Multiprocessing child processes stall during VSCode debugpy breakpoints

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#1,891 opened on May 14, 2025 0 comments 2 reactions 1 assignee View on GitHub

@heejaechang is already working on this.

Since May 14, 2025.

Assessment

This issue has not been assessed yet.

Description

bug needs repro

Environment data

debugpy version: 1.8.13
OS and version: Windows 11
Python version: 3.12+ (python standalone by uv)
Using VS Code or Visual Studio: VS Code 1.100.1 (Universal) on macOS tunneled to Windows with same version VS Code

Actual behavior

I have a running child process which draws some rectangles on screen. If I hit a breakpoint in my main process now I can't manually trigger rectangles drawing from the REPL. This isn't working with debugpy and python 3.12+ - it is working though with Python 3.11.12. Outside of VS Code with f.e. pdb.set_trace() it's working in all versions of Python I've tested.

Expected behavior

The child process keeps working and I can manually trigger things.

Steps to reproduce:

minimal example, tested with Python 3.13.3 on Windows and issue persists.

import multiprocessing
import time


def child_process(q):
    while True:
        msg = q.get()
        if msg == "STOP":
            print("Child received STOP command.")
            break
        print(f"Child received: {msg}")
        time.sleep(0.5)


if __name__ == "__main__":
    multiprocessing.freeze_support()

    queue = multiprocessing.Queue()

    child = multiprocessing.Process(target=child_process, args=(queue,))
    child.start()

    for i in range(3):
        queue.put(f"Message {i}")
        time.sleep(1)

    # Set a breakpoint on the next line in VSCode
    queue.put("Message at breakpoint")  # BREAKPOINT HERE
    # execute in REPL at breakpoint queue.put(f"Hello from REPL")
    
    # Try to continue communication after breakpoint
    queue.put("Final message")
    queue.put("STOP")

    child.join()
    print("Parent process complete.")

no launch.json in use, I've just hit the "Debug Python File" icon in VS Code.

Dominant language: Python
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