Part 3: Trust game¶
Now let’s create a 2-player Trust game, and learn some more features of oTree.
To start, Player 1 receives 10 points; Player 2 receives nothing. Player 1 can send some or all of his points to Player 2. Before P2 receives these points they will be tripled. Once P2 receives the tripled points he can decide to send some or all of his points to P1.
The completed app is here.
Create the app¶
Just as in the previous part of the tutorial, create another app, called my_trust
.
Constants¶
Go to your app’s constants class (C
).
First we define our app’s constants. The endowment is 10 points and the donation gets tripled.
class C(BaseConstants):
NAME_IN_URL = 'my_trust'
PLAYERS_PER_GROUP = 2
NUM_ROUNDS = 1
ENDOWMENT = cu(10)
MULTIPLICATION_FACTOR = 3
Models¶
Then we add fields to player and group. There are 2 critical data points to record: the “sent” amount from P1, and the “sent back” amount from P2.
Your first instinct may be to define the fields on the Player like this:
# Don't copy paste this
class Player(BasePlayer):
sent_amount = models.CurrencyField()
sent_back_amount = models.CurrencyField()
The problem with this model is that sent_amount
only applies to P1,
and sent_back_amount
only applies to P2. It does not make sense that
P1 should have a field called sent_back_amount
. How can we make our
data model more accurate?
We can do it by defining those fields at the Group
level. This makes
sense because each group has exactly 1 sent_amount
and exactly 1
sent_back_amount
:
class Group(BaseGroup):
sent_amount = models.CurrencyField(
label="How much do you want to send to participant B?"
)
sent_back_amount = models.CurrencyField(
label="How much do you want to send back?"
)
We also define a function called sent_back_amount_choices
to populate the
dropdown menu dynamically. This is the feature called
{field_name}_choices
, which is explained here: Dynamic form field validation.
def sent_back_amount_choices(group):
return currency_range(
0,
group.sent_amount * C.MULTIPLICATION_FACTOR,
1
)
Define the templates and pages¶
We need 3 pages:
- P1’s “Send” page
- P2’s “Send back” page
- “Results” page that both users see.
Send page¶
class Send(Page):
form_model = 'group'
form_fields = ['sent_amount']
@staticmethod
def is_displayed(player):
return player.id_in_group == 1
We use is_displayed() to only show this to P1; P2 skips the
page. For more info on id_in_group
, see Groups.
For the template, set the title
to Trust Game: Your Choice
,
and content
to:
<p>
You are Participant A. Now you have {{C.ENDOWMENT}}.
</p>
{{ formfields }}
{{ next_button }}
SendBack.html¶
This is the page that P2 sees to send money back.
Set the title
block to Trust Game: Your Choice
,
and the content
block to:
<p>
You are Participant B. Participant A sent you {{group.sent_amount}}
and you received {{tripled_amount}}.
</p>
{{ formfields }}
{{ next_button }}
Here is the page code. Notes:
- We use vars_for_template() to pass the variable
tripled_amount
to the template. You cannot do calculations directly in the HTML code, so this number needs to be calculated in Python code and passed to the template.
class SendBack(Page):
form_model = 'group'
form_fields = ['sent_back_amount']
@staticmethod
def is_displayed(player):
return player.id_in_group == 2
@staticmethod
def vars_for_template(player):
group = player.group
return dict(
tripled_amount=group.sent_amount * C.MULTIPLICATION_FACTOR
)
Results¶
The results page needs to look slightly different for P1 vs. P2. So, we
use the {{ if }}
statement
to condition on the current player’s id_in_group
.
Set the title
block to Results
, and the content block to:
{{ if player.id_in_group == 1 }}
<p>
You sent Participant B {{ group.sent_amount }}.
Participant B returned {{ group.sent_back_amount }}.
</p>
{{ else }}
<p>
Participant A sent you {{ group.sent_amount }}.
You returned {{ group.sent_back_amount }}.
</p>
{{ endif }}
<p>
Therefore, your total payoff is {{ player.payoff }}.
</p>
class Results(Page):
pass
Wait pages and page sequence¶
Add 2 wait pages:
WaitForP1
(P2 needs to wait while P1 decides how much to send)ResultsWaitPage
(P1 needs to wait while P2 decides how much to send back)
After the second wait page, we should calculate the payoffs.
So, we define a function called set_payoffs
:
def set_payoffs(group):
p1 = group.get_player_by_id(1)
p2 = group.get_player_by_id(2)
p1.payoff = C.ENDOWMENT - group.sent_amount + group.sent_back_amount
p2.payoff = group.sent_amount * C.MULTIPLICATION_FACTOR - group.sent_back_amount
Then in ResultsWaitPage
, set after_all_players_arrive
:
after_all_players_arrive = set_payoffs
Make sure they are ordered correctly in the page_sequence:
page_sequence = [
Send,
WaitForP1,
SendBack,
ResultsWaitPage,
Results,
]
Add an entry to your SESSION_CONFIGS
¶
Create a session config with my_trust
in the app sequence.
Run the server¶
Load the project again then open your browser to http://localhost:8000
.