"IndexError: Cannot choose from an empty sequence" between tasks

Description of issue

Spurious error messages during execution.

Environment settings

• OS: Official ubuntu:18.04 Docker container
• Python version: 3.6.7
• Locust version: 0.9.0

Steps to reproduce

I’ve registered all my tasks by looping through the OpenAPI JSON for the API like this:

class APIRequest(TaskSet):
    def setup(self) -> None:
        for api_path, properties in self.client.get('/api/doc/?format=openapi').json()['paths'].items():
            [exclude some API paths]
            self.schedule_task(self.api_task, args=[api_path])

    def api_task(self, path: str) -> None:
        response = self.client.get("/api{}?limit=1000".format(path))
        assert response.status_code == HTTPStatus.OK, response.content.decode()


class ApiRequests(HttpLocust):
    task_set = APIRequest
    min_wait = 200
    max_wait = 1000
    host = 'https://{}'.format(os.environ['WEB_HOSTNAME'])


if __name__ == '__main__':
    locust = ApiRequests()
    locust.run()

I then run Locust: locust --csv=output/ --locustfile=api.py --no-web --run-time=5m

Expected behavior

Locust should run for five minutes and return successfully, reporting progress as it runs.

Actual behavior

Even if the run is successful, Locust will frequently print one, two or three error messages such as the below between each time it prints the summary table:

[2019-02-04 00:19:23,150] 2e0a2ecc40ea/ERROR/stderr: Traceback (most recent call last):
  File "/src/app/.venv/lib/python3.6/site-packages/locust/core.py", line 358, in run
    self.schedule_task(self.get_next_task())
  File "/src/app/.venv/lib/python3.6/site-packages/locust/core.py", line 419, in get_next_task
    return random.choice(self.tasks)
  File "/src/app/.venv/lib/python3.6/random.py", line 260, in choice
    raise IndexError('Cannot choose from an empty sequence') from None
IndexError: Cannot choose from an empty sequence

[cyberw]

Setup is run only once (and thus should never have been made an object method, that is a design mistake), so only the first TaskSet instance will have self.apis value.

If you set a class variable (LoadTestBehaviour.apis instead of self.apis), it should work.

[cyberw]

Oh I see what you are trying to do now. remove the call to schedule_tasks. I dont think you are meant to call it yourself. Instead, just populate a list of paths in setup and pick from that list inside your task method.