Notes: Boston Django Meetup, Intro to Flask, 25 June 2015

On 25 June 2015, Ned Jackson Lovely spoke about Flask at the Boston Django Users Meetup Group. I took some notes and am putting them here so I don't lose them; they may or may not be useful to others. Theses notes are not exhaustive.

Functions that are decorated with @flaskapp.route() should return:

• a string
• a tuple of (response, status, headers)
• flask.Response / current_app.response_class
• a WSGI callable

• In theory, your function could return another Flask app, or spit out a WSGI callable that generated a massive CSV on the fly and streamed it to the user

• You can do testing:

  def test_splash(): client = app.test_client() response = client.get('/') assert response.status_code == 200 assert 'form' in response.data

• The Werkzeug debugger is awesome

• Defining filters for templates is possible (and ostensibly simple)

Sessions in Flask are interesting

• Session data is serialized to JSON, cryptographically signed, and set in the user's browser as a cookie
• Because it's client side, it doesn't matter which datacenter they end up in

• If you write to a "second level" in session, you need to set session.modified = True for the changes to get written out:

  session['first']['second'] = 'new thing' session.modified = True

• Flashing is done with sessions, and is useful for displaying those one-time, web app-y messages like "Your post was submitted"

• On a general Python note, contexts (with/as) are really cool -- you only have to implement two __ methods to get the benefits of them

• Some useful libraries: SQLAlchemy, WTForms

Blueprints:

• Helpful when your app gets bigger
• They're very similar to Flask objects with an additional "namespace"

• app.register_blueprint(bp, url_prefix='/counter')

• To get GET/POST data, use request.values

Deploying:

• gevent, gunicorn, nginx, ansible
• supervisor
• pip freeze