从php函数执行python脚本？

Question

I want to execute a python script,test.py, from a php function.

#!/usr/bin/env python
import random

def test() :
   print("test success")

if __name__ == '__main__':
    test()

I can successfully execute it using php command line:

php -r '$message = exec("python3 ~/../scripts/test.py"); print_r($message);' 

I've added the following to my php code:

$message = exec("python3 ~/../scripts/test.py", $output, $return_val); 

When I execute it, nothing is output (no errors in devtools). When I print the variables, $output and $message are empty and $return_val is 2 (misuse of shell builtins according to Bash documentation).

I know exec() is allowed after adding the following into the same function:

if (exec('echo TEST') == 'TEST')
    {
      echo 'exec works!';
    }

What could be causing this error? How can I execute the python script with php?