Apps & rounds


In oTree (and Django), an app is a folder containing Python and HTML code. When you create your oTree project, it comes pre-loaded with various apps such as public_goods and dictator. A session is basically a sequence of apps that are played one after the other.

Creating an app


$ otree startapp your_app_name

This will create a new app folder based on a oTree template, with most of the structure already set up for you.

The key files are,, and the HTML files under the templates/ folder.

Think of this as a skeleton to which you can add as much as you want. You can add your own classes, functions, methods, and attributes, or import any 3rd-party modules.

Then go to and create an entry for your app in SESSION_CONFIGS that looks like the other entries.

Combining apps

In your SESSION_CONFIGS, you can combine apps by setting 'app_sequence'.

Passing data between apps

See participant.vars and session.vars.


You can make a game run for multiple rounds by setting Constants.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 self.round_number (this attribute is present on subsession, group, player, and page 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 self.player.my_field = True in round 1. In round 2, if you try to access self.player.my_field, you will find its value is None (assuming that is the default value of the field). 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, which work similarly:

  • in_previous_rounds()

  • in_all_rounds()

  • in_rounds()

  • in_round()

player.in_previous_rounds() and player.in_all_rounds() each return a list of players representing the same participant in previous rounds of the same app. The difference is that in_all_rounds() includes the current round’s player.

For example, if you wanted to calculate a participant’s payoff for all previous rounds of a game, plus the current one:

cumulative_payoff = sum([p.payoff for p in self.player.in_all_rounds()])

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 self.player.in_round(self.round_number - 1).payoff.

Similarly, subsession objects have methods in_previous_rounds(), in_all_rounds(), in_rounds(m,n) and in_round(m) that work the same way.

Group objects also have methods in_previous_rounds(), in_all_rounds(), in_rounds(m,n) and in_round(m), but note that if you re-shuffle groups between rounds, then these methods may not return anything meaningful.


If you want to pass data between different apps, you should store this data on the participant, 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.)

participant.vars is is a dictionary that can store any data. For example, you can set an attribute like this:

self.participant.vars['blah'] = [1, 2, 3]

Later in the session (e.g. in a separate app), you can retrieve it like this:

# the below line returns [1, 2, 3]
# or try printing:
print('vars is', self.participant.vars)

As described here, the current participant can be accessed from a Page or Player:

class MyPage(Page):
    def before_next_page(self):
        self.participant.vars['foo'] = 1
class Player(BasePlayer):
    def some_method(self):
        self.participant.vars['foo'] = 1

You can also access it from Group or Subsession, as long as you retrieve a Player instance (e.g. using get_players() or get_player_by_role(), etc.).

class Group(BaseGroup):
    def some_method(self):
        for p in self.get_players():
            p.participant.vars['foo'] = 1

If your key may or may not exist, you can use the .get() method. For example, self.participant.vars.get('my_var', DEFAULT_VALUE). More here.

or you can test if 'my_var' exists with 'my_var' in self.participant.vars.

Note: participant.vars is not included in the Excel/CSV data export, or in the “Data” tab in the session admin. If you want that, you can create a StringField on your player (for example, called participant_vars_dump) and then at the end of your session, assign:

self.player.participant_vars_dump = str(self.participant.vars)

(The same concept applies for session.vars below.)


For global variables that are the same for all participants in the session, you can use self.session.vars. This is a dictionary just like participant.vars. The difference is that if you set a variable in self.session.vars, it will apply to all participants in the session, not just one.

As described here, the session object can be accessed from a Page object or any of the models (Player, Group, Subsession, etc.).

Variable number of rounds

If you want a variable number of rounds, consider setting 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.