django-rest-framework: How to prepopulate django-rest-framework serializer fields before validation?

Hello I am new to django and I am using django-rest-framework for making res APIs. Is there any hook or any Signals like django for executing a callback on pre validation state?

I need to prepopulate some data outside the admin area before the validation and then I am doing this stuff:

  if serializer.is_valid():
      serializer.save()

Here is my serializer class:

class HeroSerializer(serializers.HyperlinkedModelSerializer):
  class Meta:
    model = Hero
    fields = ['name', 'alias', 'gender']

Do I need another serializer for write operations? or should I create a custom Signal for it? Please suggest me an efficient way to do this.