Printing subprocess stdout lines output

August 4, 2024

I created a simple Python function:

import subprocess
from io import TextIOWrapper


def run_shell_command(command: list, debug: bool = False):
    '''
    Run shell command

    :param command: Shell command
    :param debug: Debug mode
    :return: Result code and message
    '''
    try:
        process = subprocess.run(
            command, check=True, text=True, timeout=5,
            stdout=subprocess.PIPE, stderr=subprocess.STDOUT
        )
        if debug:
            for line in TextIOWrapper(process.stdout, encoding='utf-8'):
                print(line)
        message = 'Shell command executed sucessfully'
        return ({'code': 200, 'msg': message, 'stdout': process.stdout})
    except subprocess.CalledProcessError as e:
        return ({'code': 500, 'msg': e.output})


if __name__ == "__main__":
    command = run_shell_command(['ls', '-lah'], True)
    print(command)

When I run it in debug mode, I get the following error:

Traceback (most recent call last):
  File "/tmp/command.py", line 28, in <module>
    command = run_shell_command(['ls', '-lah'], True)
  File "/tmp/command.py", line 19, in run_shell_command
    for line in TextIOWrapper(process.stdout, encoding="utf-8"):
AttributeError: 'str' object has no attribute 'readable'

Running Python 3.9 on a Linux server, I was wondering if you can provide some insight where the issue might be. With debug disabled, I get a proper text output. Thank you for your help.

Edit: Based on the comments below, the fix is quite simple:

        if debug:
            print(process.stdout.rstrip())

>Solution :

Unfortunately "simple" doesn’t cut it with all of the corner cases involved – you’ll need to read the subprocess stdout as it’s being streamed, print it out and accumulate it in a buffer, and keep track of time so you can timeout correctly. Note that this too has a possible bug (though not very dangerous) if the 4096 byte read happens to end at a multi-line character.

import subprocess
import time


def run_shell_command(command: list, debug: bool = False, timeout: float = 5):
    """
    Run shell command

    :param command: Shell command
    :param debug: Debug mode
    :return: Result code and message
    """
    process = subprocess.Popen(
        command,
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT,
    )
    start_time = time.time()
    output = b""
    while True:
        buf = process.stdout.read(4096)
        if debug:
            # could fail if `buf` happens to end in a multi-byte character
            print(buf.decode("utf-8", "ignore"))
        output += buf

        if time.time() - start_time > timeout:
            process.kill()
            message = "Shell command timed out"
            return {"code": 500, "msg": message, "stdout": output}

        if process.poll() is not None:
            break
    if process.returncode != 0:
        message = "Shell command failed"
        return {"code": 500, "msg": message, "stdout": output}

    message = "Shell command executed successfully"
    return {"code": 200, "msg": message, "stdout": output}


if __name__ == "__main__":
    command = run_shell_command(["ls", "-lah"], True)
    print(command)