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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 get(self, key):
 """ Retrieves previously stored key from the storage :return value, stored in the storage """

    if key not in self._keystore:
      return None

    rec = self._keystore[key]
    """:type rec InMemoryItemValue"""

    if rec.is_expired:
      self.delete(key)
      return None

    return rec.value