<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_callback(self, callback=None, errorback=None):
 """ Adds a callback that will be called when the upload finishes successfully or when error is raised. """

        self._callback = callback
        self._errorback = errorback