How to Run a Flask Application

After successfully creating a Flask app, we can run it on the development server using the Flask CLI or by running the Python script.

Simply execute one of the following commands in the terminal:

• flask --app app_name run
• python app_name

File Structure

Here, we are using the following folder and file.

Demonstration with an Example

In this example, we have a basic application called helloworld.py, below is the code for it and we will run this app with the debugger mode on. 

# import flast module
from flask import Flask

# instance of flask application
app = Flask(__name__)

# home route that returns below text when root url is accessed
@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

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

Now let's run the app using both the commands seperately.

Using flask --app <app_name> run

Using the python app_name.py

Below is a snippet of the live app running on the development server. You can open it by Ctrl + Click on the link: "http://127.0.0.1:5000" or by typing the URL directly into your browser.

Run the app in the debugger

With debug mode on, Flask automatically detects errors and shows a detailed traceback, helping developers quickly find and fix issues. We will use the below command to run the flask application with debug mode as on.

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