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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def __json(self):

 """
 Using the exclude lists, convert fields to a string.
 """

        if self.exclude_list is None:
            self.exclude_list = []
        fields = {}
        for key, item in vars(self).items():
            if hasattr(self, '_sa_instance_state'):
                # load only deferred objects
                if len(orm.attributes.instance_state(self).unloaded) > 0:
                    mapper = inspect(self)
                    for column in mapper.attrs:
                        column.key
                        column.value
            if str(key).startswith('_') or key in self.exclude_list:
                continue
            fields[key] = item
        obj = Json.safe_object(fields)
        return str(obj)