You can write “bots” that simulate participants playing your app, so that you can test that it functions properly and can handle the traffic of a real experiment.

A lot of oTree users skip writing bots because they think it’s complicated or because they are too busy. But bots are possibly the easiest part of oTree. For a game with 2 pages, it’s just 2 yield statements:

class PlayerBot(Bot):

    def play_round(self):
        yield (pages.Contribute, {'contribution': 10})
        yield (pages.Results)

Then, each time you make a change to your app, you can just re-run this test. It’s a great time saver.

Running tests

Let’s say you have this session config:

    app_sequence=['ultimatum', 'survey'],

If each app in the app_sequence has bots, you can run them with:

otree test mysession

To test with a specific number of participants (otherwise it will default to num_demo_participants):

otree test mysession 6

To run tests for all session configs:

otree test

Exporting data

Use the --export flag to export the results to a CSV file:

otree test mysession --export

To specify the folder where the data is saved, do:

otree test mysession --export=myfolder

Writing tests

Submitting pages

Go to your app’s It should make one yield per page submission. For example:

class PlayerBot(Bot):
    def play_round(self):
        yield (pages.Start)
        yield (pages.Offer, {'offer_amount': 50})

Here, we first submit the Start page, which does not contain a form. The next page is Offer, which contains a form whose field is called offer_amount, which we set to 50.

We use yield, because in Python, yield means to produce or generate a value. You could think of the bot as a machine that yields (i.e. generates) submissions.

If a page contains several fields, use a dictionary with multiple items:

yield (pages.Offer, {'first_offer_amount': 50, 'second_offer_amount': 150, 'third_offer_amount': 150})

The test system will raise an error if the bot submits invalid input for a page, or if it submits pages in the wrong order.

You use if statements to play any player or round number. For example:

if self.round_number == 1:
    yield (pages.Introduction)
if self.player.id_in_group == 1:
    yield (pages.Offer, {'offer': 30})
    yield (pages.Accept, {'offer_accepted': True})

Your if statements can depend on self.player,, self.round_number, etc.

You should ignore wait pages when writing bots. Just write a yield for every page that is submitted. The bot will wait until any wait pages are cleared, then it will execute the next yield.


As the name indicates, play_round() should just play 1 round at a time. oTree will automatically execute it num_rounds times.


You can use assert statements to ensure that your code is working as you expect.

For example:

class PlayerBot(Bot):

    def play_round(self):
        assert self.player.money_left == c(10)
        yield (pages.Contribute, {'contribution': c(1)})
        assert self.player.money_left == c(9)
        yield (pages.Results)

If the asserted condition does not hold, an error will be raised. (You can read about assert in the Python documentation.)

The assert statements are executed immediately before submitting the following page. For example, let’s imagine the page_sequence for the game in the above example is [Contribute, ResultsWaitPage, Results]. The bot submits pages.Contribute, is redirected to the wait page, and is then redirected to the Results page. At that point, the Results page is displayed, and then the line assert self.player.money_left == c(9) is executed.

Testing form validation

If you use form validation, you should test that your app is correctly rejecting invalid input from the user, by using SubmissionMustFail().

For example, let’s say you have this page:

class MyPage(Page):

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

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

Here is how to test that it is working properly:

from . import pages
from otree.api import Bot, SubmissionMustFail

class PlayerBot(Bot):

    def play_round(self):
        yield SubmissionMustFail(pages.MyPage, {'int1': 0, 'int2': 0})
        yield SubmissionMustFail(pages.MyPage, {'int1': 101, 'int2': 0})
        yield (pages.MyPage, {'int1': 99, 'int2': 1})

The bot will submit MyPage 3 times. If one of the first 2 submissions succeeds, an error will be raised, because it is not supposed to succeed.


When using SubmissionMustFail on forms with multiple fields, you can use error_fields for extra thoroughness.

For example, let’s say we a submit a valid age, but an invalid weight and height:

yield SubmissionMustFail(
        'age': 20,
        'weight': -1,
        'height': -1,

What’s missing is that the bot system doesn’t tell us exactly why the submission fails. Is it an invalid weight, height, or both? error_fields can resolve the ambiguity:

yield SubmissionMustFail(
        'age': 20,
        'weight': -1,
        'height': -1,
    error_fields=['weight', 'height']

This will verify that weight and height contained errors, but age did not.

If error_message returns an error, then error_fields will be ['__all__'].

Test cases

You can define an attribute cases on your PlayerBot class that lists different test cases. For example, in a public goods game, you may want to test 3 scenarios:

  • All players contribute half their endowment

  • All players contribute nothing

  • All players contribute their entire endowment (100 points)

We can call these 3 test cases “basic”, “min”, and “max”, respectively, and put them in cases. Then, oTree will execute the bot 3 times, once for each test case. Each time, a different value from cases will be assigned to in the bot.

