displaying output to the terminal in pexpect - Python

Question

I'm automating a installer using pexpect , which works nicely. However, I would like to replace the pexpect.interact() with some sort of stdout , which would allow me to keep track of the progress bar of the installer:

Please wait while Setup installs on your computer.

 Installing
 0% ______________ 50% ______________ 100%
 #########################################

----------------------------------------------------------------------------
Setup has finished installing on your computer.

View readme file [Y/n]: n

[Errno 5] Input/output error

The source code looks like:

## A bunch of informations being given to the installer above
## Do you want install? y
child.sendline('y')
## now I keep tracking of the installation bar progress ... 
child.interact()
## View readme file [Y/n]: n
child.sendline('n')

so the last part is being done manually, become I can't get of the child.interact() once the install has been completed. How could I accomplish that?

Answer 1

I had to do the same thing once. The problem is that things are working in line buffered mode by default. Here is how I worked around it:

Right after you create your child (I am assuming this is a pexpect.spawn ) you can set the attribute child.logfile to something - this doesn't have to be a logfile literally, it can be any file handle. In your case, you can set it to sys.stdout but open this file handle in unbuffered mode.

Working example:

#!/usr/bin/env python

import pexpect
import sys
import time
import os

def potato():
    for i in range(20):
        time.sleep(0.1)
        sys.stdout.write(str(i))
        sys.stdout.flush()
    sys.stdout.write('bye\n')



if __name__ == '__main__':
    if sys.argv[-1] == 'potato':
        potato()
    else:
        child = pexpect.spawn(__file__ + ' potato')
        child.logfile = os.fdopen(sys.stdout.fileno(), 'w', 0)
        child.expect('bye')