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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_value(self, entity):
 """Compute and store a default value if necessary."""

    value = super(_ClassKeyProperty, self)._get_value(entity)
    if not value:
      value = entity._class_key()
      self._store_value(entity, value)
    return value