Question

This is a follow up question from here specifically concerning its answer.

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From a python module I am calling a Hello World executable that simply prints Hello World to the stdout. I am interested in redirecting that output to a python StringIO and ran into this answer which almost brings me all the way to the solution.

The critical part of this answer is this code segment:

1. def redirect_stdout():
2.     print "Redirecting stdout"
3.     sys.stdout.flush() # <--- important when redirecting to files
4.     newstdout = os.dup(1)
5.     devnull = os.open('/dev/null', os.O_WRONLY)
6.     os.dup2(devnull, 1)
7.     os.close(devnull)
8.     sys.stdout = os.fdopen(newstdout, 'w')

Also I would like to restore the stdout as it was before the redirection.

Questions

1. What exactly is going on in the function above?
   • What is dup and dup2 doing?
   • What is /dev/null?
   • What is line 8 doing? (sys.stdout = os.fdopen(newstdout, 'w'))
2. How can I store the stdout in a StringIO object?
3. How can I restore the stdout after the call to my Hello World program?

I am pretty sure that once I have the answer for my question 1 that the answers of questions 2 and 3 will be easy. I decided to post them anyway to maybe push the answer of question 1 into the direction where I want to go.

Answer 1

I’ve written below a few additional comments that should make clearer what it’s going on inside the redirect_stdout function:

def redirect_stdout():
    print "Redirecting stdout"
    sys.stdout.flush() # <--- important when redirecting to files

    # Duplicate stdout (file descriptor 1)
    # to a different file descriptor number
    newstdout = os.dup(1)

    # /dev/null is used just to discard what is being printed
    devnull = os.open('/dev/null', os.O_WRONLY)

    # Duplicate the file descriptor for /dev/null
    # and overwrite the value for stdout (file descriptor 1)
    os.dup2(devnull, 1)

    # Close devnull after duplication (no longer needed)
    os.close(devnull)

    # Use the original stdout to still be able
    # to print to stdout within python
    sys.stdout = os.fdopen(newstdout, 'w')

One important thing to note is that a process gets three different file descriptors from the OS when launched:

• stdin: 0
• stdout: 1
• stderr: 2

As explained in the comments, the code above takes advantage of the file descriptor for stdout and the file descriptor duplication functions to make trick the C code into using a different stdout while still keeping a reference to the original stdout in the python code to be able to print.