When starting to learn oTree, you may face uncertainty about where to put a particular piece of code. Should it go in before_next_page, after_all_players_arrive, creating_session, etc? Should it be a player, group, or subsession function?

Here is a quick guide comparing these various functions.

• creating_session: code that is put here will execute immediately when you click the "Create session" button. Therefore, when this function executes, all fields will be blank, because the session has not started yet. creating_session is where you can put "setup" code, such as predetermining treatment groups. This function cannot depend on anything that happens during the session. For that, you should instead use before_next_page or after_all_players_arrive.
• before_next_page: This function executes after the player finishes the page, before proceeding to the next page.
• after_all_players_arrive: This function can only be defined on a WaitPage, not a regular page. It's a good place for doing computations that depend on other players in the group, like picking the winner in a multi-player game. after_all_players_arrive executes just once for the whole group, whereas before_next_page executes separately for each player.
• vars_for_template: This function executes when a page is loaded. Be aware that if the user reloads the page, this code will get re-run. This can be convenient during development, since you can easily edit your code and reload the page to see the result. But make sure the code you put here is idempotent; otherwise, it could be exploited by participants. For example, if you have code that randomly chooses the participant's payoff, the participant could keep reloading the page until they see a payoff that they like. It's safer to use before_next_page, which is guaranteed to only execute once.
• User-defined functions: in oTree apps, you will frequently see user-defined functions such as def set_payoffs(group): You can define player/group/subsession functions with whatever name you want, but unlike built-in functions, oTree won't execute these functions automatically. It's up to you to call these functions from a built-in function such as creating_session, before_next_page, etc.