Apps & rounds


An oTree app is a folder containing Python and HTML code. A project contains multiple apps. A session is basically a sequence of apps that are played one after the other.

Combining apps

You can combine apps by setting your session config’s app_sequence.

Passing data between apps

See participant fields and session fields.


You can make a game run for multiple rounds by setting C.NUM_ROUNDS. For example, if your session config’s app_sequence is ['app1', 'app2'], where app1 has NUM_ROUNDS = 3 and app2 has NUM_ROUNDS = 1, then your sessions will contain 4 subsessions.

Round numbers

You can get the current round number with player.round_number (this attribute is present on subsession, group, and player objects). Round numbers start from 1.

Passing data between rounds or apps

Each round has separate subsession, Group, and Player objects. For example, let’s say you set player.my_field = True in round 1. In round 2, if you try to access player.my_field, you will find its value is None. This is because the Player objects in round 1 are separate from Player objects in round 2.

To access data from a previous round or app, you can use one of the techniques described below.

in_rounds, in_previous_rounds, in_round, etc.

Player, group, and subsession objects have the following methods:

  • in_previous_rounds()
  • in_all_rounds()
  • in_rounds()
  • in_round()

For example, if you are in the last round of a 10-round game, player.in_previous_rounds() will return a list with 9 player objects, which represent the current participant in all previous rounds.

player.in_all_rounds() is almost the same but the list will have 10 objects, because it includes the current round’s player.

player.in_rounds(m, n) returns a list of players representing the same participant from rounds m to n.

player.in_round(m) returns just the player in round m. For example, to get the player’s payoff in the previous round, you would do:

prev_player = player.in_round(player.round_number - 1)

These methods work the same way for subsessions (e.g. subsession.in_all_rounds()).

They also work the same way for groups, but it does not make sense to use them if you re-shuffle groups between rounds.

Participant fields

If you want to access a participant’s data from a previous app, you should store this data on the participant object, which persists across apps (see Participant). (in_all_rounds() only is useful when you need to access data from a previous round of the same app.)

Go to settings and define PARTICIPANT_FIELDS, which is a list of the names of fields you want to define on your participant.

Then in your code, you can get and set any type of data on these fields:

participant.mylist = [1, 2, 3]

(Internally, all participant fields are stored in a dict called participant.vars. is equivalent to participant.vars['xyz'].)

Session fields

For global variables that are the same for all participants in the session, add them to the SESSION_FIELDS, which works the same as PARTICIPANT_FIELDS. Internally, all session fields are stored in session.vars.

Variable number of rounds

If you want a variable number of rounds, consider using Live pages.

Alternatively, you can set NUM_ROUNDS to some high number, and then in your app, conditionally hide the {{ next_button }} element, so that the user cannot proceed to the next page, or use app_after_this_page. But note that having many rounds (e.g. more than 100) might cause performance problems, so test your app carefully.