Executing a Python file from within Python, feeding it input and retrieving output

Question

I'm writing a web application where users may attempt to solve various programming problems. The user uploads an executable .py file and the application feeds it some pre-determined input and checks whether the output is correct. (similar to how Codeforce works)

Assuming I have uploaded the user's script, how do I execute the user's script from within my own Python script, feeding the user's script input that would be captured by a normal input() function, and retrieving the output from print() functions so that I can verify its correctness?

What have you tried. Please make an effort to solve your problem before asking a question. More often then not, you'll end up solving your own problem. — Chris
– Chris, Commented Aug 29, 2016 at 3:34

mario_sunny · Accepted Answer · 2016-08-30 00:06:34Z

1

Figured it out.

Note: If you are going to use this in a production environment, make sure you place restrictions on what the user can execute.

executor.py

import subprocess

# create process
process = subprocess.Popen(
        ['python3.4','script_to_execute.py'], # shell command
        stdin=subprocess.PIPE,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE
)

# encode string into bytes, since communicate function only accepts bytes
input_data = "Captured string".encode(encoding='UTF-8')

# send the input to the process, retrieve output of print() function
stdoutdata, stderrdata = process.communicate(input=input_data)

# decode output back into string
output_data = stdoutdata.decode(encoding='UTF-8')

print(output_data)

script_to_execute.py

input_data = input()

print("Input recieved!\nData: " + input_data)

Output of executor.py

Input recieved!
Data: Captured string

edited Aug 30, 2016 at 0:06

answered Aug 29, 2016 at 3:42

mario_sunny

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4 Comments

Torben Klein Over a year ago

If you execute any script that somebody upload, you can count the seconds until your server will do something bad. You should at least start the subprocess in a restricted user account.

boardrider Over a year ago

... or a virtualenv.

ShadowRanger Over a year ago

If you pass universal_newlines=True to the Popen constructor, you can pass str and receive str without explicit encoding to bytes or decoding from bytes; communicate will take str as an argument, and return a two-tuple of str. Mind you, it uses the encoding returned by locale.getpreferredencoding(False), so on some machines (particularly Windows where it's usually something terrible like cp1252 instead of utf-8), you may want to stick with explicit encode and decode steps.

ShadowRanger Over a year ago

@boardrider: virtualenv provides no security guarantees, it's just to provide a predictable separated environment for Python. Arbitrary user submitted code could effectively undo that virtualization, and even without undoing it, has no special restrictions on what it can do; if the user could do it outside a virtualenv, they can do it inside a virtualenv too.

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