Here are the steps: 1) install

debugpy

with pip 2) Add a launch configuration for Python in

.vscode/launch.json

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug SST Start",
      "type": "node",
      "request": "launch",
      "runtimeExecutable": "${workspaceRoot}/node_modules/.bin/sst",
      "runtimeArgs": [
        "start",
        "--increase-timeout"
      ],
      "console": "integratedTerminal",
      "skipFiles": [
        "<node_internals>/**"
      ],
      "env": {}
    },
    {
      "name": "Python: Attach",
      "type": "python",
      "localRoot": "${workspaceFolder}/backend/functions",
      "request": "attach",
      "host": "localhost",
      "port": 5678
    }
  ]
}

3) modify the python startup related code in node_modules/@serverless-stack/core/dist/runtime/handler/python.js and replace

"-u"

(line 76 for me) by

"-m", "debugpy", "--listen", "0.0.0.0:5678" , "--wait-for-client",

4) Make sure you increase the timeout for sst in package.json

...
  "scripts": {
    "start": "sst start --increase-timeout", // <--
     ...}

the

--wait-for-client

option means you now have to launch the debugger (F5 for me on Windows) for the function to run

@Jay @Frank could you perhaps later include an option to read an extra parameter when starting sst that could be passed to the python runtime handler and allow an easy choice for a user to wait for the debugger without my hack 🙂?

When I run as described,

debugpy

returns the following:

RuntimeError: Can't listen for client connections: [Errno 48] Address already in use

I changed the port thinking I had something running on

5678

from previous attempts with no success

Or are you pointing to a specific function in your

launch.json

?

but in worst case, you can also specify a different port (it needs to be the same in the command line parameter and your debug configuration)

The root in launch.json also has to match the folder setup you have (I just used the one from the python starter example)

I assume I need to re-run

npx sst start

after making the changes to

python.js

, right?

after the change to

node_modules/@serverless-stack/core/dist/runtime/handler/python.js

you just run

npm run start

(whihc use the modified file in the current project

the parameters for running the function are only generated when sst starts, so a new run is needed

I also updated the instructions above with 4) (just in case you didn't increase the timeout for SST in package.json yet)

An alternative would be to not modify the file + run debugpy directly in the handler itself, but perhaps it's less elegant this way - it depends a bit if you need to debug all functions or not

This is hos it would look inside the lambda function (but then you have to remove the python.js patch as they would conflict)

# ...
# for debugging only
import debugpy
debugpy.listen(('0.0.0.0', 5678))
debugpy.wait_for_client()
debugpy.breakpoint()

@Frank I also updated the open Github Issue, with a link here, just in case someone prefers searching there

I then have to trigger the lambda and run the VSCode debugger at the same time. I presume this is because the

debugpy

listener is only listening during the function run.

I ran the debugger on the JS code on the python.js hander line by line. It seems like on my environment the

debugpy

listening server is not being run during the SST start procedure.

After starting SST, I run

ps aux | grep debugpy

and there is no service running.