How to close file descriptors in python?

Question

I have the following code in python:

import os


class suppress_stdout_stderr(object):
    '''
    A context manager for doing a "deep suppression" of stdout and stderr in
    Python, i.e. will suppress all print, even if the print originates in a
    compiled C/Fortran sub-function.
       This will not suppress raised exceptions, since exceptions are printed
    to stderr just before a script exits, and after the context manager has
    exited (at least, I think that is why it lets exceptions through).

    '''
    def __init__(self):
        # Open a pair of null files
        self.null_fds = [os.open(os.devnull,os.O_RDWR) for x in range(2)]
        # Save the actual stdout (1) and stderr (2) file descriptors.
        self.save_fds = (os.dup(1), os.dup(2))

    def __enter__(self):
        # Assign the null pointers to stdout and stderr.
        os.dup2(self.null_fds[0],1)
        os.dup2(self.null_fds[1],2)

    def __exit__(self, *_):
        # Re-assign the real stdout/stderr back to (1) and (2)
        os.dup2(self.save_fds[0],1)
        os.dup2(self.save_fds[1],2)
        # Close the null files
        os.close(self.null_fds[0])
        os.close(self.null_fds[1])

for i in range(10**6):
    with suppress_stdout_stderr():
        print 'plop'
    if i % 50 == 0:
        print i

it fails at 5100 on OSX with OSError: [Errno 24] Too many open files. I'm wondering why and if there is a solution to close the file descriptor. I'm looking for a solution for a context manager which closes stdout and stderr.

Answer 1

I executed your code on a Linux machine and got the same error but at a different number of iterations. I added the following two lines in the __exit__(self, *_) function of your class:

os.close(self.save_fds[0]) os.close(self.save_fds[1])

With this change I do not get an error and the script returns successfully. I assume that the duplicated file descriptors stored in self.save_fds are kept open if you don't close them with os.close(fds) and so you get the too many files open error. Anyway my console printed "plop", but maybe this depends on my platform. Let me know if it works :)