URL Variables

URL Variables

Dynamic URLs let you capture parts of the path and use them in your function. Flask uses angle brackets to mark these segments:

@app.route("/user/")
def user_profile(name):
    return f"Hello, {name}!"

Visit /user/alice and the name parameter receives "alice". By default, these are strings. But you can constrain the type:

@app.route("/post/")
def show_post(post_id):
    return f"Viewing post #{post_id}"

If someone visits /post/abc, Flask returns a 404 because "abc" isn't an integer. This saves you from writing validation logic yourself.

You can use multiple variables in a single route:

@app.route("/user//post/")
def user_post(username, post_id):
    return f"{username}'s post #{post_id}"

After a request is handled, you can access the captured variables through request.view_args:

from flask import request

@app.before_request
def log_route_params():
    if request.view_args:
        print("URL params:", request.view_args)

Here's a tip that will save you headaches: never hardcode URLs in your templates or redirects. Use url_for() instead. It takes an endpoint name and builds the URL for you:

from flask import url_for

@app.route("/")
def index():
    # Instead of "/user/alice"
    link = url_for("user_profile", name="alice")
    return f'Go to <a href="{link}">Alice\'s profile</a>'

This is better because if you ever change a route's URL pattern, every url_for() call updates automatically. Hardcoded links break silently — url_for() doesn't.

In templates, the same principle applies:

<a href="{{ url_for('user_profile', name='alice') }}">Alice</a>