Flask Tutorial: Hello World

Welcome! In this tutorial you will learn how to create your first Web App with Python Flask. If you prefer learning with a video course, I recommend the course below:

Setup Project

Python projects live in virtual environments. Each project lives in a different virtual environment. This prevents package conflicts. Python packages should not be installed system wide.

Consider this:

• Project A needs Flask version 0.2
• Project B needs Flask version 0.3
• Project C needs Flask version 0.1

There is no way that system wide package installation would work, as there would be package conflicts.

Create Virtual Environment

Open a terminal (see below how to open one quickly). Then install python3-venv.

On Ubuntu Linux you can run this command:

sudo apt-get install python3-venv

First create a project directory with the command

$ mkdir flaskexample
cd flaskexample

Then you can create a new virtual environment with the command:

$ python3 -m venv venv

Activate Virtual Environment

The virtual environment has been created, but it's not yet active. Activate the virtual environment on Linux, use the command:

source venv/bin/activate

On Microsoft Windows use this instead:

$ venv\Scripts\activate

You should see:

(venv) ➜  flaskexample

Install Flask

The first step is to install Flask. Python comes with a package manager named pip. It uses the the official Python package repository named PyPI.

To install a Python package, you need to open a terminal. This varies per operating system.

• On Linux you can press the key combination Ctrl+Alt+T to open a terminal.
• On Mac OS X, press the keys CMD + Space to open spotlight search, and type terminal and hit return.
• On Windows, press the keys Super + R and type cmd and press the enterk ey.

You can install a Python package with the command:

pip install <package-name>

In this case you want to type the command:

pip install flask

Then verify it's installed correctly. Type the command

(venv) ➜  flaskexample python3

The output should be:

>>> import flask
>>>

If you see the output below, it means flask is not installed in the virtual enviroment.

Python 3.7.3 (default, Aug 20 2019, 17:04:43)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import flask
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'flask'
>>>

Hello World

Great! Now that everything is installed you can create your first Flask App.

Use the line below to import Flask in Python.

from flask import Flask

Create app, that hosts the application

app = Flask(__name__)

Then you need a route that calls a Python function. A route maps what you type in the browser (the url) to a Python function.

@app.route('/')
    def index():

The function should return something to the web browser,

    return 'Web App with Python Flask!'

Almost done, the server needs to be started. This starts the web app at port 81.

app.run(host='0.0.0.0', port=81)

Enter the url http://localhost:81/ in your web browser.

Code summary:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return 'Web App with Python Flask!'

app.run(host='0.0.0.0', port=81)