def __call__(self, attrs):
        self.enforce_required_fields(attrs)
        queryset = self.queryset
        queryset = self.filter_queryset(attrs, queryset)
        queryset = self.exclude_current_instance(attrs, queryset)

        # Ignore validation if any field is None
        checked_values = [
            value for field, value in attrs.items() if field in self.fields
        ]
        if checked_values not in None and queryset.exists():
            field_names = ', '.join(self.fields)
            raise ValidationError(self.message.format(field_names=field_names))
dataset/ETHPy150Open tomchristie/django-rest-framework/rest_framework/validators.py/UniqueTogetherValidator.__call__/swapped operands of `not in`