Error Handling

This document covers HTTP status codes, error handling in Flask, and best practices for returning error responses from API endpoints. It explains status code categories, custom error responses, and application-level error handlers for robust API design.

Understanding HTTP Status Codes

Every HTTP response includes a three-digit status code that indicates the result of the request. Status codes are grouped into categories:

Common codes include:

Returning Status Codes in Flask

Flask returns a 200 OK status by default when a route returns a response. To return a different status code, provide it as a tuple or use the make_response method.

 1from flask import Flask, make_response
 2
 3app = Flask(__name__)
 4
 5@app.route('/')
 6def home():
 7    return '<h1>My First Application in Action</h1>', 200
 8
 9@app.route('/custom')
10def custom():
11    response = make_response('<h1>Custom Response</h1>', 200)
12    return response

Example: Handling API Errors

When building APIs, return appropriate status codes for different outcomes:

 1@app.route('/search')
 2def search_response():
 3    query = request.args.get('q')
 4    if not query:
 5        return {'message': 'Input parameter missing'}, 422
 6    result = fetch_from_database(query)
 7    if result:
 8        return result, 200
 9    else:
10        return {'message': 'Resource not found'}, 404

This example returns 422 if a required parameter is missing, 200 if the resource is found, and 404 if not found.

Application-Level Error Handlers in Flask

Flask allows defining custom error handlers for specific status codes at the application level:

1@app.errorhandler(404)
2def not_found(error):
3    return {'message': 'API not found'}, 404
4
5@app.errorhandler(500)
6def server_error(error):
7    return {'message': 'Something went wrong on the server'}, 500

These handlers return custom messages and status codes for 404 and 500 errors.

Conclusion

HTTP status codes communicate the result of API requests. Flask provides default and custom ways to return status codes and error messages, including application-level error handlers for robust API design.

FAQ