When exactly is the update method called within the serializer's operation?

The update method in serializers (for example, in Django REST Framework) is called when updating an existing object through the serializer.

When you call serializer.save() and pass an existing model instance to the serializer (via the instance parameter), the update method is invoked inside the serializer, responsible for updating the fields of that object based on the provided data.

Example:

serializer = MySerializer(instance=existing_obj, data=request.data)
if serializer.is_valid():
    serializer.save()  # calls update

If the serializer creates a new object (no instance), the create method is called.

Thus, update is called when saving a serializer with an existing object to update that object's data.