Deploy a Flask App to Heroku

Deploy a Flask app with an encrypted .env.vault file to Heroku.

Initial setup

Install flask and gunicorn.

pip install --upgrade flask gunicorn

Create an file with the web server code.

import os
from flask import Flask

app = Flask(__name__)

def hello():
  hello = os.getenv("HELLO")
  return f"Hello {hello}"

Create your file.

from index import app

if __name__ == '__main__':

Add a Procfile to run your app on Heroku.


web: gunicorn --workers 4 --bind${PORT} wsgi:app

Create runtime.txt.



Freeze your requirements.txt.

pip freeze > requirements.txt

Set up your .gitignore file.



Commit that to code and push it to Heroku.

heroku create
git push heroku

Once deployed, your app will say 'Hello None' as it doesn't have a way to access the environment variable yet. Let's do that next.

Install python-dotenv-vault

Install python-dotenv-vault.

pip install --upgrade python-dotenv-vault

Create a .env file in the root of your project.


# .env

As early as possible in your application, import and configure dotenv-vault.

from dotenv_vault import load_dotenv

import os
from flask import Flask

Try running it locally.

gunicorn --workers 4 --bind wsgi:app
[2023-07-31 10:20:35 -0700] [49163] [INFO] Starting gunicorn 20.1.0
[2023-07-31 10:20:35 -0700] [49163] [INFO] Listening at: (49163)

It should say Hello World.

Great! os.environ now has the keys and values you defined in your .env file. That covers local development. Let's solve for production next.

Build .env.vault

Push your latest .env file changes and edit your production secrets. Learn more about syncing

npx dotenv-vault@latest push
npx dotenv-vault@latest open production

Use the UI to configure those secrets per environment.

Then build your encrypted .env.vault file.

npx dotenv-vault@latest build

Its contents should look something like this.


#/         cloud-agnostic vaulting standard         /
#/   [how it works](   /

# development

# production


Fetch your production DOTENV_KEY.

npx dotenv-vault@latest keys production
# outputs: dotenv://:[email protected]/vault/.env.vault?environment=production

Set DOTENV_KEY on Heroku using the CLI.

heroku config:set DOTENV_KEY='dotenv://:key_1234…'

Or use Heroku's UI.


Commit those changes safely to code and deploy.

That's it! On deploy, your .env.vault file will be decrypted and its production secrets injected as environment variables‚ just in time.

You'll know things worked correctly when you see Loading env from encrypted .env.vault in your logs. If a DOTENV_KEY is not set (for example when developing on your local machine) it will fall back to standard dotenv functionality.

heroku logs --tail