Q:

Where is the best place to send email in django-rest-framework? 

I want to send emails in django rest framework, for example when someone creates an account,or comments something and etc.  I found this in django documents.  But I don't know where to use it.  maybe in Viewset but in which method. 
so what is the best way and where is the best place to send emails in Django Rest Framework? 

A:

I would like to give you couple of suggestions(Of course, with reasons):

That is synchronous.  Maybe that would be not desired/efficient.  I would suggest you to use something like this package (django-anymail) , this will decouple your email sender.  Assume now you're using AWS SES, tomorrow you want to switch to sendgrid, then you'll only need to change setting variables with this. 
Assuming User, Comment each of these are separate models.  I would suggest you to override the save() method of these models and send email from there.  I am not suggesting signals because I feel (fully personal view) that signals are to be used when access to the source is not possible, like some event in 3rd party library, etc.