Forms

Each page in oTree can contain a form, which the player should fill out and submit by clicking the “Next” button. To create a form, first you need fields on the Player model. Then, in your Page class, set form_model and form_fields.

For example, here is a Player model:

class Player(BasePlayer):
    name = models.StringField(label="Your name:")
    age = models.IntegerField(label="Your age:")

And page:

class Page1(Page):
    form_model = 'player'
    form_fields = ['name', 'age'] # this means player.name, player.age

When the user submits the form, the submitted data is automatically saved to the corresponding fields on the player model.

Forms in templates

In your template, you can display the form simply with:

{% formfields %}

If you want to position the fields individually, you can instead use {% formfield %}:

{% formfield player.contribution %}

You can also put the label in directly in the template:

{% formfield player.contribution label="How much do you want to contribute?" %}

Simple form field validation

The player must submit a valid form before they go to the next page. If the form they submit is invalid (e.g. missing or incorrect values), it will be re-displayed to them along with the list of errors they need to correct.

_images/Sz34h7d.png

oTree automatically validates input. For example, if you have a form containing a IntegerField, oTree will reject inputs like 1.5 or hello.

min and max

For example, this is how you would require an integer to be between 12 and 24:

offer = models.IntegerField(min=12, max=24)

If the max/min are not fixed, you should use {field_name}_max()

choices

If you want a field to be a dropdown menu with a list of choices, set choices=:

level = models.IntegerField(
    choices=[1, 2, 3],
)

To use radio buttons instead of a dropdown menu, you should set the widget to RadioSelect or RadioSelectHorizontal:

level = models.IntegerField(
    choices=[1, 2, 3],
    widget=widgets.RadioSelect
)

If the list of choices needs to be determined dynamically, use {field_name}_choices()

You can also set display names for each choice by making a list of [value, display] pairs:

level = models.IntegerField(
    choices=[
        [1, 'Low'],
        [2, 'Medium'],
        [3, 'High'],
    ]
)

If you do this, users will just see a menu with “Low”, “Medium”, “High”, but their responses will be recorded as 1, 2, or 3.

You can do this for BooleanField, StringField, etc.:

cooperated = models.BooleanField(
    choices=[
        [False, 'Defect'],
        [True, 'Cooperate'],
    ]
)

After the field has been set, you can access the human-readable name using get_FOO_display , like this: player.get_level_display() # returns e.g. 'Medium'. However, if you define the choices dynamically with {field_name}_choices(), in order to use get_*_display() you need to also define the *_choices method on the Player/Group in models.py.

Optional fields

If a field is optional, you can use blank=True like this:

offer = models.IntegerField(blank=True)

Dynamic form field validation

The min, max, and choices described above are only for fixed (constant) values.

If you want them to be determined dynamically (e.g. different from player to player), then you can instead define one of the below methods on your Page.

Note

As of May 2019 (oTree 2.1.35), it is recommended to define the following methods on the Player (or Group) model, not the Page:

  • FIELD_min

  • FIELD_max

  • FIELD_choices

  • FIELD_error_message

For example, here is the old format:

class MyPage(Page):

    form_model = 'player'
    form_fields = ['offer']

    def offer_max(self):
        return self.player.endowment

To change this to the new format, you move offer_max into the Player model:

class Player(BasePlayer):

    offer = models.CurrencyField()

    def offer_max(self):
        return self.endowment

Note that we change return self.player.endowment to just self.endowment, because self is the player.

The old format will continue to work, so it is not urgent for you to make this change.

{field_name}_choices()

Note

It is now recommended to define this method on the Player (or Group) model, not the Page. See the note above.

Like setting choices=, this will set the choices for the form field (e.g. the dropdown menu or radio buttons).

Example:

class Player(BasePlayer):

    fruit = models.StringField()

    def fruit_choices(self):
        choices = ['apple', 'kiwi', 'mango']
        random.shuffle(choices)
        return choices

(If you’re not using oTree Studio, then you need to have import random at the top of your file.)

{field_name}_max()

Note

It is now recommended to define this method on the Player (or Group) model, not the Page. See the note above.

