# Views¶

Each page that your players see is defined by a Page class in views.py (“views” is basically a synonym for “pages”).

Your views.py must have a page_sequence variable that gives the order of the pages. For example:

page_sequence=[Start, Offer, Accept, Results]


## Pages¶

A Page class can have any of the following optional methods and attributes:

### is_displayed()¶

You can define this function to return True if the page should be shown, and False if the page should be skipped. If omitted, the page will be shown.

For example, to only show the page to P2 in each group:

class Page1(Page):
def is_displayed(self):
return self.player.id_in_group == 2


Or only show the page in round 1:

class Page1(Page):
def is_displayed(self):
return self.round_number == 1


If you need to repeat the same rule for many pages, see here.

is_displayed() may be executed more than once, so beware of code that relies on it only being executed once, e.g. incrementing a counter.

### vars_for_template()¶

You can use this to return a dictionary of variable names and their values, which is passed to the template. Example:

class Page1(Page):
def vars_for_template(self):
return {'a': 1 + 1, 'b': self.player.foo * 10}


Then in the template you can access a and b like this:

Variables {{ a }} and {{ b }} ...


oTree automatically passes the following objects to the template: player, group, subsession, participant, session, and Constants. You can access them in the template like this: {{ Constants.blah }} or {{ player.blah }}.

Note

You generally shouldn’t generate random values in vars_for_template, because if the user refreshes their page, vars_for_template will be executed again, and the random calculation might return a different value. Instead, you should calculate random values in either before_session_starts, before_next_page, or after_all_players_arrive, each of which only executes once.

### before_next_page()¶

Here you define any code that should be executed after form validation, before the player proceeds to the next page.

If the page is skipped with is_displayed, then before_next_page will be skipped as well.

Example:

class Page1(Page):
def before_next_page(self):
self.player.tripled_payoff = self.player.bonus * 3


### template_name¶

Each Page should have a file in templates/ with the same name. For example, if your app has this page in my_app/views.py:

class Page1(Page):
pass


Then you should create a file my_app/templates/my_app/Page1.html, (note that app_name is repeated). See Templates for info on how to write an HTML template.

If the template needs to have a different name from your view class (e.g. you are sharing the same template for multiple views), set template_name. Example:

class Page1(Page):
template_name = 'app_name/MyView.html'


### timeout_seconds (Remaining time)¶

The number of seconds the user has to complete the page. After the time runs out, the page auto-submits.

Example: timeout_seconds = 20

When there are 60 seconds left, the page displays a timer warning the participant.

Note

If you are running the production server (runprodserver) or using timeoutworker, the page will always submit, even if the user closes their browser window. However, this does not occur if you are running the test server (runserver).

If you need the timeout to be dynamically determined, use get_timeout_seconds.

### timeout_happened¶

This attribute is automatically set to True if the page was submitted by timeout. It can be accessed in before_next_page. For example:

class Page1(Page):
timeout_seconds = 60

def before_next_page(self):
if self.timeout_happened:
self.player.my_random_variable = random.random()


timeout_happened is undefined in other methods like vars_for_template, because the timeout countdown only starts after the page is rendered.

### Timeouts and forms¶

To control what happens with the page’s form if a timeout occurs, see timeout_submission and timeout_happened.

### get_timeout_seconds¶

Note

This is a new feature in otree-core 1.3 (May 2017).

This is a dynamic alternative to timeout_seconds, so that you can base the timeout on self.player, self.session, etc.:

For example, you can make the timeout for a page configurable by adding a parameter to the session config (see Configure sessions) and referencing it in your page. In settings.py add this:

SESSION_CONFIGS = [
{
'name': 'my_app',
'num_demo_participants': 1,
'app_sequence': ['my_app'],
'my_page_timeout_seconds': 60,
},
# etc...
]

class MyPage(Page):

def get_timeout_seconds(self):
return self.session.config['my_page_timeout_seconds']


#### Timeouts that span multiple pages¶

You can use get_timeout_seconds to create timeouts that span multiple pages, or even the entire session. The trick is to define a fixed “expiration time”, and then on each page, make get_timeout_seconds return the number of seconds until that expiration time.

