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Learn Flask in five minutes

Abel Lifaefi Mbula

Flask is a lightweight framework for quick web application development in Python. This shot aims to teach you how to quickly bootstrap a basic app with it.

Let’s get started.

Note: For this exercise, make sure you have Python 3 installed on your computer.

Workflow

First thing first, let’s see how to work with Flask the right way.

1. Create a new folder for your project

mkdir flask101
# move to the newly created folder
cd flask101

2. Create a virtual environment with the venv module

The virtual environment is important, as it helps avoid packages conflicts within your projectse.g., due to different versions. Since Python 3.3, Python comes bundled with the venv module to create a lightweight virtual environment.

Now, within our project folder, let’s run the following.

python3 -m venv .venv

This will create the target directory (.venv). This will house everything related to our virtual environment in the project, like the pyvenv.cfg file, bin: (or Scripts in Windows), and lib (or Lib in Windows).

Notice that while my target directory is named .venv, you can name it as you wish.

3. Activate the environment

The next step before we work on our project is to activate the corresponding environment.

# GNU/Linx || MacOS
. venv/bin/activate

# Windows
venv\Scripts\activate

Once you run the command above, your shell prompt will change to show the name of the activated environment; in our case, .venv.

Note:

  • If you want to quit the virtual environment, you need to type deactivate.
  • If running scripts is disabled on Windows due to the execution policy, open Powershell as administrator and run this command:
Set-ExecutionPolicy Unrestricted

Type Yes (Y) when it asks for confirmation.

4. Install Flask

Last but not least, you can now install our framework within the activated environment. To do so, use the following command.

pip install Flask

That is how you set up a working environment to work with Flask.

Let’s now learn how to say “Hello, world!” in Flask.

Say “Hello, world!” in Flask

A minimal app

Make sure you’re in the activated environment and create a file called hello.py.

touch hello.py

Within the file, add the following code.

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

This code snippet is what a minimal Flask app looks like. We first import the Flask class from the flask module and then create an instanceapplication object. The first argument for the object is __name__, a convenient shortcut for the name of the application’s module or package.

Next, we set a route (/) to tell Flask what URL should trigger our function (hello_world()). Our function returns the message we want to display in the user’s browser.

Warning: Do not call your application flask.py. This will conflict with Flask itself.

Run the app

It is not difficult to run the app. You can use either flask or the python -m flask command, like this:

flask run

Running this command will throw an error. To fix it, we need to export the FLASK_APP environment variable to tell our terminal which application to work with.

# Bash
export FLASK_APP=hello

# CMD
set FLASK_APP=hello

Now, we can run our app and enjoy the result on the browser:

flask run
 * Running on http://127.0.0.1:5000/

Our application responds on the port 5000 of http://localhost.

Conclusion

Flask is a popular framework used for web development in Python. To get started with it, make sure you have a convenient isolated environment for your project; you can use the venv module for this. After you create a minimal Flask app by importing the Flask class, you have to export the FLASK_APP environment variable before running the app with flask run.

Happy coding!

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