# Part 1: 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.)

• There are 3 players per group. So, change `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 `c(1000)`. (`c()` means it is a currency amount; see Money & Payoffs).

• Each contribution is multiplied by 2. So let’s define `multiplier` and set it to 2:

Now we have:

```class Constants(BaseConstants):
name_in_url = 'my_public_goods'
players_per_group = 3
num_rounds = 1

endowment = c(1000)
multiplier = 2
```

Now let’s define Player and Group.

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, we define a `contribution` column on the Player:

```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¶

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`. 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).

When all players have completed the `Contribute` page, the players’ payoffs can be calculated. You can trigger this calculation inside the the `after_all_players_arrive` method on the `WaitPage`, which automatically gets called when all players have arrived at the wait page. We can access the current group with `self.group` (for more info about `self`, see Conceptual overview).

```class ResultsWaitPage(WaitPage):

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

### Page 3: Results¶

Now we create the `Results` page. This page doesn’t need any properties:

```class Results(Page):
pass
```

For the template, set the `title` block to `Results`, and the `content` block 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¶

Now we specify the order in which the pages are shown:

```page_sequence = [
Contribute,
ResultsWaitPage,
Results
]
```

## Define the session config¶

We add another session config:

• name: my_public_goods

• display_name: My Public Goods (Simple Version)

• num_demo_participants: 3

• app_sequence: [‘my_public_goods’]

## Run the code¶

Again, run the server 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).