First, choose a place to start the timer. This could be a page called “Start” that displays text like “Press the button when you’re ready to start”. When the user clicks the “next” button, before_next_page will be executed and the expiry timestamp will be set:

import time

class Start(Page):

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

def before_next_page(self):
# user has 5 minutes to complete as many pages as possible
self.participant.vars['expiry_timestamp'] = time.time() + 5*60


(You could also start the timer in after_all_players_arrive or before_session_starts, and it could be stored in session.vars if it’s the same for everyone in the session.)

Then, each page’s get_timeout_seconds should be the number of seconds until that expiration time:

class Page1(Page):
def get_timeout_seconds(self):
return self.participant.vars['expiry_timestamp'] - time.time()


When time runs out, get_timeout_seconds will return 0 or a negative value, which will result in the page loading and being auto-submitted right away. This means all the remaining pages will quickly flash on the participant’s screen, which is usually undesired. So, you should use is_displayed to skip the page if time has run out, or if there’s only a few seconds remaining (e.g. 3).

class Page1(Page):
def get_timeout_seconds(self):
return self.participant.vars['expiry_timestamp'] - time.time()

def is_displayed(self):
return self.participant.vars['expiry_timestamp'] - time.time() > 3


If you have multiple pages in your page_sequence that need to share the timeout, rather than copy-pasting the above code to every page redundantly, you can create a base class for all pages:

class BasePage(Page):

def get_timeout_seconds(self):
return self.participant.vars['expiry_timestamp'] - time.time()

def is_displayed(self):
return self.participant.vars['expiry_timestamp'] - time.time() > 3

class Page1(BasePage):
pass

class Page2(BasePage):
pass

class Page3(BasePage):
pass

page_sequence = [
Start,
Page1, Page2, Page3,
]


The default text on the timer says “Time left to complete this page:”. But if your timeout spans multiple pages, you should word it more accurately, by setting timer_text:

class BasePage(Page):

timer_text = 'Time left to complete this section:'

def get_timeout_seconds(self):
return self.participant.vars['expiry_timestamp'] - time.time()

def is_displayed(self):
return self.participant.vars['expiry_timestamp'] - time.time() > 3


### def vars_for_all_templates(self)¶

This is not a method on the Page class, but rather a top-level function in views.py. It is useful when you need certain variables to be passed to multiple pages in your app. Instead of repeating the same values in each vars_for_template, you can define it in this function.

## 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.

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 NormalWaitPage(WaitPage):
pass

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 method can determine the winner and set each player’s payoff.

class ResultsWaitPage(WaitPage):
def after_all_players_arrive(self):
self.group.set_payoffs()


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. (However, you can use self.player in a wait page’s is_displayed.)

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

### get_players_for_group()¶

Note

This is a new feature in otree-core 1.3 (May 2017).

get_players_for_group() can be used in addition to group_by_arrival_time, to control exactly which players are assigned together.

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, ordered by the time they first arrived at the wait page. 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):
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:
# this is a Python "list slice"
return a_players[:2] + b_players[:2]

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


Here’s an example of a where each player has a field treatment, and 2 players can only be assigned to the same 2-player group if they have the same treatment. Note that we only need to check for possible groupings with the last player, because if any of the other 2 players matched with each other, they would have been grouped the previous time get_players_for_group was run.

class GroupByTreatment(WaitPage):
group_by_arrival_time = True

def get_players_for_group(self, waiting_players):

# since the list is ordered by arrival time,
# the last element is the newest player who just arrived

# the players who were already waiting
# (each of them was newest_player a previous time this method was called)

# check if any of the already waiting players have the same treatment
# as the newly arrived player

# if so, put them in a group together
if possible_partners:


### 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.

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.

## Randomizing page sequence¶

You can randomize the order of pages using rounds. An example is here.