The dynamic alternative to setting max= in the model field. For example:

class Player(BasePlayer):

    offer = models.CurrencyField()

    def offer_max(self):
        return self.budget

    budget = models.CurrencyField()

{field_name}_min()

Note

It is now recommended to define this method on the Player (or Group) model, not the Page. See the note above.

The dynamic alternative to setting min= on the model field.

{field_name}_error_message()

Note

It is now recommended to define this method on the Player (or Group) model, not the Page. See the note above.

This is the most flexible method for validating a field.

class Player(BasePlayer):

    offer = models.CurrencyField()

    def offer_error_message(self, value):
        print('value is', value)
        if value > self.budget / 2:
            return 'Cannot offer more than half your remaining budget'

    budget = models.CurrencyField()

Validating multiple fields together

Let’s say you have 3 integer fields in your form whose names are int1, int2, and int3, and the values submitted must sum to 100. You can enforce this with the error_message method:

class MyPage(Page):

    form_model = 'player'
    form_fields = ['int1', 'int2', 'int3']

    def error_message(self, values):
        print('values is', values)
        if values["int1"] + values["int2"] + values["int3"] != 100:
            return 'The numbers must add up to 100'

Notes: - If a field was left blank (and you set blank=True), its value here will be None. - This function is only executed if there are no other errors in the form.

form_model = ‘group’

If you set form_model = 'group', the values submitted by the user will be stored onto the group model, rather than the player. This is often useful in games where some players make decisions on behalf of the group. For example, in an ultimatum game, player 1 makes an offer and player 2 accepts or rejects. Since there is only 1 offer made per group, you would define the offer field on the group:

class Group(Group):
    offer = models.CurrencyField()

Your page would look like this:

class Offer(Page):
    form_model = 'group'
    form_fields = ['offer'] # this means it will be stored in group.offer

And in your template, you would have:

{% formfield group.offer %}

Determining form fields dynamically

Note

This feature is not yet supported in oTree Studio.

If you need the list of form fields to be dynamic, instead of form_fields you can define a method get_form_fields(self) that returns the list. For example:

class MyPage(Page):

    form_model = 'player'
    def get_form_fields(self):
        if self.player.num_bids == 3:
            return ['bid_1', 'bid_2', 'bid_3']
        else:
            return ['bid_1', 'bid_2']

But if you do this, you have to be sure to also include the same {% formfield %} elements in your template. The easiest way is to use {% formfields %}.

Widgets

The full list of form input widgets offered by Django is here.

oTree additionally offers:

  • RadioSelectHorizontal (same as RadioSelect but with a horizontal layout, as you would see with a Likert scale)

  • Slider

    • Slider is not yet supported in oTree Studio

    • To specify the step size, do: Slider(attrs={'step': '0.01'})

    • To disable the current value from being displayed, do: Slider(show_value=False)

Customizing a field’s appearance

{% formfields %} and {% formfield %} are easy to use because they automatically output all necessary parts of a form field (the input, the label, and any error messages), with Bootstrap styling.

However, if you want more control over the appearance and layout, you can use Django’s manual field rendering. Instead of {% formfield player.my_field %}, do {{ form.my_field }}, to get just the input, and then position it as you want.

Just remember to also include {{ form.my_field.errors }}, so that if there is an error in the form, the participant will see the error message.

More info here.

Example: Radio buttons in tables and other custom layouts

Let’s say you have a set of IntegerField in your model:

class Player(BasePlayer):

    offer_1 = models.IntegerField(widget=widgets.RadioSelect, choices=[1,2,3])
    offer_2 = models.IntegerField(widget=widgets.RadioSelect, choices=[1,2,3])
    offer_3 = models.IntegerField(widget=widgets.RadioSelect, choices=[1,2,3])
    offer_4 = models.IntegerField(widget=widgets.RadioSelect, choices=[1,2,3])
    offer_5 = models.IntegerField(widget=widgets.RadioSelect, choices=[1,2,3])

And you’d like to present them as a likert scale, where each option is in a separate column.

(First, try to reduce the code duplication in models.py by following the instructions in How to make many fields.)

Because the options must be in separate table cells, the ordinary RadioSelectHorizontal widget will not work here.

