      def perform_validation(self, attrs):
          """
          Run `validate_<fieldname>()` and `validate()` methods on the serializer
          """
          # TODO: refactor this so we're not determining the fields again
          fields = self.get_fields()
  
          for field_name, field in fields.items():
              try:
                  validate_method = getattr(self, 'validate_%s' % field_name, None)
                  if validate_method:
                      source = field.source or field_name
                      attrs = validate_method(attrs, source)
 
</s> remove         for key, val in self.default_fields.items():
 </s> add         for key, val in self.get_default_fields().items(): </s> remove         fields = self.get_fields()
        for field_name, field in fields.items():
 </s> add         for field_name, field in self.serialize_fields.items(): </s> remove         for field_name, field in fields.items():
 </s> add         for field_name, field in self.serialize_fields.items(): </s> remove         fields = self.get_fields()
 </s> add  </s> remove         self.default_fields = self.get_default_fields()
 </s> add         self.serialize_fields = self.get_fields()
        for field_name, field in self.serialize_fields.items():
      def perform_validation(self, attrs):
          """
          Run `validate_<fieldname>()` and `validate()` methods on the serializer
          """
        for field_name, field in self.serialize_fields.items():
        for field_name, field in self.serialize_fields.items():
        for field_name, field in self.serialize_fields.items():
        for field_name, field in self.serialize_fields.items():
              try:
                  validate_method = getattr(self, 'validate_%s' % field_name, None)
                  if validate_method:
                      source = field.source or field_name
                      attrs = validate_method(attrs, source)
keep keep keep keep replace replace replace replace keep keep keep keep keep
 Cache all fields on serializer init, not just default fields. 
rest_framework/serializers.py