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Your first Flask app | Learning Flask Ep. 1

In the first part of this series, you'll learn how to create and run your very first Flask web application

Article Posted on by in Flask
Julian Nash · 2 years ago in Flask

Welcome to the first part of the Learning Flask series. In this part, you'll be creating a very basic web application, just so you can see how simple and basic a Flask application can be.

We're going to be creating a virtual environment and using it throughout this series. So let's get started.

Creating a project directory and virtual environment

First of all, we need to create our new project directory. We're going to a new directory called app in our home directory.

Tip - I advise using the same names for your project so it's easier to follow along

Go ahead and create the directory with the following:

mkdir ~/app

Next, let's move into the app directory:

cd ~/app

Now we need create out Python virtual environment. Do so with the following command:

python -m venv env

This will create a new virtual environment called env

You'll see a new directory appear inside the app directory called env. Now we need to activate our virtual environment before we install Flask.

Go ahead and run the following command to activate the environment:

source env/bin/activate

You should see (env) appear in front of your terminal prompt indicating the virtual environment is activated!

Updating pip

It's best practice to update pip (Python's package manager) after creating a new virtual environment. We can do so with the following:

pip install --upgrade pip

You should see a success message like the following

Collecting pip
  Using cached https://files.pythonhosted.org/packages/46/dc/7fd5df840efb3e56c8b4f768793a237ec4ee59891959d6a215d63f727023/pip-19.0.1-py2.py3-none-any.whl
Installing collected packages: pip
  Found existing installation: pip 18.1
    Uninstalling pip-18.1:
      Successfully uninstalled pip-18.1
Successfully installed pip-19.0.1

Now we're ready to install Flask!

Installing Flask

You install Flask just as you would any other Python package.

pip install flask

If we now run pip list, you'll see the following:

Package      Version
Click        7.0
Flask        1.0.2
itsdangerous 1.1.0
Jinja2       2.10
MarkupSafe   1.1.0
pip          19.0.1
setuptools   40.6.2
Werkzeug     0.14.1

Note - Flask comes with several other packages so don't be alarmed when you see MarkupSafe or itsdangerous!

Ok so we've got everything we need to start building our very simple application. Let's get to it.

Creating a Flask app

This guide is just to show you the most basic Flask application possible. You'll learn the correct way to structure a Flask application over the next couple of parts in this series.

The most basic Flask app can be just a single file. We're going to call it app.py. Make sure you're in the app directory and run the following to create it:

touch app.py

Let's write some code! Go ahead and open up app.py in your favourite editor and follow along.

First of all, we need to import Flask from flask

from flask import Flask

Now we need to create our Flask application. We're going to pass __name__ to Flask and assign it to the variable app

Don't worry about exactly why we're doing this. We'll cover it in a more advanced episode in this series.

from flask import Flask

app = Flask(__name__)

Next up, we need to create a route or view (route and view are used interchangeably)

Let's create a route and explain it line by line after:

from flask import Flask

app = Flask(__name__)

def index():
    return "Hello world!"

Let's talk through the 3 lines we just added:


Routes in Flask are created using the @app.route decorator and passing in a URL or path.

In this example, we've passed "/" into the @app.route decorator. "/" is the root of the website or application.

This route will be triggered when someone goes to the root or index of our website, for example http://example.com.

def index():
    return "Hello world!"

Under the @app.route decorator, we simply write a standard Python function with a return statement.

Flask will return whatever we pass to the return statement! In this case, just a short "Hello world!" string.

We need to add 2 more lines of code before we can run our app. Add the following:

from flask import Flask

app = Flask(__name__)

def index():
    return "Hello world!"

if __name__ == "__main__":

Let's take a look at what we added:

if __name__ == "__main__":

Again, I don't want you to worry too much about what's happening here. For now just know that __name__ is a special variable used by the Python interpreter to understand if a file is the main program.

Just as we passed __name__ into the Flask() class, the special variable __name__ is equal to __main__. You'll learn more about this principle as you advance through the series.

Running the Flask app

Time to see the app in action! We can run our Flask app in a couple of ways.

In your terminal, make sure you're in the same directory as app.py and run the following:

python app.py

You'll see the following message in your terminal to let you know Flask is running:

 * Serving Flask app "app" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on (Press CTRL+C to quit) - - [03/Feb/2019 14:35:04] "GET / HTTP/1.1" 200 - - - [03/Feb/2019 14:35:04] "GET /favicon.ico HTTP/1.1" 404 -

Great, our app is running! Ignore any of the warning messages in your terminal and open up a new browser tab and head to the following URL:

You should see Hello world! in your browser!

Head back over to your terminal and stop the app by hitting Ctrl + c

So you've seen one way to run your Flask app but it's not recommended. There's a better way.

Flask environment variables

To make running our app even easier, we're going to set a couple of environment variables in our shell. Run the following commands and we'll talk through them after:

export FLASK_APP=app.py
export FLASK_ENV=development

Running export FLASK_APP=app.py will set the FLASK_APP variable to app.py

Running export FLASK_ENV=development tells Flask we want to run our app in development mode

Warning - Never run a live Flask application in production using development mode

We're quite a way from deploying our app to the web but I want to drill it home early, just so you know. We'll cover the reasons why later in the series.

We can now run our app using the following simple command:

flask run

You'll see:

 * Serving Flask app "app.py" (lazy loading)
 * Environment: development
 * Debug mode: on
 * Running on (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 201-167-879

You'll notice we don't get any of the warnings and that Environment is set to development

Head back to and you'll see the same Hello world! message as before.

Use Ctrl + c in your terminal to stop the app when you're ready.

To deactivate your virtual environment, simply enter:


Wrapping up

You now know how to create and run a very basic Flask application, along with some best practices for setting environment variables and using the flask run command.

Next up, learning how to structure your Flask application properly!

Last modified · 28 Feb 2019
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