Instead, you should simply loop over the choices in the field as follows:

<tr>
    <td>{{ form.offer_1.label }}</td>
    {% for choice in form.offer_1 %}
        <td>{{ choice }}</td>
    {% endfor %}
</tr>

If you have many fields with the same number of choices, you can arrange them in a table:

<table class="table">
    {% for field in form %}
        <tr>
            <th>{{ field.label }}</th>
            {% for choice in field %}
                <td>{{ choice }}</td>
            {% endfor %}
        </tr>
    {% endfor %}
</table>

You can also get choices individually by using their 0-based index, e.g. {{ form.my_field.0 }} gives you the radio button of the first choice. For more granular control, as described here, you can use the choice_label and tag attributes on a field choice.

Advanced: Raw HTML widgets

If {% formfield %} and manual field rendering are still not flexible enough for you, you can write the raw HTML for your form input. However, you will lose the convenient features handled automatically by oTree. For example, if the form has an error and the page re-loads, all entries by the user may be wiped out.

To use raw HTML, just ensure that each field in your Page’s form_fields has a corresponding <input> element with a matching name attribute.

Remember that for any field my_field, you should include {{ form.my_field.errors }}, so that if there is an error in the form, the participant will see the error message.

Raw HTML example: custom user interface with JavaScript

Let’s say you don’t want users to fill out form fields, but instead interact with some sort of visual app, like a clicking on a chart or playing a graphical game. Or, you want to record extra data like how long they spent on part of the page, how many times they clicked, etc.

You can build these interfaces in any front-end framework you want. Simple ones can be done with jQuery; more complex ones would use something like React or Polymer.

Then, use JavaScript to record the relevant data points and store it in a hidden form field. For example:

# models.py
my_hidden_input = models.IntegerField()

# pages.py
form_fields = ['my_hidden_input']

# HTML template
<input type="hidden" name="my_hidden_input" id="id_my_hidden_input"/>

Then you can use JavaScript to set the value of that input, by selecting the element by id id_my_hidden_input, and setting its value attribute.

When the page is submitted, the value of your hidden input will be recorded in oTree like any other form field.

Buttons

Button that submits the form

If your page only contains 1 decision, you could omit the {% next_button %} and instead have the user click on one of several buttons to go to the next page.

For example, let’s say your Player model has offer_accepted = models.BooleanField(), and rather than a radio button you’d like to present it as a button like this:

_images/yes-no-buttons.png

First, put offer_accepted in your Page’s form_fields as usual. Then put this code in the template (the btn classes are just for Bootstrap styling):

<p><b>Do you wish to accept the offer?</b></p>
<div>
    <button name="offer_accepted" value="True" class="btn btn-primary btn-large">Yes</button>
    <button name="offer_accepted" value="False" class="btn btn-primary btn-large">No</button>
</div>

You can use this technique for any type of field, not just BooleanField.

Button that doesn’t submit the form

If the button has some purpose other than submitting the form, add type="button" to the <button>:

<button>
    Clicking this will submit the form
</button>

<button type="button">
    Clicking this will not submit the form
</button>

Miscellaneous & advanced

Forms with a dynamic vector of fields

Let’s say you want a form with a vector of n fields that are identical, except for some numerical index, e.g.:

contribution[1], contribution[2], ..., contribution[n]

Furthermore, suppose n is variable (can range from 1 to N).

Currently in oTree, you can only define a fixed number of fields in a model. So, you should define in models.py N fields (contribution_1...contribution_N...), and then use get_form_fields as described above to dynamically return a list with the desired subset of these fields.

For example, let’s say the above variable n is actually an IntegerField on the player, which gets set dynamically at some point in the game. You can use get_form_fields like this:

class MyPage(Page):

    form_model = 'player'
    def get_form_fields(self):
        return ['contribution_{}'.format(i) for i in range(1, self.player.n + 1)]

Form fields with dynamic labels

If the label should contain a variable, you can construct the string in pages.py:

class Contribute(Page):
    form_model = 'player'
    form_fields = ['contribution']

    def vars_for_template(self):
        return {
            'contribution_label': 'How much of your {} do you want to contribute?'.format(self.player.endowment)
        }

Then in the template, set the label to this variable:

{% formfield player.contribution label=contribution_label %}

If you use this technique, you may also want to use Dynamic form field validation.