Debug python multiprocess program with VSCode

How to debug a program using python's multiprocessing module on VSCode

Assumed reader

--The python code using the multiprocessing module works fine on the console, but VSCode Those who say that it doesn't work when I try to debug with --Those who want to understand python's multi-process program but don't understand without a debugger --When debugging with VS Code ↓ Those who are in trouble with such an error

Exception escaped from start_client  

failed to launch debugger for child process  

AssertionError: can only join a child process  

RuntimeError: already started

How to do

Debugging a program using the multiprocessing module requires the following three points:

・ 1. If \ _ \ _ name__ == "\ _ \ _ main__": Write

If \ _ \ _ name \ _ \ _ == "\ _ \ _ main \ _ \ _": is required for the file (top level module) to start debugging. If you don't see this statement, add it to format your code. It can be a daunting task in some cases, but it is essential.

・ 2. Write the statement multiprocessing.set_start_method ('spawn', True)

For example, writing the following directly under the main statement will work.

`Description example`


if __name__ == "__main__": 
    import multiprocessing 
    multiprocessing.set_start_method('spawn', True)

For windows freeze_support()
It may be necessary to add a line

・ 3. Write the section'"subProcess": true'in launch.json

Press Ctrl + Shift + D to enter debug mode and edit the configuration launch.json file.

Add a section called "subProcess": true here.

`Configuration example`


{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python: Current File",
      "type": "python",
      "request": "launch",
      "program": "${file}",
      "subProcess": true,    //Postscript
      "console": "integratedTerminal"
    }
  ]
}

Debugging the jupyter notebook file (.ipynb)

VSCode can open and run jupyter notebook files (.ipynb). However, there is still no ability to debug open code. If you want to debug a multi-process program, please apply the above method after converting it to a python script with VSCode function.

Confirmed environment

OS: Debian 10 python: 3.7.4 Visual Studio Code: 1.44.0