Flask 2.3 removed before_first_request, and the replacement runs at import not at first request

@app.before_first_request was deprecated in Flask 2.2 and removed in 2.3 (April 2023). Applications using it fail with an AttributeError on the Flask object at import time.

There is no drop-in replacement, because the hook was a bad fit for multi-worker servers: it ran once per worker process, at an unpredictable moment, and any exception it raised turned into a failed request for whichever unlucky client arrived first.

Move one-time setup into the application factory, so it runs deterministically before any worker serves traffic, or run it explicitly during startup inside with app.app_context():. For work that must happen once across all workers, such as a schema migration or a cache warm, run it as a separate command in the deployment pipeline rather than inside the web process. If you genuinely need lazy initialisation, guard it yourself with a lock and a flag stored on the app object, and remember that each worker process still needs its own copy.