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).

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)

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

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)

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!'

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

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.

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)