Easy!

Either Flask persists the data into a database or a file, or it does not.

Wait, not so fast.

There is also

  • session scoped data (e.g. storing data in a cookie or similar)
  • app scoped data (e.g. store data on the app object)

The caveats

  • session scoped data is only available for one client, and as long the session lasts
  • app scoped data is only available within a single app - usually, e.g. with the help of gunicorn, you start more than one app instance