Flask “Hello, World” Application


  • Pip3
  • Virtualenv

Install pip first.

For python 2.7

sudo apt-get install python-pip

For python 3

sudo apt-get install python3-pip

Install virtual environment

sudo pip3 install virtualenv

Now create a virtual environment

virtualenv venv

You can also use a Python interpreter of your choice

virtualenv -p /usr/bin/python2.7 /virtualenvspath/
virtualenv -p /usr/bin/python3 /virtualenvspath/

Activate your virtual enviornment

source path-to-virtualenvs/bin/activate


. path-to-virtualenvs/bin/activate

To deactivate virtual enviorment

Let’s start Flask project

We have already get information, how can setup our virtual environment. So let’s create a virtual environment. For this example am using python3 and virtual environment name is flask_demo. Let do that.

virtualenv -p /user/bin/python3.5 /path/flask_demo

Let’s activate virtual environment and install flask.

. /path/flask_demo/bin/activate

You can confirm like this:

(flask_demo) rs@think:~/....

Am assuming you have already setup pip/pip3, if not please above instructions to setup pip. So let’s install Flask

pip3 install flask

You can confirm with pip freeze command, that basically list down all the packages installed in virtual environment. Here is my pip freeze command output after flask installed.


Greet ! Let’s create your first Flask project

mkdir myproject
cd myproject
mkdir app
cd app
vi __init__.py

Let write some few lines of code in __init__.py under myproject/app folder. This is basically a main file that initialize a flask application.

from flask import Flask

# you can choose any name instead app. 
app = Flask(__name__)

from app import routes

Right now project has no any routes file, let’s create that. This file will basically hold url route and related code.

Under the same directory myproject/app/routes.py

 from app import app

def index():
    return "Welcome to flask demo application"

To complete the application, you need to have a Python script at the top-level that defines the Flask application instance. Let’s call this script main.py, and define it as a single line that imports the application instance.


from app import app

Let’s do last thing but not least. Install one new package that will help to set environment variable that use by flask to start application. The name of the python-dotenv

pip install python-dotenv

Then you can just write the environment variable name and value in a .flaskenv file in the top-level directory of the project

flask run

* Serving Flask app "main"
* Running on (Press CTRL+C to quit)

Flask “Hello, World” Application
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