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Introduction to Flask

Flask is a Python framework for building web apps. It's small, light and simple compared with the other widely used Python framework, Django.

This site is the home of Flask.

We will install Flask in a new Python3 virtual environment. Students have already installed both Python3 and virtualenv.

Install Python3, virtualenv, Jupyter


Setup for Flask

Create a directory and change into it

The first step is to create a new folder (directory) for all your Flask projects. Mine is:


Change into that directory. For me, the command would be:

cd Documents/python/flask

Create a new virtualenv in that directory and activate it

Create a new virtualenv there (this is done only once):

Mac OS/bash

$ virtualenv --python=/usr/local/bin/python3 env

Windows PowerShell

PS> virtualenv --python=C:\Python36\python.exe env

Activate the virtualenv:

Mac OS/bash

$ source env/bin/activate

Windows PowerShell

PS> env\Scripts\activate.bat

Important: You should now see (env) at the far left side of your prompt. This indicates that the virtualenv is active. Example (Mac OS/bash):

(env) mcadams flask $

Install Flask

In Mac OS or Windows, at the $ bash prompt (or Windows PS>), type:

pip install Flask

This is how you install any Python library that exists in the Python Package Index. Pretty handy. pip is a tool for installing Python packages, which is what you just did.

Note: You installed Flask in the Python3 virtualenv that is currently active. When that virtualenv is not active, Flask will not be available to you. This is ideal, because you will create different virtual environments for different Python projects, and you won't need to worry about updated libraries in the future breaking your (past) code.

Test Flask

Using Atom, create a file in your flask directory, copy/paste the code into it, and save it with the name hello.py.

from flask import Flask
app = Flask(__name__)

def hello():
    return "Hello World!"

In Terminal, at the $ bash prompt, type this:

FLASK_APP=hello.py flask run

You'll see this:

Results of FLASK_APP=hello.py flask run

Open your web browser and, in the address bar, type: localhost:5000

Result in the browser

Flask includes a built-in web server, for development use. What you've done is:

  1. With the hello.py file, you have written a small (and essentially useless) web app in Flask.
  2. With the command FLASK_APP=hello.py flask run, you started the server and ran the app on it.

To shut down the server, press Control-C in Terminal.

Deconstruct the code in a small Flask app

Import Flask and create an application object

from flask import Flask
app = Flask(__name__)

The first line is a typical Python import statement. Flask is a Python library, and it must be imported. As always, case matters, so note the lowercase f and the uppercase F. (We are importing the Flask class from the flask module, and they are two different things.)

The second line, which is new to you, begins with a new variable, app, which will be used in every Flask app. The value of that new variable, Flask(__name__), is a new object that inherits from the class Flask — meaning that it gets all the attributes and methods built into that class, which we have imported.

__name__ is a built-in variable in Python. Python has many double-underscore entities, and they always have this pattern: two underscores, a word, and two underscores. These double underscore entities are referred to with the slang dunder — for __name__, we can say “dunder name.”

What does __name__ do? Every Python module has a name, and __name__ used in a module contains the name of that module. The value of __name__ is not always the filename, as demonstrated in a common Python statement:

if __name__ == '__main__':

When that statement returns True, it means the program (the file) is being run by itself, and was not imported.

app = Flask(__name__) creates a Flask application object, app, in the current Python module. A Python module is just a Python file, filename.py.

Add a route and some action

This next part of your first Flask app is what does the work.

def hello():
    return "Hello World!"

It consists of two parts: the decorator and the function that is “decorated.”

A decorator begins with @ and is a unique feature of the Python language. It modifies the function that follows it. Let that sink in.

@app.route("/") is a decorator.

  • Remember that app is a Flask application object. It has all the methods and attributes of the Flask class, and one of those is route(), which expects to be used in exactly this way — in a decorator.
  • The contents of the parentheses are a path — a partial URL.
  • Your Flask application will perform different actions depending on which URL is sent to it. "/" is the root of the website, the top, the home page. @app.route('/index') indicates a URL such as localhost:5000/index or (on a live server) https://mydomain.com/index. Note that there is no file there — no .html.
  • The action that will be performed at that URL depends on what is written in the function that immediately follows the decorator.
def hello():
    return "Hello World!"

All this function does is return a simple string: "Hello World!" Our Flask app performs this action when the server is running, the app is running, and we open localhost:5000 in the browser.

Note that there does not need to be any relationship between the decorator and the function except proximity:

def hello():
    return "Hello World!"

Often people writing a Flask app use the same word for the route and the function, like so:

def foobar():
    return "Hello World!"

There's nothing wrong with this, but it's not necessary. Use it if you like it.

You're ready now to move on to flask/part2.