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 the user must navigate through. 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, and so on.
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.
In the below example,
self refers to a
class Player(object): def my_method(self): return self.my_field
In the next example, however,
self refers to a
class Group(object): def my_method(self): return self.my_field
self is conceptually similar to the word “me”. You refer to yourself
as “me”, but others refer to you by your name. And when your friend says
the word “me”, it has a different meaning from when you say the word
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’.
Self: extended examples¶
Here are some code examples to illustrate:
class Session(...) # this class is defined in oTree-core def example(self): # current session object self self.config # child objects self.get_subsessions() self.get_participants() class Participant(...) # this class is defined in oTree-core def example(self): # current participant object self # parent objects self.session # child objects self.get_players()
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