Issue with running `sudo` in `subprocess.Popen` if `/etc/sudoers` enables `use_pty`

[Wenzel]

Bug report

Bug description:

Summary

Terminal output becomes corrupted/shifted when running a subprocess with sudo using subprocess.Popen. This is a regression introduced in Python 3.7 and still present in Python 3.14-rc.

Python Versions Affected

• Works correctly: Python 3.6.15
• Broken: Python 3.7.17, Python 3.14-rc (and presumably all versions in between)

Environment

• OS: Linux (Docker containers)
• Tested on: python:3.6, python:3.7, python:3.14-rc-bookworm
• Terminal: Docker TTY
• Subprocess command: sudo ping 192.0.2.1

Expected Behavior

Terminal output should remain properly formatted, with log messages appearing on their correct line positions (as seen in Python 3.6).

Actual Behavior

Terminal output becomes visually corrupted with text appearing in wrong positions, overwriting previous lines or shifting unexpectedly. Text appears to ignore carriage returns or line positioning, causing output to overlay incorrectly.

Reproduction Code

Dockerfile (adapt FROM line to test different versions):

FROM python:3.14-rc-bookworm
# Also reproducible with: FROM python:3.7

RUN apt-get update && apt-get install -y sudo inetutils-ping
RUN useradd --create-home --uid 1000 --user-group --groups sudo --shell /bin/bash reporter && \
    echo "reporter ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
USER reporter
WORKDIR /home/reporter
COPY --chown=reporter:reporter corruption.py .
ENTRYPOINT [ "python" ]
CMD [ "corruption.py" ]

corruption.py

#!/usr/bin/env python3

import logging
import subprocess
from typing import List
import time
import os


def _build_proc_cmdline() -> List[str]:
    cmd = [
        "sudo",
        "ping",
        "192.0.2.1"
    ]
    return cmd


logging.basicConfig(level=logging.INFO)
logging.info("Current Python version: %s", ".".join(map(str, os.sys.version_info[:3])))
cmd = _build_proc_cmdline()
logging.info("Running command: %s", cmd)
proc = subprocess.Popen(cmd, stdout=None, stderr=None)

for i in range(5):
    time.sleep(1)
    logging.info("Message %s: proc.status = %s", i, proc.poll())
proc.terminate()

Steps to reproduce

Steps to Reproduce

1. Build Docker image: docker build -t python_corruption .
2. Run container with TTY: docker run -it python_corruption
3. Observe corrupted terminal output
4. Change Dockerfile to FROM python:3.6 and repeat - output is clean

Tested Workarounds

• Changing stderr parameter: No effect
• Changing TERM variable: No effect (initially suspected Docker/terminal issue)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[picnixz]

Changing TERM variable: No effect (initially suspected Docker/terminal issue)

Locally, I can't reproduce this on Python 3.14.

[picnixz]

Also, does it happen with print and without sudo?

[Wenzel]

@picnixz and thanks for the quick reply.

I confirmed that I could reproduce the issue without Docker, on Ubuntu 24.04, Python 3.12.3

The Dockerfile was here to rease reproducibility on multiple Pythons.

Also without sudo, the bug goes away.

And replacing the logging calls by print() doesn't change the issue:

def _build_proc_cmdline() -> List[str]:
    cmd = [
        "sudo",
        "ping",
        "192.0.2.1"
    ]

    return cmd


print("Current Python version:", ".".join(map(str, os.sys.version_info[:3])))
cmd = _build_proc_cmdline()
print("Running command:", cmd)
proc = subprocess.Popen(cmd, stdout=None, stderr=None)

for i in range(5):
    time.sleep(1)
    print(f"Message {i}: proc.status = {proc.poll()}")
proc.terminate()

Happy to provide more info if needed, thanks for taking a look !

[picnixz]

Out of curiosity, could it be because of your PS1/PS2? I see that you have a nice design on your terminal. Can you try with a non-fancy terminal?

[Wenzel]

@picnixz thanks for the suggestion
I have a repro here on /bin/sh with a default PS1:

[picnixz]

Ok, now I don't really know what to suggest. Here are some possible tricks:

• Use universal_newlines=True and bufsize=1.
• Try to add flush=True to the print call.
• Set subprocess._USE_VFORK = False after importing subprocess.
  and set subprocess._USE_POSIX_SPAWN = False (try setting them to False separately).