How to Run a Flask Application

Introduction

Running a Flask application is a fundamental skill for Python web developers. Flask offers a straightforward way to start a local server so you can test and debug your web apps before deploying them. This guide explains how to run your Flask app efficiently.

Prerequisites

Before running a Flask application, make sure you have Python and Flask installed on your system. You can install Flask using pip if you haven't already:

pip install Flask

Basic Structure of a Flask App

A typical Flask app has a simple structure, usually a single Python file like app.py with the following code snippet:

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Hello, Flask!'

if __name__ == '__main__':
    app.run(debug=True)

Running the Flask Application

To run your Flask app, open your command prompt or terminal, navigate to the directory containing your application file, and execute:

python app.py

This command starts the Flask development server, which by default runs on http://127.0.0.1:5000/. Open this URL in your browser to see your app in action.

Using Environment Variables (Alternative Method)

You can also run Flask apps by setting environment variables and using the flask run command. This method separates app code from server commands.

For Windows Command Prompt:

set FLASK_APP=app.py
set FLASK_ENV=development
flask run

For Windows PowerShell:

$env:FLASK_APP = "app.py"
$env:FLASK_ENV = "development"
flask run

For macOS/Linux Terminal:

export FLASK_APP=app.py
export FLASK_ENV=development
flask run

This will also start the server with debug mode enabled, allowing automatic reloads when you change your code.

Understanding the Parameters

• FLASK_APP: Specifies the Python file to run.
• FLASK_ENV=development: Enables debug mode for easier development.
• flask run: Starts the Flask development server.

Stopping the Flask Server

To stop the Flask development server, simply press Ctrl + C in the terminal or command prompt window where the server is running.

Summary

Running a Flask application is simple and can be done either by executing the Python file directly or by using Flask’s built-in command line tool with environment variables. Both methods allow quick testing and debugging of your web apps during development.