URL Building (url_for)

Hardcoding routes like "/user/123""/user/123" is fragile.

If you rename the route later, you must update every place that used the string.

Flask’s best practice is to generate URLs with url_for()url_for().

Basic usage

from flask import Flask, url_for
 
app = Flask(__name__)
 
 
@app.route("/")
def home():
    profile_url = url_for("profile", username="ravi")
    return f"Profile link: {profile_url}"
 
 
@app.route("/user/<username>")
def profile(username):
    return f"Profile: {username}"

from flask import Flask, url_for
 
app = Flask(__name__)
 
 
@app.route("/")
def home():
    profile_url = url_for("profile", username="ravi")
    return f"Profile link: {profile_url}"
 
 
@app.route("/user/<username>")
def profile(username):
    return f"Profile: {username}"

Key idea:

You pass the endpoint name (usually the function name)
And any variables required by the route

Why url_for is important

avoids broken links after refactors
supports query parameters cleanly
supports Blueprints (namespaced endpoints)

Query parameters with url_for

url_for("search", q="flask", page=2)

url_for("search", q="flask", page=2)

Output (conceptually):

/search?q=flask&page=2/search?q=flask&page=2

Static files

You’ll use:

url_for("static", filename="styles.css")

url_for("static", filename="styles.css")

That’s how templates should reference CSS/JS.

Debugging: what is my endpoint name?

Run:

flask routes

flask routes

It lists endpoints and URL rules.

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