How to auto reload & debug Django and Celery workers running in Docker (VS Code)

I run Django projects in Docker containers and use Visual Studio Code as my IDE. In this article, I share how I debug and auto-reload both Django and Celery workers. The solutions are based on debugpy, watchdog, and django.utils.autoreload.

In this article:

• Example docker-compose
• 🔄🐞 Debugging a Django app running in Docker
• 🐞 Debugging a Django celery worker running in Docker (no auto reload)
• 🔄 Auto-reloading a Django celery worker running in Docker (no debug)
• 🔄🐞 Debugging a Django celery worker running in Docker with auto-reload

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Example docker-compose

To better understand the rest of the post, let's assume your docker-compose.yml looks similar to this:

services:
  web: # Django App
    build: .
    command: ./manage.py runserver 0.0.0.0:80
    volumes:
      - .:/app # mount the code inside the container
    links:
      - postgres

  worker: # Celery Worker
    build: .
    command: >-
         celery -A my.package.worker worker 
         -l info --concurrency=6 --queues a,b
    volumes:
      - .:/app
    links:
      - postgres

  postgres: # Database
    image: postgres:15-alpine
    ports:
      - '5432'
    volumes:
      - .data/postgres:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: ...
      POSTGRES_USER: ...
      POSTGRES_PASSWORD: ...

🔄🐞 Debugging a Django app running in Docker

Live reload is enabled when running manage.py runserver, but what about debugging?

The easiest way to make breakpoints work is to install debugpy in the container, and open a remote port for debugging that vscode can connect to.

Since the docker-compose file and the Dockerfile are usually version controlled, let's use a docker-compose.override.yml for the debug setup. From docker compose's documentation:

If you don’t provide the -f flag on the command line, Compose traverses the working directory and its parent directories looking for a docker-compose.yml and a docker-compose.override.yml file. [...] If both files are present on the same directory level, Compose combines the two files into a single configuration.

(see also Merge and override).

Create a docker-compose.override.yml alongside your docker-compose.yml with the following:

# in docker-compose.override.yml
services:
  web:
     command: >- 
        sh -c "pip install debugpy && 
        python -m debugpy --listen 0.0.0.0:3000 
        manage.py runserver 0.0.0.0:80"
     ports:
       - 3003:3000 # set 3003 to anything you want

Now, when running docker compose up, you should see the following in the logs of the web container, before the normal startup logs:

web-1  | Collecting debugpy
web-1  |   Downloading debugpy-1.6.7-py2.py3-none-any.whl (4.9 MB)
web-1  | Installing collected packages: debugpy
web-1  | Successfully installed debugpy-1.6.7

On vscode, create a new debug configuration, either using the UI (debug > add configuration) or by creating the file .vscode/launch.json:

{
    "version": "0.2.0",
    "configurations": [
      {
        "name": "CHANGEME",
        "type": "python",
        "request": "attach",
        "pathMappings": [
          {
            "localRoot": "${workspaceFolder}",
            "remoteRoot": "/app"
          }
        ],
        "port": 3003,
        "host": "127.0.0.1",
        "django": true,
        "justMyCode": false,
      }
    ]
  }

The important things:

• pathMappings.remoteRoot should match the folder where your code is mounted in the container

• port should match the one you mapped to the container's port 3000, i.e. the port debugpy listens to

• justMyCode determines if breakpoints outside of your code (e.g. in libraries you use) work or not.

With this configuration, you can start a debug session (or hit F5) whenever you want and all your breakpoints should work. Once you are done debugging, simply "detach" the debugger using the detach icon.

ℹ️ if you need to debug something that only happens during startup, pass -wait-for-client to debugpy. When set, the Django app won't start until you start the debugger.

🐞 Debugging a Django celery worker running in Docker (no auto reload)

For the celery workers, the same principles apply. Simply change the docker-compose.override.yml to:

services:
  worker:
    command: >-
      sh -c "pip install debugpy && 
      python -m debugpy --listen 0.0.0.0:3000
      /usr/local/bin/celery -A my.package.worker worker
      -l info -P solo --queues a,b"
    ports:
      - 3003:3000

Compared to the initial celery command, the big differences are:

• celery must be called using an absolute path (e.g. /usr/local/bin/celery) since debugpy looks for scripts in the working directory (in my case /app). Using just celery will raise:
  

   No such file or directory: '/app/celery'
  

• passing -P solo to celery instead of --concurrency N simplifies debugging, as only one celery thread is used.

❗⚠️❗ This won't AUTO RELOAD your worker ❗⚠️❗(keep reading 😉)

🔄 Auto-reloading a Django celery worker running in Docker (no debug)

To only auto-reload a celery worker, it is possible to use the awesome utility watchmedo auto-restart from the watchdog package. watchmedo watches a set of files and/or directories and automatically restarts a process upon file changes.

The docker-compose.override.yml becomes:

services:
  worker:
    command: >-
      sh -c "pip install "watchdog[watchmedo]" && 
      python -m watchdog.watchmedo auto-restart 
      -d src/ -p '*.py' --recursive
      celery -A my.package.worker worker -l info -P solo --queues a,b"
    ports:
      - 3003:3000

Common options of watchmedo are:

• the -d or --directory option is the directory to watch. It can be repeated.

• the -p or --patterns option restricts the watch to the matching files. Use ; to list multiple patterns, for example *.py;*.json

• the -R or --recursive option monitors the directories recursively.

Note that we do not need to specify an absolute path for celery anymore.

❗⚠️❗ The debugger won't work ❗⚠️❗(keep reading 😉)

🔄🐞 Debugging a Django celery worker running in Docker with auto-reload

To make live reload and the debugger work, combining debugpy and watchmedo is not enough. I believe it has to do with debugpy restarting every time (thanks to watchmedo), hence losing the connection and context. In other words, we need the restart to happen in the debugged context.

How does Django do auto-reload? Looking at the source code, we can see that the runserver command uses django.utils.autoreload under-the-hood (see runserver.py::run). The cool thing is, this utility can also be used to run other processes!

Here is a simple python file that uses autoreload to run a celery worker:

import django
# ❗ needs to be called *before* importing autoreload
django.setup()

from django.utils import autoreload

def run_celery():
    # ↓ import the Celery app object from your code
    from my_package.celery.app import app as celery_app
    # ↓ usual celery arguments
    args = "-A my.package.worker worker -l info -P solo --queues a,b"

    celery_app.worker_main(args.split(" "))

print("Starting celery worker with autoreload...")
autoreload.run_with_reloader(run_celery)

Don't forget to adapt the args and Celery object import to suit your needs.

Assuming this file is saved at the root of your project, the docker-compose.override.yml becomes:

services:
  worker:
    command: >-
      sh -c "pip install debugpy && 
      python -m debugpy --listen 0.0.0.0:3000 worker.py"
    ports:
      - 3003:3000

For other ways of achieving the same, have a look at [StackOverflow] Celery auto reload on ANY changes.