Handling Errors In Flask API

In the referenced chat application, you will notice that there is a mechanism to handle errors. Whenever the errors occur, the application will return error templates suitable for a web client. When an API returns an error, we need a "machine-friendly" type of error that the client (whatever it is) can interpret.

Browse the completed code on GitHub.

For your reference, these are the topics in our discussion:

• Overview
• API Blueprint
• Resource Representation
• Error Handling (this article)
• Unique Resource Identifiers
• API Authentication
• API-friendly Error Messages
• API Testing Using Postman

Table Of Content

This article is broken down into the following subsections:

• Error Representation
• Error Response
• Bad Request Error

Error Representation

First, let us decide on how to represent an API error message.

{
    "error": "short error description",
    "message": "error message (this is optional)"
}

Error Response

Besides the error payload (what the error is), we can use status codes from the HTTP protocol to indicate the general class of the error.

app/api/errors.py: Generate error responses

from flask import jsonify
from werkzeug.http import HTTP_STATUS_CODES

def error_response(status_code, message=None):
    payload = {"error": HTTP_STATUS_CODE.get(status_code, "Unknown error")}
    if message:
        payload["message"] = message
    response = jsonify(payload)
    response.status_code = status_code
    return response

The error_response() method utilizes the HTTP_STATUS_CODE dictionary from Flask's Werkzeug to provide a short descriptive name for each HTTP status code. We have used these names for the error field in our error representation so now we can only worry about the status codes and the optional long description. jsonify() returns a Flask Response object with a default status code of 200, so after creating that response, we set the status code to the correct one for the error.

Bad Request Error

The most commonly returned error in APIs is the 400 Bad Request error. It indicates that the client sent a request with invalid data in it. Since it is so common, we can create a dedicated function that only requires a long descriptive message as an argument.

app/api/errors.py: Bad request response

def bad_request(message):
    return error_response(400, message)