# Wait pages¶

Wait pages are necessary when one player needs to wait for others to take some action before they can proceed. For example, in an ultimatum game, player 2 cannot accept or reject before they have seen player 1’s offer.

If you have a WaitPage in your sequence of pages, then oTree waits until all players in the group have arrived at that point in the sequence, and then all players are allowed to proceed.

class NormalWaitPage(WaitPage):
pass


If your subsession has multiple groups playing simultaneously, and you would like a wait page that waits for all groups (i.e. all players in the subsession), you can set the attribute wait_for_all_groups = True on the wait page, e.g.:

class AllGroupsWaitPage(WaitPage):
wait_for_all_groups = True


Wait pages can define the following methods:

## after_all_players_arrive()¶

Any code you define here will be executed once all players have arrived at the wait page. For example, this is a good place to set the players’ payoffs or determine the winner.

class ResultsWaitPage(WaitPage):
def after_all_players_arrive(self):
print('in after_all_players_arrive')
for player in self.group.get_players():
player.payoff = c(100)


Note, you can’t reference self.player inside after_all_players_arrive, because the code is executed once for the entire group, not for each individual player.

## is_displayed()¶

Works the same way as with regular pages. If this returns False then the player skips the wait page.

If some or all players in the group skip the wait page, then after_all_players_arrive() may not be run.

## group_by_arrival_time¶

If you set group_by_arrival_time = True on a WaitPage, players will be grouped in the order they arrive at that wait page:

class MyWaitPage(WaitPage):
group_by_arrival_time = True


For example, if players_per_group = 2, the first 2 players to arrive at the wait page will be grouped together, then the next 2 players, and so on.

This is useful in sessions where some participants might drop out (e.g. online experiments, or experiments with consent pages that let the participant quit early), or sessions where some participants take much longer than others.

A typical way to use group_by_arrival_time is to put it after an app that filters out participants. For example, if your session has a consent page that gives participants the chance to opt out of the study, you can make a “consent” app that just contains the consent pages, and then have an app_sequence like ['consent', 'my_game'], where my_game uses group_by_arrival_time. This means that if someone opts out in consent, they will be excluded from the grouping in my_game.

If a game has multiple rounds, you may want to only group by arrival time in round 1:

class MyWaitPage(WaitPage):
group_by_arrival_time = True

def is_displayed(self):
return self.round_number == 1


If you do this, then subsequent rounds will keep the same group structure as round 1. Otherwise, players will be re-grouped by their arrival time in each round. (group_by_arrival_time copies the group structure to future rounds.)

Notes:

• id_in_group is not necessarily assigned in the order players arrived at the page.
• group_by_arrival_time can only be used if the wait page is the first page in page_sequence
• If you use is_displayed on a page with group_by_arrival_time, it should only be based on the round number. Don’t use is_displayed to show the page to some players but not others.
• If group_by_arrival_time = True, then in creating_session, all players will initially be in the same group. Groups are only created “on the fly” as players arrive at the wait page.

If you need further control on arranging players into groups, use get_players_for_group().

## get_players_for_group()¶

If you’re using group_by_arrival_time and want more control over which players are assigned together, you can use get_players_for_group().

Let’s say that in addition to grouping by arrival time, you need each group group to consist of 1 man and 1 woman (or 2 “A” players and 2 “B” players, etc).

If you define a method called get_players_for_group, it will get called whenever a new player reaches the wait page. The method’s argument is the list of players who are waiting to be grouped (not people who have disconnected or closed their browser). If you select some of these players and return them as a list, those players will be assigned to a group, and move forward. If you don’t return anything, then no grouping occurs.

Here’s an example where each group has 2 A players, 2 B players.

class GroupingWaitPage(WaitPage):
group_by_arrival_time = True

def get_players_for_group(self, waiting_players):
print('in get_players_for_group')
a_players = [p for p in waiting_players if p.participant.vars['type'] == 'A']
b_players = [p for p in waiting_players if p.participant.vars['type'] == 'B']

if len(a_players) >= 2 and len(b_players) >= 2:
return [a_players[0], a_players[1], b_players[0], b_players[1]]
print('not enough players to create a group')

def is_displayed(self):
return self.round_number == 1


## Preventing players from getting stuck on wait pages¶

A common problem especially with online experiments is players getting stuck waiting for another player in their group who dropped out or is too slow.

Here are some things you can do to reduce this problem:

### Use group_by_arrival_time¶

As described above, you can use group_by_arrival_time so that only players who are actively playing around the same time get grouped together.

group_by_arrival_time works well if used after a “lock-in” task. In other words, before your multiplayer game, you can have a single-player effort task. The idea is that a participant takes the effort to complete this initial task, they are less likely to drop out after that point.

### Use page timeouts¶

Use timeout_seconds on each page, so that if a player is slow or inactive, their page will automatically advance. Or, you can manually force a timeout by clicking the “Advance slowest participants” button in the admin interface.

## Customizing the wait page’s appearance¶

You can customize the text that appears on a wait page by setting the title_text and body_text attributes, e.g.:

class MyWaitPage(WaitPage):
title_text = "Custom title text"
body_text = "Custom body text"


You can also make a custom wait page template. For example, save this to my_app/templates/my_app/MyWaitPage.html (this template must extend ‘otree/WaitPage.html’):

{% extends 'otree/WaitPage.html' %}
{% block title %}{{ title_text }}{% endblock %}
{% block content %}
{{ body_text }}
<p>
My custom content here.
</p>
{% endblock %}


Then tell your wait page to use this template:

class MyWaitPage(WaitPage):
template_name = 'my_app/MyWaitPage.html'


Then you can use vars_for_template in the usual way. Actually, the body_text and title_text attributes are just shorthand for setting vars_for_template; the following 2 code snippets are equivalent:

class MyWaitPage(WaitPage):
body_text = "foo"

class MyWaitPage(WaitPage):
def vars_for_template(self):
return {'body_text': "foo"}


If you want to apply your custom wait page template globally, save it to _templates/global/WaitPage.html. oTree will then automatically use it everywhere instead of the built-in wait page.

## CSS and JavaScript in Wait Pages¶

Wait pages have the same block structure as regular pages (global_scripts, app_scripts, scripts, etc...), so you can follow the same instructions described in JavaScript and CSS and Customizing the theme.

For example, to apply CSS to your custom wait page at _templates/global/WaitPage.html, put a block global_scripts in the template.

You can even make other custom wait pages inherit from _templates/global/WaitPage.html, just the way regular pages inherit from _templates/global/Page.html, and they can define the blocks app_scripts and scripts, etc.