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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 task_log(self):
 """Get task log. :rtype: str :returns: The task log as a string. """

        if self.task_id is None:
            raise ValueError('task_id is None')
        return self.get_task_log(self.task_id, self.session, self.request_kwargs)