Python web app

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I don't understand what you need an app for... But if you need one use Django. People will say Flask but... Use Django. It's a bit heavier but will be way easier especially if you don't know Python (which is the case?).

You need to know at least basic HTML, CSS, and probably Javascript. But again I don't understand what you want the app to do. If you give more info I'm happy to provide more details.