Handling Query Parameters

Query parameters are the ?key=value part of a URL.

Example:

• /search?q=flask&page=2

diagram query parameters are strings, and missing is not an error mermaid

  flowchart TD
  A["/search?q=ada&page=2"] --> B["request.args"]
  B --> C["get('q') -> 'ada'"]
  B --> D["get('page') -> '2'  (a STRING)"]
  B --> E["get('page', type=int) -> 2"]
  B --> F["get('missing') -> None, no error"]
  G["/search?page=notanumber"] --> H["get('page', type=int)"]
  H --> I["None -- the conversion failed silently"]
  I --> J["supply a default: get('page', 1, type=int)"]

Everything in a query string arrives as text. request.args.get returns None for a key that is absent rather than raising, so a typo in the parameter name looks exactly like a user who did not supply it. Asking for a type converts the value -- and, importantly, also returns None when the conversion fails.

type=int returns None on bad input, it does not raise

Verified:

Request	request.args.get("page", type=int)
?page=2	2
?page=notanumber	None
?q=ada (no page)	None

A malformed value and a missing value are indistinguishable. If the next line is page * 20, both cases raise TypeError somewhere unrelated to the actual problem.

Give it a default and the failure mode disappears:

python

page = request.args.get("page", 1, type=int)

If a bad value should be an error rather than quietly falling back, check it explicitly — the argument parsing will not do it for you.

Accessing query parameters

Flask stores query parameters in request.args.

python

from flask import Flask, request
 
app = Flask(__name__)
 
 
@app.route("/search")
def search():
    q = request.args.get("q", "")
    page = request.args.get("page", default=1, type=int)
    return {"q": q, "page": page}

Why use type=?

• Converts the string to the type you expect
• If conversion fails, Flask returns the default (instead of crashing)

Handling multiple values

Some URLs have repeated params:

• /filter?tag=python&tag=flask

python

tags = request.args.getlist("tag")

Validation basics

Even with type=int, you still should validate:

• ranges (page >= 1)
• allowed values (sort in top)

If invalid, return a clear error or a 400 status code.

🧪 Try It Yourself

Exercise 1 – Create a Flask App

pch.exerciseTag pch.exerciseTitle

Exercise 2 – Dynamic Route

pch.exerciseTag pch.exerciseTitle

Exercise 3 – Return JSON

pch.exerciseTag pch.exerciseTitle