The Flask (Werkzeug) debugger is really good, but it only triggers on exceptions — sometimes it could be useful for debugging behavior that isn't raising anything. In these situations you can simply raise an exception intentionally.
@app.route('/') def index(): do_something_wrong() raise return 'Ohnoes'
This use of
raise is actually wrong, which means it… raises an exception. ☺
If you're afraid you might accidentally leave this in the code, here's one that only enters the debugger in debug-mode:
assert app.debug == False
This snippet by Dag Odenhall can be used freely for anything you like. Consider it public domain.