test_errorhandler.py

from flask import Flask, Response

from werkzeug.exceptions import HTTPException, NotFound

app = Flask('test')

def handler(e):
    return Response('error')

app.register_error_handler(HTTPException, handler)

@app.route('/')
def index():
    raise NotFound()

def test_errorhandler():
    client = app.test_client()

    response = client.get('/')

    assert response.data == "error"

Based on how _find_error_handler recurses through the mro, i would expect this to pass, as the second item in NotFound's mro is HTTPException.

Instead this fails showing the default response for the NotFound exception.

_find_error_handler is used to fetch the handler for a http exception, at line 1468 it appears to be None though.

Flask==0.12.2
Werkzeug==0.12.2
Python==3.6.1