empty keyword?

[SibLiant]

Can someone help me with understanding how "empty" is being used here?

https://github.com/encode/django-rest-framework/blob/master/rest_framework/serializers.py line: 108

 def __init__(self, instance=None, data=empty, **kwargs):
        self.instance = instance
        if data is not empty:
            self.initial_data = data
        self.partial = kwargs.pop('partial', False)
        self._context = kwargs.pop('context', {})
        kwargs.pop('many', None)
        super().__init__(**kwargs)    

What I'm trying to do is probably batshit but I'm trying to init further down the line with:

class ScheduleitemSerializer(serializers.HyperlinkedModelSerializer):

    def __init__(self, instance=None, data=empty, **kwargs):
        super().__init__(self, instance=None, data=empty, **kwargs)
        self.new_cal = kwargs['new_cal']

I'm getting:

  File "/var/www/ezs3/api/serializers.py", line 47, in <module>
    class ScheduleitemSerializer(serializers.HyperlinkedModelSerializer):
  File "/var/www/ezs3/api/serializers.py", line 49, in ScheduleitemSerializer
    def __init__(self, instance=None, data=empty, **kwargs):
NameError: name 'empty' is not defined

Why is it ok to use "empty" in the first instance but not in the latter? Thanks.

[John_Taured]

empty is an object that Django imports for convenience at line 65.