For example:

from . import pages
from otree.api import Bot, SubmissionMustFail

class PlayerBot(Bot):

    cases = ['basic', 'min', 'max']

    def play_round(self):
        yield (pages.Introduction)

        if == 'basic':
            assert self.player.payoff == None

        if == 'basic':
            if self.player.id_in_group == 1:
                for invalid_contribution in [-1, 101]:
                    yield SubmissionMustFail(pages.Contribute, {'contribution': invalid_contribution})
        contribution = {
            'min': 0,
            'max': 100,
            'basic': 50,

        yield (pages.Contribute, {"contribution": contribution})
        yield (pages.Results)

        if self.player.id_in_group == 1:

            if == 'min':
                expected_payoff = 110
            elif == 'max':
                expected_payoff = 190
                expected_payoff = 150
            assert self.player.payoff == expected_payoff

cases needs to be a list, but it can contain any data type, such as strings, integers, or even dictionaries. Here is a trust game bot that uses dictionaries as cases.

from . import pages
from otree.api import Bot, SubmissionMustFail

class PlayerBot(Bot):

    cases = [
        {'offer': 0, 'return': 0, 'p1_payoff': 10, 'p2_payoff': 0},
        {'offer': 5, 'return': 10, 'p1_payoff': 15, 'p2_payoff': 5},
        {'offer': 10, 'return': 30, 'p1_payoff': 30, 'p2_payoff': 0}

    def play_round(self):
        case =
        if self.player.id_in_group == 1:
            yield (pages.Send, {"sent_amount": case['offer']})

            for invalid_return in [-1, case['offer'] * Constants.multiplication_factor + 1]:
                yield SubmissionMustFail(pages.SendBack, {'sent_back_amount': invalid_return})
            yield (pages.SendBack, {'sent_back_amount': case['return']})

        yield (pages.Results)

        if self.player.id_in_group == 1:
            expected_payoff = case['p1_payoff']
            expected_payoff = case['p2_payoff']

        assert self.player.payoff == expected_payoff

Checking the HTML

In the bot, self.html will be a string containing the HTML of the page you are about to submit. This is useful for asserts:

if self.player.id_in_group == 1:
    assert self.player.is_winner
    assert 'you won the game' in self.html
    assert not self.player.is_winner
    assert 'you did not win' in self.html
yield pages.Results
# etc...

self.html is updated with the next page’s HTML, after every yield statement. Linebreaks and extra spaces are ignored.

Automatic HTML checks

An error will be raised if the bot is trying to submit form fields that are not actually found in the page’s HTML, or if the page’s HTML is missing a submit button.

However, the bot system is not able to see fields and buttons that are added dynamically with JavaScript. In these cases, you should disable the HTML check by using Submission with check_html=False. For example, change this:

class PlayerBot(Bot)
    def play_round(self):
        yield (pages.MyPage, {'foo': 99})

to this:

from otree.api import Submission

class PlayerBot(Bot)
    def play_round(self):
        yield Submission(pages.MyPage, {'foo': 99}, check_html=False)

(If you used Submission without check_html=False, the two code samples would be equivalent.)

If many of your pages incorrectly fail the static HTML checks, you can bypass these checks globally by setting BOTS_CHECK_HTML = False in

Simulate a page timeout

You can use Submission with timeout_happened=True:

from otree.api import Submission

class PlayerBot(Bot)
    def play_round(self):
        yield Submission(pages.MyPage, {'foo': 99}, timeout_happened=True)

Misc note

In bots, it is risky to assign player = self.player (or participant = self.participant, etc), even though that kind of code is encouraged in

Because if there is a yield in between, the data can be stale:

def play_round(self):
    player = self.player
    assert player.money_left == c(10) # OK
    yield (pages.Contribute, {'contribution': c(1)})
    # don't do this!
    # "player" variable still has the data from BEFORE pages.Contribute was submitted.
    assert player.money_left == c(9)

It’s safer to use self.player.money_left directly, because doing self.player gets the most recent data from the database.

Browser bots

Bots can run in the browser. They run the same way as command-line bots.

The advantage is that they test the app in a more full and realistic way, because they use a real web browser. Also, while it’s playing you can briefly see each page and notice if there are visual errors.

Basic use

  • Write your as described above.

  • In, set use_browser_bots=True for your session config(s).

  • Run your server and create a session. The pages will auto-play with browser bots, once the start links are opened.

  • If using Heroku, make sure the runprodserver2of2 dyno is enabled.

Command-line browser bots (running locally)

You can launch browser bots from the command line, using otree browser_bots.

  • Make sure Google Chrome is installed, or set BROWSER_COMMAND in (more info below).

  • Run your server (e.g. otree devserver)

  • Close all Chrome windows.

  • Run this:

    otree browser_bots mysession

This will launch several Chrome tabs and run the bots. When finished, the tabs will close, and you will see a report in your terminal window.

If Chrome doesn’t close windows properly, make sure you closed all Chrome windows prior to launching the command.

Command-line browser bots on a remote server (e.g. Heroku)

If the server is running on a host/port other than the usual http://localhost:8000, you need to pass --server-url. For example, if it’s on Heroku, you would do like this:

otree browser_bots mysession --server-url=


On my PC, running the default public_goods session with 3 participants takes about 4-5 seconds, and with 9 participants takes about 10 seconds.

Choosing session configs and sizes

You can specify the number of participants:

otree browser_bots mysession 6

To test all session configs, just run this:

otree browser_bots

Browser bots: misc notes

You can use a browser other than Chrome by setting BROWSER_COMMAND in Then, oTree will open the browser by doing something like subprocess.Popen(settings.BROWSER_COMMAND).

(Optional) To make the bots run more quickly, disable most/all add-ons, especially ad-blockers. Or create a fresh Chrome profile that you use just for browser testing. When oTree launches Chrome, it should use the last profile you had open.