how to run a command in python, supplying input, then reading the output

Question

I want to use Popen from subprocess to execute the command: 'python3 test.py'

# The following is test.py code:

string = input('Enter Something')
if string == 'mypassword':
    print('Success')
else:
    print('Fail')

In my program, I want to execute 'python3 test.py' multiple times, each time supplying input, and reading the output ('Success' or 'Fail') and storing it in a variable.

My program that is suppose to execute 'python3 test.py' is as follows:

from subprocess import Popen, PIPE

# Runs test.py
command = Popen(['python3', 'test.py'], stdin=PIPE)
# After this, it prompts me to type in the input, 
# but I want to supply it from a variable

# I want to do something like
my_input = 'testpassword'
command.supplyInput(my_input)
result = command.getOutput()

# result will have the string value of 'Success' or 'Fail'

I would normally use that command if I am not going to be prompted an input, but I have to follow it with an input — AznBoyStride
– AznBoyStride, Commented Jul 19, 2018 at 6:26
You can pass input as command line arguments along with the command — Gireesh
– Gireesh, Commented Jul 19, 2018 at 6:29
I understand, but in this case there aren't command line arguments. The command must be ran first, then it prompts you to manually enter something — AznBoyStride
– AznBoyStride, Commented Jul 19, 2018 at 6:32
Just to be clear, the test.py file is not going to change. I want to modify the python program that will execute test.py correctly supplying it the input and getting the output — AznBoyStride
– AznBoyStride, Commented Jul 19, 2018 at 6:33

blhsing · Accepted Answer · 2018-07-19 06:56:19Z

1

You can add parameter stdout=PIPE to Popen, and use Popen.communicate to supply input and read the output instead.

from subprocess import Popen, PIPE
command = Popen(['python3', 'test.py'], stdin=PIPE, stdout=PIPE)
my_input = 'testpassword\n'
result, _ = command.communicate(my_input)

Please read Popen.communicate's documentation for more details: https://docs.python.org/3/library/subprocess.html#subprocess.Popen.communicate

edited Jul 19, 2018 at 6:56

answered Jul 19, 2018 at 6:48

blhsing

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