How to send a django signal from other signal

Question

TL;DR:
I need a way to trigger a custom signal after the post_save signal, automatically, is there any way of doing it?

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I'm currently developing a library for django that requires a lot of comes and goes with the post_save signal in django and I was wondering if it's possible to trigger another signal after the post_save so I could implement my own and not intervene the post_save in case a project that uses the library needs to do it.

So far I know that signals are expected to receive a class as a sender argument, and if I trigger manually the signal from the post_save I would be doing nothing (I'd still be intervening it). Is there any workaround for this? Am I missing something in the docs?

Answer 1

Although this may be possible by calling a signal manually from inside another signal like this:

post_save.send(MyModel, instance=a_mymodel_instance)

There are easier ways to do something like that:

Let us assume that you follow the file organization that I use in this answer: Django Create and Save Many instances of model when another object are created

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Suggestion 1:

• Let us assume that your first post_save does something to MyModel1 and another post_save signal does something to MyModel2 after some processing on the instance that triggered the first signal.
• post_save is always send at the end of a save() method.

• Organize your signals as follows:

  @receiver(post_save, sender=MyModel1)
  def mymodel1_signal (sender, instance, **kwargs):
      Do stuff on MyModel1 instance...
      Do also stuff on MyModel2 and then call MyModel2.save()
  
  
  @receiver(post_save, sender=MyModel2)
  def mymodel2_signal (sender, instance, **kwargs):
      Do stuff on MyModel2 instance...
  

  This way the mymodel2_signal signal will get triggered after the call to MyModel2.save() from mymodel1_signal.

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Suggestion 2:

• Use a mix of signals to achieve the correct "timing".
• Let us assume that you want to start a process on MyModel2 before a MyModel1 get's saved

• Use a pre_save and a post_save signal:

  @receiver(pre_save, sender=MyModel1)
  def mymodel1_signal (sender, instance, **kwargs):
      Do stuff on MyModel1 instance...
      Do also stuff on MyModel2 and then call MyModel2.save()
  
  
  @receiver(post_save, sender=MyModel2)
  def mymodel2_signal (sender, instance, **kwargs):
      Do stuff on MyModel2 instance...
  

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Suggestion 3:

Use a MyModel2 method directly inside a MyModel1 post_save signal.