How to Run a Flask App: Simple Steps to Start Your Server

To run a Flask app, create a Python file with your app code and use flask run in the terminal after setting the FLASK_APP environment variable. Alternatively, run your script directly with python your_app.py if it includes app.run().

Syntax

To run a Flask app, you typically set the FLASK_APP environment variable to your app's Python file and then run flask run in your terminal. Alternatively, you can run the Python script directly if it contains app.run().

• FLASK_APP=your_app.py: Tells Flask which file to run.
• flask run: Starts the Flask development server.
• app.run(): Starts the server when running the script directly.

bash

export FLASK_APP=app.py
flask run

Output

 * Serving Flask app 'app.py'
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Example

This example shows a minimal Flask app that returns 'Hello, World!' when you visit the home page. You can run it by setting FLASK_APP and using flask run, or by running the script directly.

python

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return 'Hello, World!'

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

Output

 * Serving Flask app 'app.py'
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)

Common Pitfalls

Common mistakes when running Flask apps include:

• Not setting the FLASK_APP environment variable before running flask run.
• Trying to run flask run without an app file or with a wrong filename.
• Forgetting to include app.run() when running the script directly.
• Running Flask in production mode without proper server setup (Flask's built-in server is for development only).

bash

Wrong way:
flask run
# Error: "Could not locate a Flask application"

Right way:
export FLASK_APP=app.py
flask run

Quick Reference

Command	Description
export FLASK_APP=app.py	Set the app file to run
flask run	Start the Flask development server
python app.py	Run the app script directly if it has app.run()
CTRL+C	Stop the running Flask server

Key Takeaways

Set the FLASK_APP environment variable before running flask run.

Use flask run for development; use app.run() if running script directly.

Flask's built-in server is for development only, not production.

Common errors come from missing FLASK_APP or wrong filenames.

Stop the server anytime with CTRL+C in the terminal.