Celery Chord Callback error

Question

Suppose i have a celery code like this:

@celery.task()
def A(a):
    try:
        .......
        .......
    except Exception,e:
        raise

@celery.task()
def B(b):
    try:
        .......
        .......
    except Exception,e:
        raise

def C(c):
    try:
        .......
        .......
    except Exception,e:
        raise

def final():
    callback = C.s()
    header = [A.s(something), B.s(something)]
    result = chord(header)(callback)
    result.get()

Now when i try to run the task final() i always get an error like C.s() takes exactly 1 argument (2 given) because callback is applied with the return value of each task in the header. so how can i fix this so that the task C() runs fine????

score 2 · Accepted Answer · edited May 23 '17 at 10:32

You can use immutable tasks to make independant tasks and not forward the result.

def final():
    # C needs an argument, note the use of the shortcut `si`
    callback = C.si(something)
    header = [A.s(something), B.s(something)]
    result = chord(header)(callback)
    result.get()

As a completely side-note, the use of except Exception as e is preferred if you're running Python 2.6+ or required for Python 3.x (see here for more details).

Celery Chord Callback error

1 Answers1