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Intro to Flask

Learning Objectives#

After this lesson, you will be able to:

  • Write a basic Flask application.

Discussion: Commonalities#

What do you think these websites have in common?

They're each:

  • High on user interactivity.
  • Handling a large server load.

What else?

They All Use Flask#

Some quick notes about Flask:

  • It's a Python micro web framework.
  • It can create and write the entire back-end in Python!
  • It can do small tasks (e.g., create a microblog or stand up a simple API).
  • It can do complex tasks (e.g., Pinterest's API or create a Twitter clone).

Talking Points:

  • Flask is classified as a microframework because it does not require particular tools or libraries.
  • Open the lesson by describing what students are going to do (build a Flask app), and why this is so exciting ("We are using Flask to actually put your stuff on the internet!").
  • Why do we use a lighter web framework like Flask?
  • Talk about how these sites work (lots of interaction and data) and why it is helpful to use Flask for these (get to focus on the interactivity/data and not just getting the thing up on to the internet and staying there).

Flask Syntax#

How?

We just make a normal Python app.

It looks like:

# Import Flask class from flask library. (Note the upper/lowercase convention.)from flask import Flask
# Initialize an instance of the Flask class.# This starts the website!app = Flask(__name__)
# The default URL ends in / ("my-website.com/").# Could be instead "my-website.com/about" or anything - more on this later.@app.route('/')
# Function that returns the page: Display "Hello, World!"def index():  return 'Hello, World!'
# Run the app when the program starts!if __name__ == '__main__':    app.run(debug=True)

Talking Points:

<Note: This is copied from the Flask docs.>

  • First we imported the Flask class.
  • Next, we create an instance of this class. We use __name__ so that Flask knows where to look for templates, static files, and so on.
  • We then use the route() decorator to tell Flask what URL should trigger our function.
  • The function is given a name, which is also used to generate URLs for that particular function and returns the message we want to display in the user’s browser.

We Do: Let's Try#

We'll run the Flask app like any other app.

  • We need to install Flask!
    • pip install flask

Create a file called my_website.py.

Start with:

# Import Flask class from flask library.from flask import Flask

Talking Points:

  • Flask (and lots of web frameworks) can be launched on the command line, giving developers more control and clarity into what is going on.
  • Set global variable (so Flask knows where our main application logic lives).

Teaching Tips:

  • Make sure everyone has done these steps!
  • If there are difficulties with pip, check sudo.
  • Demo these so students have the idea, then let them experiment on their own.
  • For more advanced students, write longer scripts in the index() function, or, if time, assign them the task.

We Do: The Main Flask App#

Let's add:

# Initialize an instance of the Flask class.# This starts the website!app = Flask(__name__)
# The default URL ends in / ("my-website.com/").@app.route('/')
# Function that returns the page: Display "Hello, World!"def index():  return 'Hello, World!'
# Run the app when the program starts!if __name__ == '__main__':    app.run(debug=True)

Teaching Tip:

  • Continuously walk through the code.

We Do: Flask App — Try it#

Run the app like normal:

python my_website.py

Go to:

http://localhost:5000/

You made a web app!

Let's change the string:

def index():  # The "return" determines what's displayed.  return 'Hello, World!'

Teaching Tip:

  • Change around what's returned in index(), so they can see that that's what makes the display.

I Do: Displaying the App#

It's just Python — we can write any code.

  • But return essentially just takes strings.
def index():  my_list = ["Hey", "check", "this", "out"]  return my_list[0] # Works!

Conversely:

def index():  my_list = ["Hey", "check", "this", "out"]  return my_list # WON'T WORK

Teaching Tips:

  • Change around what's returned in index() in a more advanced way.

We Do: Flask Variations#

app and index are just naming conventions.

  • def index(): could be def monkey():.
  • app could be guitar.
    • Be sure to change it in all places!

But, naming variables sensibly is important!

from flask import Flask
guitar = Flask(__name__)@guitar.route('/')
def monkey():  return 'Hello, World!'
if __name__ == '__main__':    guitar.run(debug=True)

Flask History#

Let's back up. Where did Flask come from?

  • Before 2010:
    • No easy method for Python websites.
  • 2010:
    • A few developers built Flask to fix this.

Flask is built on two libraries:

  • Werkzeug:
    • Interfaces with the web.
    • Helps handle request and connections.
  • Jinja:
    • We'll be using this later!
    • We can write templates for all pages across our web app.

we're hitting Jinja later!

Talking Points:

  • Around 2010, a group of open-source Python developers release the first version of Flask!.
  • Before this, there was no easy way to use Python on the internet/for web apps.
  • Flask is built using two libraries (already written bundles of code).
  1. Werkzeug is a library to interface with the web. It helps to handle request and connections.
  2. Jinja is a templating engine, which lets us write an HTML file once and then apply that file to all of our site.

Summary: Flask#

  • A Python micro web framework
  • Developed in 2010

Looks like this:

# Import Flask class from flask library.from flask import Flask
# Initialize an instance of the Flask class.app = Flask(__name__)
# The default URL ends in / ("my-website.com/").@app.route('/')
# Function that returns the page: Display "Hello, World!"def index():  return 'Hello, World!'
# Run the app when the program starts!if __name__ == '__main__':    app.run(debug=True)

Additional Reading#