FlaskHow-ToBeginner · 3 min read

How to Use Route Decorator in Flask: Simple Guide

In Flask, use the @app.route() decorator to link a URL path to a Python function that handles requests to that path. This decorator tells Flask which function to run when a user visits the specified URL.

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Syntax

The @app.route() decorator is placed above a function to define the URL path it responds to. The string inside route() is the URL path, and the decorated function returns the response shown on that path.

@app.route('/path'): Defines the URL path.
def function_name(): The function that runs when the URL is visited.
return: Sends the response back to the browser.

python

from flask import Flask
app = Flask(__name__)

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

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

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Example

This example shows a simple Flask app with two routes: the home page and a greeting page. Each route uses the @app.route() decorator to connect a URL to a function that returns text.

python

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to the Home Page!'

@app.route('/greet')
def greet():
    return 'Hello, visitor!'

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

Output

Running on http://127.0.0.1:5000/ - Visiting '/' shows: Welcome to the Home Page! - Visiting '/greet' shows: Hello, visitor!

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Common Pitfalls

Common mistakes when using @app.route() include:

Forgetting the leading slash / in the route path, which can cause the route not to match.
Defining multiple routes with the same path, which leads to only the last one working.
Not returning a response from the function, causing errors.
Running the app without the if __name__ == '__main__' guard, which can cause issues when importing.

python

from flask import Flask
app = Flask(__name__)

# Wrong: missing leading slash
@app.route('hello')
def hello_wrong():
    return 'This will not work as expected'

# Right: with leading slash
@app.route('/hello')
def hello_right():
    return 'This works correctly'

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

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Quick Reference

Use this quick reference to remember how to use the route decorator:

Usage	Description
@app.route('/path')	Defines the URL path to handle
def function():	Function that runs when URL is visited
return 'response'	Sends text or HTML back to the browser
@app.route('/path', methods=['GET', 'POST'])	Specify HTTP methods allowed
app.run(debug=True)	Run app with debug mode for easier development

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Key Takeaways

Use @app.route('/path') above a function to link URLs to code in Flask.

Always include a leading slash '/' in the route path string.

Return a response string or HTML from the route function to display content.

Avoid duplicate routes with the same path to prevent conflicts.

Run your Flask app inside the if __name__ == '__main__' block.