Conceptual overview

Sessions

In oTree, a session is an event during which multiple participants take part in a series of tasks or games. An example of a session would be:

“A number of participants will come to the lab and play a public goods game, followed by a questionnaire. Participants get paid EUR 10.00 for showing up, plus their earnings from the games.”

Subsessions

A session is a series of subsessions; subsessions are the “sections” or “modules” that constitute a session. For example, if a session consists of a public goods game followed by a questionnaire, the public goods game would be subsession 1, and the questionnaire would be subsession 2. In turn, each subsession is a sequence of pages. For example, if you had a 4-page public goods game followed by a 2-page questionnaire:

_images/session_subsession.png

If a game is repeated for multiple rounds, each round is a subsession.

Groups

Each subsession can be further divided into groups of players; for example, you could have a subsession with 30 players, divided into 15 groups of 2 players each. (Note: groups can be shuffled between subsessions.)

Object hierarchy

oTree’s entities can be arranged into the following hierarchy:

Session
    Subsession
        Group
            Player
                Page
  • A session is a series of subsessions
  • A subsession contains multiple groups
  • A group contains multiple players
  • Each player proceeds through multiple pages

Participant

In oTree, the terms “player” and “participant” have distinct meanings. The relationship between participant and player is the same as the relationship between session and subsession:

_images/participant_player.png

A player is an instance of a participant in one particular subsession. A player is like a temporary “role” played by a participant. A participant can be player 2 in the first subsession, player 1 in the next subsession, etc.

What is “self”?

In Python, self is an instance of the class you’re currently under. If you are ever wondering what self means in a particular context, scroll up until you see the name of the class.

For example, in this code, self is a player (an instance of the Player class).

class Player(BasePlayer):

    def set_payoff(self):
        self.payoff = 100

The name self is just shorter and more convenient than player.

This is similar to how people don’t use their own names when they talk about themselves; they just use pronouns like “me”, “myself”, and “I”. So, self is basically a pronoun.

Functions vs. attributes

Classes have attributes and functions.

Here is an example of a page with an attribute:

class Results(Page):
    # this is an attribute
    timeout_seconds = 60

This means that this page has a time limit of 60 seconds.

But what if you want the time limit to be dynamic? Maybe it should depend on the current round number, or on the player’s performance so far.

To solve this, we need to make it a function of the current page, like this:

class Results(Page):
    # this is a function
    def get_timeout_seconds(self):
        if self.round_number == 1:
            return 60
        else:
            return 30

First, let’s look at the line def get_timeout_seconds(self):. The def means we are defining a function called get_timeout_seconds. It is a function, so it has input and output. The input (i.e. the argument) is called self, which is the current instance of the page. What do we mean by “instance”? Although the Results page will be viewed many times by many players, self has specific properties about the current page view. For example, self.round_number gives us the current round number, self.player gives us the player currently viewing the page, and self.session gives us the session that is currently taking place.

In conclusion, if you define an attribute, then it will be same for all players. If you want something to be different from player to player, you need to use a function that takes a parameter self. Sometimes, oTree gives both options. For example, oTree provides both the timeout_seconds attribute (for simple pages with fixed time limits), and the get_timeout_seconds function (for complex pages with dynamic time limits).

Self: extended examples

What properties can you access through self?

Here is a diagram of how you can refer to objects in the hierarchy within your code:

_images/object_model_self.png

For example, if you are in a method on the Player class, you can access the player’s payoff with self.payoff (because self is the player). But if you are inside a Page class in pages.py, the equivalent expression is self.player.payoff, which traverses the pointer from ‘page’ to ‘player’.

Here are some code examples to illustrate:

in your models.py

class Subsession(BaseSubsession):
    def example(self):

        # current subsession object
        self

        # parent objects
        self.session

        # child objects
        self.get_groups()
        self.get_players()

        # accessing previous Subsession objects
        self.in_previous_rounds()
        self.in_all_rounds()

class Group(BaseGroup):
    def example(self):

        # current group object
        self

        # parent objects
        self.session
        self.subsession

        # child objects
        self.get_players()

class Player(BasePlayer):

    def example(self):

        # current player object
        self

        # method you defined on the current object
        self.my_custom_method()

        # parent objects
        self.session
        self.subsession
        self.group
        self.participant

        self.session.config

        # accessing previous player objects
        self.in_previous_rounds()

        # equivalent to self.in_previous_rounds() + [self]
        self.in_all_rounds()

in your pages.py

class MyPage(Page):
    def example(self):

        # current page object
        self

        # parent objects
        self.session
        self.subsession
        self.group
        self.player
        self.participant
        self.session.config