Lecture 38

Web app wrap-up

MCS 275 Spring 2021
David Dumas

Lecture 38: Web app wrap-up

Course bulletins:

  • Project 4 description posted. Make a Flask+SQLite app. Very flexible rules (e.g. can give and receive help, use online resources, base it on Yellaro/Whinge or not).
  • Project 4 is due 6pm CDT Friday April 30.

This is the last in our lecture series focused on live coding two web applications (source here):

  • Yellaro - Simple chat application
  • Whinge - Submit your pet peeves, vote on others' submissions

Chat app todo

  • ✓ HTML mockup
  • ✓* Stylesheet
  • ✓ Database schema & test data
  • ✓ Flask route to generate front page
  • ✓* Add form to post a message to HTML
  • ✓ Flask route for new message submission

* Additional refinements outside lecture.

Vote app todo

  • ✓* HTML mockup
  • ✓* Stylesheet
  • ✓* Database schema & test data
  • ✓* Flask route to generate front page
  • ✓* Form to submit new item
  • ✓* Flask route to create new item
  • Make the + and - buttons work
  • Second view (chrono vs score)

* Copied from chat app with minor changes.

Chat app routes

  • / - (GET) show message feed
  • /post - (POST) add message

Vote app routes

  • /top/ - (GET) show items, ranked
  • /new/ - (GET) show items, chrono
  • /post - (POST) submit item
  • /plus?postid=15 - (GET) score += 1
  • /minus?postid=15 - (GET) score -= 1

Flask functions

  • url_for(func_name,param1=val1,param2=val2,...) - Get URL corresponding to a function within this application, with optional query parameters, e.g. url_for("record_score",postid=5,score=11) might return "/setscore?postid=5&score=11" if your app contains: 
    
        @app.route("/setscore")
        def record_score():
            print("recording score {} for postid {}".format(
                flask.request.values.get("score"),
                flask.request.values.get("postid"),
            )

Flask functions

  • redirect(url) - Returning this object from a route will cause the HTTP server to issue a 302 response, redirecting the client to url. (Basically, it means "ask them to load a different URL")
  • abort(http_error_code) - Immediately stop and return a HTTP error code (usually 400 bad request, 401 not authorized, 403 forbidden, or 404 not found).

Retrospective

Some of the things you'd do differently in a "real" application:

  • Templates: Instead of strings containing HTML tags embedded in the Python application code, we'd write HTML templates that the Flask app would render.
  • Cookies: login page checks credentials against DB, sets browser cookie. Auth-required pages check for it, redirect to login page if not found.
  • JavaScript: e.g. to check for new messages in real time, post new message without loading a new page
  • Pagination: Links to show next/prev page of messages or posts.
  • Better HTML+CSS: Bottom of the message feed flush with bottom of div. Maybe infinite scrollback with JS?
  • Better vote schema: Instead of storing vote counts (or in addition to them), store a table of user-vote tuples. Allows vote policy enforcement (e.g. one vote per user).

References

Revision history

  • 2021-04-16 Initial publication