HTTPException handler catches non-error exceptions like RequestRedirect

This is related to https://github.com/pallets/flask/issues/2841, but the emphasis is different.

Expected Behavior

Non-error exceptions should not be caught by registered error handlers.

app.register_error_handler(HTTPException, my_error_handler)

@app.route('/path-with-slash/')
def my_route():
    return '', 204

Attempting to visit /path-with-slash should redirect to /path-with-slash/, without invoking my_error_handler.

Actual Behavior

As the RequestRedirect raised internally in Werkzeug is a subclass of HTTPException, my_error_handler will get called. This is unexpected because no error has occurred, and the only appropriate way to handle this case is to add boilerplate to re-raise any Werkzeug RoutingExceptions.

Environment

• Python version: 3.7.0
• Flask version: 1.0.2
• Werkzeug version: 0.14.1

[taion]

I’ve added https://github.com/pallets/flask/pull/2986 as a strawman to demonstrate what the fix would look like, and updated the test to demonstrate the behavior that it enables.

[taion]

I disagree with that statement.

For example, returning flask.redirect does not in general raise a RequestRedirect, so one cannot use error handlers on RequestRedirect to implement custom request behavior in the first place.

If one wishes to customize Werkzeug’s routing logic, the correct way to handle that would be to do so in the router, rather than by using what should be the app’s error handling logic.