Part 2: Public goods game

(A video of this tutorial is on YouTube )

We will now create a simple public goods game. The public goods game is a classic game in economics.

This is a three player game where each player is initially endowed with 100 points. Each player individually makes a decision about how many of their points they want to contribute to the group. The combined contributions are multiplied by 2, and then divided evenly three ways and redistributed back to the players.

The full code for the app we will write is here.

Create the app

Just as in the previous part of the tutorial, create another app, called my_public_goods.

Constants

Go to your app’s Constants. (For more info, see Constants.)

  • Set players_per_group to 3. oTree will then automatically divide players into groups of 3.

  • The endowment to each player is 1000 points. So, let’s define endowment and set it to a currency value of 1000.

  • Each contribution is multiplied by 2. So define an integer constant called multiplier = 2:

Now we have the following constants:

players_per_group = 3
num_rounds = 1
endowment = c(1000) # c() means it's a currency
multiplier = 2

After the game is played, what data points will we need about each player? It’s important to record how much each person contributed. So, go to the Player model and define a contribution column:

class Player(BasePlayer):
    contribution = models.CurrencyField(
        min=0,
        max=Constants.endowment,
        label="How much will you contribute?"
    )

We also need to record the payoff the user makes at the end of the game, but we don’t need to explicitly define a payoff field, because in oTree, the Player already contains a payoff column.

What data points are we interested in recording about each group? We might be interested in knowing the total contributions to the group, and the individual share returned to each player. So, we define those 2 fields on the Group:

class Group(BaseGroup):
    total_contribution = models.CurrencyField()
    individual_share = models.CurrencyField()

Pages

This game has 3 pages:

  • Page 1: players decide how much to contribute

  • Page 2: Wait page: players wait for others in their group

  • Page 3: players are told the results

Page 1: Contribute

First let’s define Contribute. This page contains a form, so we need to define form_model and form_fields. Specifically, this form should let you set the contribution field on the player. (For more info, see Forms.)

class Contribute(Page):

    form_model = 'player'
    form_fields = ['contribution']

Now, we create the HTML template. (If using PyCharm, create the template file Contribute.html)

Set the title block to Contribute, and the content block to:

<p>
    This is a public goods game with
    {{ Constants.players_per_group }} players per group,
    an endowment of {{ Constants.endowment }},
    and a multiplier of {{ Constants.multiplier }}.
</p>

{% formfields %}

{% next_button %}

Page 2: ResultsWaitPage

When all players have completed the Contribute page, the players’ payoffs can be calculated. Go to your Group model and add a method called set_payoffs (note that self refers to the group):

def set_payoffs(self):
    players = self.get_players()
    contributions = [p.contribution for p in players]
    self.total_contribution = sum(contributions)
    self.individual_share = self.total_contribution * Constants.multiplier / Constants.players_per_group
    for p in players:
        p.payoff = Constants.endowment - p.contribution + self.individual_share

After a player makes a contribution, they cannot see the results page right away; they first need to wait for the other players to contribute. You therefore need to add a WaitPage. Let’s call it ResultsWaitPage. When a player arrives at a wait page, they must wait until all other players in the group have arrived. Then everyone can proceed to the next page. (For more info, see Wait pages).

Add after_all_players_arrive method to ResultsWaitPage, and set it to trigger the set_payoffs method:

def after_all_players_arrive(self):
    self.group.set_payoffs()

Page 3: Results

Now we create a page called Results. Set the template’s content to:

<p>
    You started with an endowment of {{ Constants.endowment }},
    of which you contributed {{ player.contribution }}.
    Your group contributed {{ group.total_contribution }},
    resulting in an individual share of {{ group.individual_share }}.
    Your profit is therefore {{ player.payoff }}.
</p>

{% next_button %}

Page sequence

Make sure your page_sequence is correct:

page_sequence = [
    Contribute,
    ResultsWaitPage,
    Results
]

Define the session config

We add another session config:

  • name: my_public_goods

  • num_demo_participants: 3

  • app_sequence: [‘my_public_goods’]

Run the code

Load the project again then open your browser to http://localhost:8000.

Troubleshoot with print()

I often read messages on programming forums like, “My program is not working. I can’t find the mistake, even though I have spent hours looking at my code”.

The solution is not to re-read the code until you find an error; it’s to interactively test your program.

The simplest way is using print() statements. If you don’t learn this technique, you won’t be able to program games effectively.

You just need to insert a line in your code like this:

print('group.total_contribution is', self.group.total_contribution)

Put this line in the part of your code that’s not working, such as the payoff function defined above. When you play the game in your browser and that code gets executed, your print statement will be displayed in your command prompt window (not in your web browser).

You can sprinkle lots of prints in your code

print('in payoff function')
contributions = [p.contribution for p in players]
print('contributions:', contributions)
group.total_contribution = sum(contributions)
group.individual_share = group.total_contribution * Constants.multiplier / Constants.players_per_group
print('individual share', group.individual_share)
for p in players:
    print('payoff before', p.payoff)
    p.payoff = Constants.endowment - p.contribution + group.individual_share
    print('payoff after', p.payoff)

If you don’t see the output in your console window, that means your code is not getting executed! (Which is why it isn’t working.)

Maybe it’s because your code is inside an “if” statement that is always False. Or maybe your code is in a function that never gets called (executed).