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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 dispatch(self, value, callback=None):
 """Dispatch an item to the workqueue and optionally wait. This is the only way to add work to the background work queue. Unless you also pass a callback object, this method will synchronously wait for the work to finish and return the result. If the work raises an exception, the exception will be reraised in this method. If you pass an optional callback(exc_info, return_value), this method will not block and instead your callback will be called when the work finishes. If an exception was raised during processing, exc_info will be set with the contents of sys.exc_info(). Otherwise, exc_info will be None and whatever the work_queue handler returned will be passed as the return_value parameter to the supplied callback. Args: value (object):
 Arbitrary object that will be passed to the work queue handler. callback (callable):
 Optional callback to receive the result of the work queue when it finishes. If not passed, this method will be synchronous and return the result from the dispatch() method itself Returns: object: The result of the work_queue handler function or None. If callback is not None, then this method will return immediately with a None return value. Otherwise it will block until the work item is finished (including any work items ahead in the queue) and return whatever the work item handler returned. """


        done = None

        if callback is None:
            done = threading.Event()
            shared_data = [None, None]

            def _callback(exc_info, return_value):
                shared_data[0] = exc_info
                shared_data[1] = return_value

                done.set()

            callback = _callback

        workitem = WorkItem(value, callback)
        self._work_queue.put(workitem)
        if done is None:
            return None

        done.wait()
        exc_info, return_value = shared_data
        if exc_info is not None:
            self.future_raise(*exc_info)

        return return_value