Capture output from subprocess.call that I have no control over

Question

I'm testing a piece of Python code that uses subprocess.call(), so I have no control over that function call. I need to capture the output from that system call to do assertions. I tried to set os.stdout to a StringIO object, but that doesn't capture the system call outputs. How do I solve this problem? Here's my code so far:

Code to test (I have no control over this):

def runFile(filename):
    check_call("./" + filename)

My attempt to capture system call output:

import StringIO
oldout = sys.stdout
try:
    sys.stdout = StringIO.StringIO()
    runFile("somefile")
    output = sys.stdout.getvalue()
    assert output == "expected-output"
finally:
    sys.stdout = oldout

Answer 1

You could redirect your own stdout by

rd, wr = os.pipe()
oldstdout = os.dup(1)
os.dup2(wr, 1)
os.close(wr)

and then read from rd while the external process is running.

This might not work, as the external process might write more than fits in the pipe buffer. In this case, you would have to spawn a reading thread.

Afterwards, you restore the old state with

os.dup2(oldstdout, 1)
os.close(oldstdout)
os.close(rd)

and continue normally.