How to Get Query Parameters in Flask: Simple Guide

In Flask, you get query parameters using request.args, which is a dictionary-like object containing all URL query parameters. Access a parameter by its name like request.args.get('param_name') to safely retrieve its value.

Syntax

Use request.args to access query parameters in Flask. It behaves like a dictionary where keys are parameter names and values are parameter values.

• request.args.get('key'): Returns the value for the given key or None if not present.
• request.args['key']: Returns the value but raises an error if the key is missing.
• request.args.getlist('key'): Returns a list of values if the parameter appears multiple times.

python

from flask import request

# Access single query parameter safely
value = request.args.get('param')

# Access single query parameter, raises error if missing
value = request.args['param']

# Access multiple values for the same parameter
values = request.args.getlist('param')

Example

This example shows a Flask app that reads a query parameter named name from the URL and returns a greeting message. If name is missing, it returns a default greeting.

python

from flask import Flask, request

app = Flask(__name__)

@app.route('/greet')
def greet():
    name = request.args.get('name', 'Guest')
    return f"Hello, {name}!"

if __name__ == '__main__':
    app.run(debug=True)

Output

When you visit http://localhost:5000/greet?name=Alice, the output is: Hello, Alice!
When you visit http://localhost:5000/greet without parameters, the output is: Hello, Guest!

Common Pitfalls

Common mistakes when getting query parameters in Flask include:

• Using request.args['key'] without checking if the key exists, which raises a KeyError.
• Not providing a default value with get(), leading to None values that may cause errors later.
• Ignoring that parameters can have multiple values and using get() instead of getlist() when needed.

python

from flask import Flask, request

app = Flask(__name__)

@app.route('/wrong')
def wrong():
    # This will raise KeyError if 'param' is missing
    value = request.args['param']
    return f"Value is {value}"

@app.route('/right')
def right():
    # Safe access with default
    value = request.args.get('param', 'default')
    return f"Value is {value}"

Quick Reference

Summary tips for getting query parameters in Flask:

• Always use request.args.get('key', default) to avoid errors.
• Use request.args.getlist('key') to handle multiple values.
• request.args is read-only and reflects the URL query string.
• Query parameters are strings; convert types as needed.

Key Takeaways

Use request.args.get('param') to safely get query parameters in Flask.

Provide a default value in get() to avoid None results causing errors.

Use getlist() when expecting multiple values for the same parameter.

Avoid accessing parameters directly with request.args['param'] unless sure it exists.

Query parameters are strings; convert them to needed types explicitly.