [Edit: I started with with open(...) and then switched to os.open and did not test the final version. Fixed now.]

All that said, a good cron really should have stdout and stderr connected somewhere, and should email the output/error-output to you. Not all cron versions are this nice though.