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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 setdefault(self, key, default=None):
 """Set `default` if the key is not in the cache otherwise leave unchanged. Return the value of this key. """

        self._wlock.acquire()
        try:
            try:
                return self[key]
            except KeyError:
                self[key] = default
                return default
        finally:
            self._wlock.release()