providing input to shell from python script

Question

First of all, I'm very new to programming in general and Python in particular, I've been studying it for about 1.5 weeks, so any detailed criticism and/or explained corrections are welcome.

We are producing a certain device at my company and it is being tested at the QA station by using a shell script that is provided by the developer. I'm not allowed to change the said script.

At a certain point of the test, a comparison between two images is needed, it is done by showing the pictures to the operator of the station and asking for his/her input in y/n format.

I'm trying to come up with a python script that would be able to provide this input automatically based on the score that I got from the image comparison.

I have 2 problems, first being that I can't seem to catch the moment of the script reading the input, only an echo before or after. Second is that I can't seem to find a way to provide the input and let the script continue to do it's thing.

Now onto the tech details: I'm using this module to handle the image comparison.

from skimage.measure import compare_ssim as ssim
import cv2 as cv2

class Compare_images:

    def __init__(self):
        self.s = None
        self.original = cv2.imread("img1.png")
        self.contrast = cv2.imread("img2.png")

    def compare_images(self):
        self.s = ssim(self.original, self.contrast, multichannel = True)
        return self.s

This is the example of the shell script

echo "Welcome!"

#something happens here

read -p "Script is asking for user input as in y/n:" userinput

if [ "$userinput" = "n" ]
then 
    #script continues 1 way
else 
    #script continues another way
fi

And this is my main script.

from subprocess import Popen, PIPE
from compare_images import Compare_images
reply = None

def run(command):
    process = Popen(command, stdout=PIPE, shell=True)
    while True:
        line = process.stdout.readline().rstrip()
        if not line:
            break
        yield line


if __name__ == "__main__":
    for path in run("./shellscript.sh"):
        if 'Script is asking for user input as in y/n:' in path.decode():
            a = Compare_images()
            a.compare_images()
            result = a.s
            if result<0.9:
                reply = "n"
                #input to shell script is given
                print(f'comparison result is {result}')

            else:
                reply = "y"
                #input to shell script is given
                print(f'comparison result is {result}')

I'm using python 3.6 and running on ubuntu 18.04. Thank you for your help in advance!

nimrodm · Accepted Answer · 2019-09-14 19:43:20Z

In order to send data to the external process you need to add an stdin = PIPE argument to the Popen call. Then use write and flush to send the data to the process (flush is required since the stream is buffered).

process.stdin.write('yes\n')
process.stdin.flush()

Also pass text=True to the Popen call to let Python handle binary/text decoding for you. Then you no longer need to call decode() yourself.

The more difficult part here is that the read -p "prompt" var does not print a newline after the prompt. So your readline is stuck waiting for a complete line. You probably need to use read here to read a few characters at a time and compare to what you expect...

You may want to explore the Pexpect module as a simpler alternative.

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