How can I check with doctest that a program produced certain output?

Question

In one of my functions I'm calling an external program, using subprocess.check_call, which will produce output. How could I use doctest to make sure the output it's producing is the one I'm expecting?

Answer 1

Maybe this can help:

import sys
import tempfile
import subprocess


def example(output):
    r""" Do something ...

    >>> output = example('Processing file ...')
    >>> print output # doctest:+ELLIPSIS
    'Processing file ...'

    Check how many file was processed.
    >>> [line.startswith('Processing file')
    ... for line in output.splitlines()].count(True)
    1    

    """
    cmd = "print '%s'" % (output, )
    with tempfile.TemporaryFile() as output:
        subprocess.check_call([sys.executable, '-c', cmd], stdout=output)
        output.seek(0)
        res = output.read()

    return res

if __name__ == '__main__':
    import doctest
    doctest.testmod()

As you can see i used the argument stdout of the subprocess.check_call function so to be able to get the output of the command , beside that if you are not using the stdout argument (which i assume that is your case) i think it very hard to capture the command output.

Hope this was hopeful :)