Python Subprocess - Doesn't return anything

Question

I've seen on here a few other issues running subprocesses with Python, but none have solved the issue I'm having at the moment. Relatively new to Python, so just playing around and I'm sure it's a simple solution.. But I just can't make it work!

I want a subprocess to return some data, and it isn't. So I wrote this basic example to try and get it working but it still isn't. Where am I going wrong? I'm getting no errors or anything, it's just not doing anything.

sp_test.py

import os
import subprocess
def main():
 subp = subprocess.check_call(['python', 'sp.py'])
 print subp
if __name__ == '__main__':
 main()

and...

sp.py

def do_something():
 return "Hello World!"
do_something()
exit()

• Gohn67: Check call returns to the returncode only. Also you will need to print Hello World. Since the only way is to capture the printed output. Use check_output maybe?
• Padraic Cunningham: If you want to see the output use print(do_something())

Answer 1

There are two issues with your implementation.

1.

check_call only returns the returncode of 0 or throws an exception otherwise. (See https://docs.python.org/2/library/subprocess.html#subprocess.call)

2.

Also you are trying to capture the the value of Hello World! by simply returning a value. That won't work using subprocess. If you want to do that, you may want to look into something like Pyro4 (https://pythonhosted.org/Pyro4/intro.html)

---

Here is one solution if you want to use subprocess still.

First you can use check_output (https://docs.python.org/2/library/subprocess.html#subprocess.check_output). This will capture any output from your subprocess.

Then in your sp.py program, you will need to print the result do_something.

sp_test.py

import os
import subprocess
def main():
 subp = subprocess.check_output(['python', 'sp.py'])
 print subp
if __name__ == '__main__':
 main()

sp.py

def do_something():
 return "Hello World!"
print do_something()
exit()

• mino: Thanks for the info, I'll try and get it working with it! My current implementation prints out nothing. Shouldn't it at least print out the return code?
• Gohn67: Yeah it should. I tested your script on my system and it worked. It printed 0
• mino: Interesting.. It prints nothing for me!
• Gohn67: Shouldn't doesn't make a difference, but what OS are you using?
• Gohn67: Maybe try this simple example: subprocess.check_call(['echo', 'Hello World!'])