Demonstrate inconsistent behavior of celery.canvas.chord in celery 4.1.0

minimal_chord_chain.py

"""Demonstrate inconsistent chord header result values:

Run by::

  pip install celery redis

  python minimal_chord_chain.py worker --loglevel INFO --config=minimal_chord_chain
  
  # Separate shell
  python minimal_chord_chain.py shell --config=minimal_chord_chain
  from minimal_chord_chain import test; test()
  
Results are inspectable in logging in worker's shell. Note that sometimes
``vs`` is a scalar (an int, in this case) and sometimes it's a list. This seems
like it does not match the documented behavior of ``chord``:

> The body is applied with the return values of all the header tasks as a list.
"""

import celery
from celery.canvas import chord, group

app = celery.Celery()

CELERY_RESULT_BACKEND = "redis://localhost/0"


@app.task()
def starter(v):
    return v


@app.task()
def stepper(v):
    return v + 1


@app.task()
def resulter(vs, test):
    return vs, repr(type(vs)), test


def test():
    chord([
        starter.s(1) |
        stepper.s(),
    ])(resulter.s('single task, scalar result'))

    chord([
        starter.s(1) |
        stepper.s(),
        starter.s(11) |
        stepper.s(),
    ])(resulter.s('multiple tasks, list result'))


def main():
    app.start()


if __name__ == '__main__':
    main()