Variable Rules (Dynamic URLs)

Dynamic routes let you capture parts of the URL.

diagram a converter validates before your view runs mermaid

  flowchart TD
  A["GET /post/abc"] --> B["match against /post/<int:pid>"]
  B --> C{"does 'abc' parse as an int?"}
  C -->|no| D["404 -- the view is never called"]
  C -->|yes| E["pid = 42, a real int"]
  E --> F["your view runs"]
  G["converters"] --> H["string  -- default, no slashes"]
  G --> I["int / float -- numeric, and converted"]
  G --> J["path -- like string but DOES match slashes"]
  G --> K["uuid -- a canonical UUID"]

The type in a rule is not documentation -- it is a filter. A request whose segment does not fit the converter never reaches the function, so the view can assume the value is already the right type. That is why a bad id returns 404 rather than raising inside your code.

Basic dynamic route

python

from flask import Flask
 
app = Flask(__name__)
 
 
@app.route("/user/<username>")
def user_profile(username: str):
    return f"Hello, {username}!"

/user/ravi → username = "ravi"

Type converters

Flask supports converters that validate and convert values:

<int:id>
<float:price>
<path:subpath> (can include slashes)

Example:

python

@app.route("/post/<int:post_id>")
def post_detail(post_id: int):
    return f"Post id: {post_id}"

Now:

/post/10 works
/post/abc returns 404 (no match)

Multiple variables

python

@app.route("/category/<string:name>/page/<int:page>")
def category(name, page):
    return {"category": name, "page": page}

Common pitfalls

Using <path:...> when you don’t want slashes (it can match too much)
Forgetting to validate values when types are not enough (e.g., allowed usernames)
Returning user input directly without escaping in HTML templates (XSS risk)

Variable Rules (Dynamic URLs)

Basic dynamic route

Type converters

Multiple variables

Common pitfalls

🧪 Try It Yourself

Exercise 1 – Create a Flask App

Exercise 2 – Dynamic Route

Exercise 3 – Return JSON

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