Contents

Practice: Managing roles

Fixed number in roles in fixed groups

  • Let’s create a group with 2 buyers and 2 sellers.

otree startapp role_practice

  • Go to models.py and change players_per_group in Constants model to 4.

class Constants(BaseConstants):
    name_in_url = 'role_practice'
    players_per_group = 4
    num_rounds = 1

  • Go to Player class and define roles:

class Player(BasePlayer):

    def role(self):
        if self.id_in_group <= 2:
            return 'buyer'
        if self.id_in_group > 2:
            return 'seller'

  • Let’s go to pages.py and remove the pages MyPage and ResultsWaitPage. Having a single page would be enough for our purposes and we can check the role from oTree admin panel. Our pages.py should look like this:

from otree.api import Currency as c, currency_range
from ._builtin import Page, WaitPage
from .models import Constants


class Results(Page):
    pass


page_sequence = [Results]

  • And let’s add our app to settings.py:

SESSION_CONFIGS = [
    # other configs
     dict(
        name='role_practice',
        display_name="role_practice",
        num_demo_participants=4,
        app_sequence=['role_practice'],
     ),
]

So when we create the session with 8 participants from oTree control panel

../_images/setting_up_session.png

And when we go to the data we see that roles are set as we want them to be:

../_images/roles_regular.png

We can also shorten the if conditions as the following:

    def role(self):
        return {1: 'buyer', 2: 'seller'}[self.id_in_group]

Balanced number of roles in a variable group size

  • Imagine that we have a variable group size (for instance group size is a treatment variable). And we would like to balance our 3 roles. (say buyer, seller, observer). We have several possibilities to go for.

Python Break - Modulus

  • Modulus operator is used to get the remainder of a division.

  • Usage: Number % Divisor

    10 % 2  # returns 0
10 % 4  # returns 2

We will use modulus operator % in order to give a balanced number of roles:

    def role(self):
        roles = ['buyer', 'seller', 'observer']
        num_roles = len(roles)
        if self.id_in_group % num_roles == 0:
            return roles[0]
        if self.id_in_group % num_roles == 1:
            return roles[1]
        if self.id_in_group % num_roles == 2:
            return roles[2]

Which can be reduced to:

    def role(self):
        roles = ['buyer', 'seller', 'observer']
        num_roles = len(roles)
        return roles[(self.id_in_group -1) % num_roles]  # added -1 to start from the first element in the list

  • Then for the size 3:

../_images/roles_3_players.png

  • And for the size 7:

../_images/roles_7_players.png

Fixed roles in each round/app

In order to fix a role, we can store it in our participant.vars.

class Subsession(BaseSubsession):
    def creating_session(self):
        self.group_randomly() # This randomizes the groups
            
        # Some role assignment below
        for p in self.get_players():
            if self.round_number == 1:
                if p.id_in_group % 2 == 0:
                    p.participant.vars['type'] = 'buyer'
                else:
                    p.participant.vars['type'] = 'seller'
    
            p.type = p.participant.vars['type']

class Player(BasePlayer):
    def role(self):
        return self.participant.vars['type']

    type = models.StringField()

One final note: For the multiple round cases, you can fix id_in_group in a random matching by group_randomly(fixed_id_in_group=True).

Roles Matching