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.”
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:
If a game is repeated for multiple rounds, each round is a subsession.
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.)
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
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:
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”?¶
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
class Player(BasePlayer): def set_payoff(self): self.payoff = 100
self is just shorter and more convenient than
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 means we are defining a function called
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”?
Results page will be viewed many times
by many players,
self has specific properties about the current page view.
self.round_number gives us the current round number,
self.player gives us the player currently viewing the page,
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
Sometimes, oTree gives both options.
For example, oTree provides both the
(for simple pages with fixed time limits), and the
function (for complex pages with dynamic time limits).
Self: extended examples¶
What properties can you access through
Here is a diagram of how you can refer to objects in the hierarchy within your code:
For example, if you are in a method on the
Player class, you can
access the player’s payoff with
self is the
player). But if you are inside a
Page class in
equivalent expression is
which traverses the pointer from ‘page’ to ‘player’.
Here are some code examples to illustrate:
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()
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