Simple flask app on heroku – all steps (almost)

Note – instructions assume Ubuntu Linux.

See Getting Started with Python on Heroku (Flask) for the official instructions. The instructions below tackle things differently and include redis-specific steps.

Don’t need postgresql for my app even though needed for heroku demo app. Using redis for simple key-value store.

Main reason for each step is indicated in bold at start. There are lots of steps but there are lots of things being achieved. And each purpose only requires a few steps so probably hard to streamline any further.

    >> sudo apt-get install python3 python3-pip python-virtualenv git ruby redis-server redis-tools
    Get free heroku account
    Install heroku toolbelt Heroku setup. Sets up virtualenvwrapper for you too (one less thing to figure out)
    Do the once-ever authentication
    >> heroku login
  5. APP
    Make project folder e.g.
    >> mkdir ~/projects/myproj
  6. APP
    >> cd ~/projects/myproj
    >> echo “web: python” > Procfile
    >> git init
    >> mkvirtualenv sticky

    So requirements for specific project can be separated from other project – lets heroku identify actual requirements. Normally “workon sticky” thereafter; deactivate to exit virtual env

  10. APP
    >> pip3 install flask
    Note – installed within virtualenv
  11. HEROKU
    Save the following as requirements.txt – needed by heroku so it knows the dependencies. Update version of redis as appropriate. gunicorn is a better approach than the flask test server
  12. HEROKU
    So we can use Python 3.4 instead of the current default of 2.7:
    >> echo “python-3.4.3” > runtime.txt
  13. APP & HEROKU

    Make a toy app to get started from.

    Note – modify the standard demo flask app to add a port to ease eventual heroku deployment. Otherwise the app will fail because of a problem with the port when running

    heroku ps:scale web=1

    Starting process with command `python`
    Web process failed to bind to $PORT within 60 seconds of launch

    Here is an example (may need updating if flask changes):

    import os
    from flask import Flask
    app = Flask(__name__)

    def hello():
        return "Hello World!"

    if __name__ == "__main__":
        port = int(os.environ.get("PORT", 33507))'', port=port)

    >> deactivate
  15. Make a module to make it easier to work with redis – let’s call it

    import os
    import urllib
    import redis

    url = urllib.parse.urlparse(os.environ.get('REDISTOGO_URL',
    redis = redis.Redis(host=url.hostname, port=url.port, db=0,

    We can then use redis like this:
    from store import redis

  16. APP
    Keep building app locally. The following is good for redis: Redis docs. And flasks docs are always good: Flask Docs – Minimal Application

    Before deploying to production:

    1. Update git otherwise you’ll be deploying old code – heroku uses git for deployment
    2. set app.debug to False (although no rush when just getting started and not expecting the app to get hit much)
    3. probably switch to gunicorn sooner or later (will need to change ProcFile to
      web: gunicorn main:app --workers $WEB_CONCURRENCY
    4. Example nginx.conf:

      # As long as /etc/nginx/sites-enable/ points to
      # this conf file nginx can use it to work with
      # the server_name defined (the name of the file
      # doesn't matter - only the server_name setting)
      # sudo ln -s /home/vagrant/src/nginx.conf ...
      #     ... /etc/nginx/sites-enabled/
      # Confirm this link is correct
      # e.g. less /etc/nginx/sites-enabled/

      server {
          listen 80;
          server_name localhost;

          location /static { # static content is

              # handled directly by NGINX which means
              # the nginx user (www-data) will need
              # read permissions to this folder
              root /home/vagrant/src;


          location / { # all else passed to Gunicorn to handle
              # Pass to wherever I bind Gunicorn to serve to
              # Only gunicorn needs rights to read, write,
              # and execute scripts in the app folders

    5. Example gunicorn.conf
      import multiprocessing

      bind = "" # ensure nginx passes to this port
      logfile = "/home/vagrant/gunicorn.log"
      workers = multiprocessing.cpu_count() * 2 + 1

  18. HEROKU
    >> heroku create

    Should now be able to browse to the url supplied as stdout fom command e.g. Note – not working yet – still need to deploy to new app

    >> git push heroku master

    Must then actually spin up the app:

    >> heroku ps:scale web=1

    A shortcut for opening is

    >> heroku open

  19. HEROKU
    Add redis support (after first deployment – otherwise

    ! No app specified.
    ! Run this command from an app folder or specify which app to use with --app APP.
    >> heroku addons:create redistogo

    Note – need to register credit card to use any add-ons, even if free ones. Go to

Some other points: when developing on a different machine, I needed to supply my public key to heroku from that other machine (Permission denied (publickey) when deploying heroku code. fatal: The remote end hung up unexpectedly).

heroku keys:add ~/.ssh/

And the full sequence for upgrading your app after the prerequisites have been fulfilled is:

  1. git commit to local repo
  2. Then git push to heroku
  3. Then run heroku ps:scale web=1 again

And I had a problem when I switched from Python 2 to 3 with redis – my heroku push wouldn’t work. By looking at the logs (>> heroku logs –tail) I found that import imap wouldn’t work and searching on that generally found I needed a newer version of redis than I had specified foolishly in requirements.txt.