What is URL building and how do you use url_for()?

URL building means generating a URL from an endpoint name instead of typing the path by hand. url_for() takes the endpoint, usually the view function's name, plus a keyword argument for each URL variable.

from flask import url_for

@app.route('/user/<username>')
def user_profile(username):
    return f'Profile for {username}'

@app.route('/')
def index():
    # Generate URL for user_profile endpoint
    profile_url = url_for('user_profile', username='john')
    return f'<a href="{profile_url}">John\'s Profile</a>'

Here url_for('user_profile', username='john') returns /user/john. Arguments that match no URL variable become query parameters. This reverse lookup has three benefits:

• Changing a route updates every link that uses its endpoint.
• Special characters are escaped for you.
• The paths are absolute, and they stay correct when the app is mounted under a prefix.