Getting information from Django custom signal receiver

Question

This is my second question today actually but what I want to know...Is it possible to retrieve information from a signal handler.

I have a list of items, call it list and each item is in AppA. Each item has a couple of characteristics which are saved in a different app, AppB.

So, I figured that I could maybe create a dictionary, dict and iterate over the items in list. In each iteration, I was hoping to send a signal to AppB and retrieve the information, i.e. have something like

def blob(request):
    dict = {}
    for item in list:
        signal.send(sender=None, id=item.id)
        dict[item] = (char1, char2)
    ...some html request

My signal handler looks something like this:

def handler(sender, id, **kwargs):
    model2 = Model2.objects.get(id=id)
    a = model2.char1
    b = model2.char2
    return (a, b)

Then I was hoping to be able to just produce a list of the items and their characteristics in the webpage...THe problem is that obviously the signal sender has to send the signal, and get the information back which I want....is that even possible :S?

Currently, I get an error saying "global name 'char1' is not defined....and I have imported the handlers and signals into the view.py where blob resides....so is my problem just unsolvable? / Should it clearly be solved in another way? Or have I almost certainly made a stupid error with importing stuff?

You want to send some data via the signal and then get the data back from the signal when it's done? — Henrik Andersson, Jul 09 '13 at 18:29
Yes, and hopefully use said data in the html request at the end of blob. — user2564502, Jul 09 '13 at 22:30
You want to start looking into Message Queues for this. Celery being THE client for Django. — Henrik Andersson, Jul 10 '13 at 09:07
Ok, thanks. I mean that looks reasonably tricky....felt like it should be really simple? How about if I pass a and b, with default values in the signal sender. Then the handler updates them?... — user2564502, Jul 10 '13 at 09:50
I mean the current result is that I get a Name Error, "global name 'blah' is not defined" when I try to use 'blah' in my blob function — user2564502, Jul 10 '13 at 10:28
This is an attempt to abuse the signal system. If you want a return value, use a method or function call. The intent of a signal is that it is sent without concern for how it gets used such that one or more 'observers' can make use of the signal to do their own thing. — Paul Whipp, Nov 17 '16 at 03:29

score 0 · Answer 1 · answered Jul 18 '13 at 12:39

This wasn't actually so tricky. Thought I should perhaps post how it was solved. In my views, I actually wrote

    response_list=signal.send(sender=None, list=list_of_items)

I then iterated over my response_list, adding the items to a fresh list like so:

    snippets = []
    for response in response_list:
    logger.error(response)
    snippets.append(response[1])

And could then call the responses in snippets like a dictionary in my template. When I asked the question, I didn't appreciate that I could equate something with the signal sending...

Getting information from Django custom signal receiver

1 Answers1

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