Using PDB and docker-compose

I’ve recently migrated my web application to run inside docker containers and use docker-compose to spin up all the required dependencies in their own containers (Postgres and Redis, in my case). This application uses Python’s Flask framework and I use pdb to debug issues.

While researching solutions, I first ran into this article by Vladyslav Krylasov that suggested adding some configuration to my docker-compose.yaml and using a module called remote-pdb with a telnet client. This worked, but it was a little more cumbersome than I wanted.

My solution was to use docker attach. First, I still needed to edit my docker-compose.yaml file to add the stdin_open and tty attributes to my application container:

application:
  ...
  stdin_open: true
  tty: true

Next, I needed to change the way I started my service. I edited my docker-compose up line to run in the background with docker-compose up -d and then ran docker attach application to show all the container’s output in my console window. Now I get the app’s output (and it looks identical to how it did when I was running the Flask server locally) and can interact with it in pdb.

To test, I added a debugging line to a route:

@bp.route('/', methods=['GET'])
def index():
  import pdb; pdb.set_trace()
  return render_template(‘site/index.html’)

Then I pointed my browser to it and checked my console…

-> return render_template(‘site/index.html’)
(Pdb)

Beautiful!