for loop + transaction.on_commit + celery = Unexpected Behaviour

[Emilio Jimenez Canalejo]

Hi, I've been looking for some time around the internet a problem I have, maybe I'm just dull from all the searching and the answer is pretty obvious, but I can't find it.

For example, I have code like this:

@app.task
def foo(idx):
    print("This is iteration {}".format(idx))

def save_model(self, request, obj, form, change):
    data = request.POST
    result = super(Model, self).save_model(request, obj, form, change)

    for pk in data['_models_changed']:
        transaction.on_commit(lambda: foo(pk))

To my astonishment, I thought I would get:

"This is iteration 1"
"This is iteration 2"
"This is iteration 3"
"This is iteration 4"

But I got:

"This is iteration 4"

"This is iteration 4"

"This is iteration 4"

"This is iteration 4"

I'm using python 2.7, Django 1.10.5 and Celery 4.0.2.

'models_changed' is a field in the post added by a custom clean method.

Anyone knows a way to do this?

Thanks for all!

[m712 - Developer]

I'm not too familiar with Celery, but I think that for loop continuously keeps changing transaction's commit event handler. Maybe you are looking for something like transaction.commit instead of on_commit?

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[Emilio Jimenez Canalejo]

The commit event handler is to commit a transaction, the on_commit event is for running a function when the commit is done. That is my intention, but maybe there is something I don't see.

I want to execute a celery task, foo.delay(...), when the save of Model is finished. That way if I modify Model inside the task, it will have the changes applied. If that is not the case, there are sometimes that I could get the Model instance before the save finished, and it would be the old instance.

[Jani Tiainen]

Hi,

I think your problem is to use POST['_models_changed']. Django uses specialized version of dictionary there which return _last_ value from list. You need to use special "get_list()" method to get whole list of objects.

Your code should start working if you do data['_models_changed'].get_list()  in a for-loop.

Also, why you modify request.POST instead of adding class attribute?

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[Martin Pätzold]

This is an older topic, but for everyone who found this discusion and had the same issue: The problem is most likely using "lambda" inside a for-loop. If you switch out "lambda" for "functools.partial" it should work as expected.