Blueprint 404 errorhandler doesn't activate under blueprint's url prefix

The documentation mentions that 404 error handlers will not behave as expected on blueprints. The app handles routing and raises a 404 before the request gets to the blueprint. The 404 handler will still activate for abort(404) because that is happening after routing at the blueprint level.

This is something that could possibly be fixed in Flask (there's an open issue about it). As a workaround, you can do your own error routing within the top-level 404 handler.

from flask import request, render_template@app.errorhandler(404)def handle_404(e):    path = request.path    # go through each blueprint to find the prefix that matches the path    # can't use request.blueprint since the routing didn't match anything    for bp_name, bp in app.blueprints.items():        if path.startswith(bp.url_prefix):            # get the 404 handler registered by the blueprint            handler = app.error_handler_spec.get(bp_name, {}).get(404)            if handler is not None:                # if a handler was found, return it's response                return handler(e)    # return a default response    return render_template('404.html'), 404