Learn Python Flask #01: Install and say Hello Flask!

Learn Python Flask #01: Install and say Hello Flask!

Before getting started, let’s find out some concepts:

What is Flask?

Flask is a “micro” Python framework for developing web services and REST APIs, which is lightweight, simple but extensible.

”Micro” means it does not includes any other tools or libraries, no database abstraction layer as other related frameworks like Django.

What is REST API?

REST (REpresentational State Transfer) as a set of conventions taking advantage of the HTTP protocol to provide CRUD (Create, Read, Update, and Delete) behaviour on things and collections of those things.

You can perform these actions on a thing or resource. A URL should identify a unique resource on the web, something that will work with the same thing over and over again for the same URL

Getting started!

Setting up Virtual Python Environment

To avoid any conflict with other existing Python installations, we should set up a virtual and “isolated” Python environment.

You can use Python venv to create a new virtual environment (env):

python3 -m venv <env_name>  

Then activate that env by running the following command

. <env_name>/bin/activate

My favourite VE is conda. The VE creation and activation is similar:

Create conda environment

conda create -n <env_name> python=<version>    

Activate and deactivate the environment

# To activate this environment, use
conda activate flask

# To deactivate an active environment, use
conda deactivate

Install Flask

Using pip to install Flask

pip install Flask


Install python-dotenv to embed Flask environment variables into .env file

pip install python-dotenv

Freeze (embed) all installed pip packages to requirements.txt file:

pip freeze > requirements.txt

Write “Hello Flask” program

Create a app.py and start writing the first Flask program

from flask import Flask

app = Flask(__name__)

def index():
    return "Hello Flask!"

Let’s run!

At the same folder of app.py (the default Flask app name), run the following command to start the “Hello Flask” program locally

flask run

By default, our program will use port 5000. We can open the browser and access the URL http://localhost:5000/ or


Turn on Development mode (auto-reload if there are any changes in source code) and run the Flask program by the combined command as below

FLASK_ENV=development flask run

If the Flask python filename is not app.py (for example api.py), the flask run will not work. So, we need to set another environment FLASK_APP=api.py

We can also include all environment variables in .flaskenvfile and put it in the same folder with api.py file.


Then, just flask run to execute the Flask program.

If you can see “Hello Flask!”, then Congratulations, all done!

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