https://www.tutorialspoint.com/flask/index.htm
Flask Python Web application framework tutorial (1 of 3)
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https://www.youtube.com/playlist?list=PL6gx4Cwl9DGDi9F_slcQK7knjtO8TUvUs
Tutorial and specially on submit button
https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-iii-web-forms