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How to Use URL Query Strings in Flask (Beginner-Friendly Guide + Examples) Uploaded: Dec. 3 2025 | Modified: Dec. 3 2025

How to Deal with URL Query Strings in Flask (Beginner-Friendly Guide)

When building Flask applications, you will often need to extract values from the URL — such as filters, IDs, search keywords, or pagination numbers. These pieces of information are passed using URL query strings, and learning how to work with them is essential for building dynamic and interactive web applications.

In this guide, you’ll learn what a URL query string is, how to read it in Flask, how to avoid common errors, and the best practices when retrieving URL parameters.

What is a URL Query String?

A URL query string is a set of parameters added at the end of a URL after a question mark ?.

Example:

https://flask.blog/blog?id=1

Here:

Query strings are commonly used for things like pagination (?page=2), search filters (?search=python), or retrieving specific resources (?id=10).

How to Read URL Query Strings in Flask

Flask provides two main ways to read query strings:

1. request.query_string (Raw Query String)

This returns the raw query string as bytes.

request.query_string

Output:

b'id=1'

This is the unparsed version. To work with this data, you would normally need to parse it — which is why Flask provides a simpler option…

2. request.args (Parsed Query Parameters)

request.args automatically parses the query string and stores it in an ImmutableMultiDict.

Example:

request.args

Output:

ImmutableMultiDict([('id', '1')])

To get the value of a query parameter:

request.args['id']

Returns:

'1'

Avoiding BadRequestKeyError When Key Is Missing

If you try to access a non-existent parameter using square brackets ([]), Flask raises:

BadRequestKeyError: 400 Bad Request

To avoid this, always use .get() with an optional fallback value:

request.args.get('id', None)

This returns:

None

No exception is thrown — making it the safe way to access query parameters.

Complete Example: Reading Multiple Query Parameters

from flask import Flask, request

app = Flask(__name__)

@app.route('/search')
def search():
    keyword = request.args.get('keyword')
    page = request.args.get('page', 1)  # default value = 1
    return f"Search: {keyword}, Page: {page}"

Visiting:

/search?keyword=flask&page=2

Outputs:

Search: flask, Page: 2

Frequently Asked Questions About URL Query Strings in Flask

1. Can I access multiple values from the same query parameter?

Yes. If a parameter appears multiple times (e.g., ?tag=python&tag=flask), Flask stores them all.

request.args.getlist('tag')

Returns:

['python', 'flask']

2. How do I check if a query parameter exists?

Use:

'id' in request.args

Returns True or False.

3. How do I convert query parameter values into integers?

Query parameters are strings by default.

user_id = int(request.args.get('id'))

Always validate first if unsure.

4. What happens if the user enters an invalid value like ?id=abc?

You must handle conversion errors:

try:
    user_id = int(request.args.get('id'))
except (TypeError, ValueError):
    user_id = None

5. Can I use default values for query parameters?

Yes:

page = request.args.get('page', 1)

6. Are query strings secure?

Query strings are visible in the URL, so avoid sending sensitive data like passwords or tokens.

7. What is the difference between request.args and request.form?