Flask 2.0.0 Error Handler not called when Blueprint Name Contains a Period

[jeffsawatzky]

When you have a blueprint, and it's name contains a . and it has error handlers registered, they are never called.

We have been using blueprints to help organize our application, and we have used . to help namespace the blueprint names. And we have registered error handlers on the blueprints. However, when an error occurs, they are never called.

Below is a simple example where you might have an app that has a UI and an API, and for UI errors you may have a custom error view, and for API errors you may want to return JSON.

from flask import Blueprint, Flask, jsonify

app = Flask(__name__)
ui = Blueprint("app.ui", __name__, url_prefix="/ui")
api = Blueprint("app.api", __name__, url_prefix="/api")


@ui.errorhandler(Exception)
def ui_error_handler(error):
    print("ui error handler called")
    return str(error)


@ui.get("/")
def ui_index():
    print("ui index called")
    raise Exception("this is an unhandled exception from the ui")


@api.errorhandler(Exception)
def api_error_handler(error):
    print("api error handler called")
    return jsonify(error=str(error))


@api.get("/")
def api_index():
    print("api index called")
    raise Exception("this is an unhandled exception from the api")


app.register_blueprint(ui)
app.register_blueprint(api)

Then when you run and do a get on one of the index endpoints you get:

 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
api index called
[2021-05-13 13:14:48,849] ERROR in app: Exception on /api/ [GET]
Traceback (most recent call last):
  File "/lib/python3.9/site-packages/flask/app.py", line 2051, in wsgi_app
    response = self.full_dispatch_request()
  File "/lib/python3.9/site-packages/flask/app.py", line 1501, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/lib/python3.9/site-packages/flask/app.py", line 1499, in full_dispatch_request
    rv = self.dispatch_request()
  File "/lib/python3.9/site-packages/flask/app.py", line 1485, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "app.py", line 29, in api_index
    raise Exception("this is an unhandled exception from the api")
Exception: this is an unhandled exception from the api
127.0.0.1 - - [13/May/2021 13:14:48] "GET /api/ HTTP/1.1" 500 -

You can see the api index called printed, but not the api error handler called printed which shows the error handler is never called.

Environment:

• Python version: 3.9
• Flask version: 2.0.0

[davidism]

Blueprint names should not contain dots. I thought this was already raising an error.

[davidism]

We won't be changing this at this point, as dots are reserved for separating blueprint names (which can be nested now) and endpoint names. You'll need to change the names to something like app_ui and app_api.

[jeffsawatzky]

@davidism if you are referring to #2450 that is only restricting the view function name to not have a dot in it. As far as I know, there has not been any restriction on the actual blueprint name to not have a dot in it.

[davidism]

Seems to have been an oversight then. Regardless, you'll need to update your code, the most we'll do is make this an explicit error.

[jeffsawatzky]

We have some monitoring/alerting setup that uses the view_func name, and the view_func name is prefixed by the blueprint name. Changing the blueprint names will mean we will need to change all the alerts that we have configured. Doable, but I was hoping that this function here could just be modified slightly to also check:

handler_map = self.error_handler_spec[_request_ctx_stack.top.request.blueprint]

Or something like that.

[davidism]

We could do that, technically, but I don't plan to. Every . separates a blueprint name now, due to implementing nested blueprints. Sorry for the inconvenience, but you'll need to do a big search and replace on your code to avoid using dots in names.

[davidism]

Closing for #4041