  
          ret = SortedDict()
          is_pk = True  # First field in the list is the pk
  
          for model_field in fields:
              if is_pk:
                  field = self.get_pk_field(model_field)
 
</s> remove         fields = self.default_fields(nested)
        for key, val in fields.items():
 </s> add         for key, val in self.default_fields.items(): </s> remove         plus the set of fields returned by default_fields().
 </s> add         plus the set of fields returned by get_default_fields(). </s> remove         fields = self.get_fields(nested=bool(self.opts.depth))
 </s> add         fields = self.get_fields() </s> remove         fields = self.get_fields(nested=bool(self.opts.depth))
 </s> add         fields = self.get_fields() </s> remove         # TODO: Modify this so that it's called on init, and drop
        #       serialize/obj/data arguments.
        #
        #       We *could* provide a hook for dynamic fields, but
        #       it'd be nice if the default was to generate fields statically
        #       at the point of __init__
 </s> add 
        nested = bool(self.opts.depth)
  
          ret = SortedDict()
        nested = bool(self.opts.depth)
          is_pk = True  # First field in the list is the pk
  
          for model_field in fields:
              if is_pk:
                  field = self.get_pk_field(model_field)
keep add keep keep keep keep keep
 Cache default fields per serializer instance for improved performance 
rest_framework/serializers.py