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This is Beverly's Flask tutorial

This tutorial is meant for people who have a good understanding of Python, but who have never done web programming before. You should know static HTML, but it is not necessary to know about forms, requests, and methods.

I finally "understood" Flask when I realized

  • how the different functions are called based on "GET" and "POST" requests
  • what a "request" was
  • how HTML pages are rendered and returned
  • how to use Python as the machinery to decide which function to call (and thus which HTML template to return)

After a short visit with a simple "Hello World" application, we will build a quiz application that will demonstrate "GET" and "POST" requests, rendering HTML templates, and redirecting to other functions that will render the appropriate HTML templates. User input will determine the HTML pages that are returned, and the data will be written to a text file on the server side.

Some housekeeping

I hate programming tutorials that use function and instance names that look like offical names. There are so many function names and instances in tutorials that are totally user-defined. Basically:

Name it whatever-you-want, and it will still work!

Instead of using willy-nilly names like "lulu", I will append "_lulu" to the end of logical names, so it will be entirely obvious which names can be changed with no penalty. And, it will give you an idea of what "real" programmers might use to call a certain function (hint: just drop the "_lulu"). Naming things well is certainly a skill to be desired.

Just realize, if you change the name of the file from hello_lulu.py to something else, like flufflepuff.py, make sure to mentally make that change for the rest of the tutorial.

A Quick Note about Debug Mode

In this tutorial, you'll be running your app in Debug Mode. This is very helpful for you - it means that if the server hits an error, it will give you a detailed message instead of just "500 - Internal Server Error" or something similarly non-obvious. However, it's important to note that debug mode is HIGHLY INSECURE, and should absolutely never be used on production machines. If that doesn't mean much to you, don't worry about it - you won't be running on production machines in this tutorial.

Hello world

Open a file called hello_lulu.py. Type the following:

from flask import Flask
app_lulu = Flask(__name__)

@app_lulu.route('/hello_lulu2')
def hello_world_lulu():
    # this is a comment, just like in Python
# note that the function name and the route argument
# do not need to be the same.
    return 'Hello World!'

if __name__ == '__main__':
    app_lulu.run(debug=True)

Now, run the code from command line using:

>> python hello_lulu.py

Running this code will start a server at: 127.0.0.1:5000/hello_lulu2. If you visit that address in a browser window, you should see a blank screen with text at the top left corner, which says "Hello World!"

Note that the @app_lulu.route('/hello_lulu2') is showing which code to run (in this case, hello_world_lulu) when the URL /hello_lulu2 is requested. (A request is made when the URL is visited.)

When the URL is visited, the code is run, and a string is returned. The string is the HTML code for the page. Since HTML does not parse white space, one long string will translate to an HTML page easily.

Short note on browsers

127.0.0.1 is Home, or your own computer. 5000 is the port number. Flask puts the server on Port 5000 as the default. I'm not going to talk a lot about this (actually I'm not going to talk at all about this), because you can play with this on your own later.