Running a string command using exec with popen

Question

I have a simple cmd_str containing a set of lines. Using exec, I can run those lines juts fine. However, running those lines in a separate process when shell=True is failing. Is this dues to missing quotes? what is happening under the hood?

import subprocess

cmd_str = """
import sys
for r in range(10):
    print('rob')
"""

exec(cmd_str) # works ok


full_cmd = f'python3 -c "exec( "{cmd_str}" )"'

process = subprocess.Popen([full_cmd],
                            shell=True,
                            stdout=subprocess.PIPE,
                            stderr=subprocess.PIPE)

(output, error) = process.communicate()
exit_code = process.wait()
output_msg = output.decode("utf-8", 'ignore')
error_msg = error.decode("utf-8", 'ignore').strip()

Answer 1

Your approach is slightly inaccurate. I believe the problem you're having has to do with the subprocess usage. The first thing you must realise is that exec is a way to send and execute python code to and from the interpreter directly. This is why it works inside python programs(and it is generally not a good approach). Subprocesses on the other hand handle command like they are being called directly from the terminal or shell. This means that you no longer need to include exec cause you are already interacting with the python interpreter when you call python -c.

To get this to run in a subprocess environment all you need to do is

full_cmd = f'python3 -c "{cmd_str}"'
process = subprocess.Popen(full_cmd,
                            shell=True,
                            stdout=subprocess.PIPE,
                            stderr=subprocess.PIPE)

Also, notice the absence of square brackets when calling subprocess.Popen, this is because that particular approach works slightly different and if you want to use the square brackets your command will have to be

full_cmd = ['python3', '-c', f'{cmd_str}']

process = subprocess.Popen(full_cmd,
                                stdout=subprocess.PIPE,
                                stderr=subprocess.PIPE)

And with these few changes, everything should work OK