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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 GetAttribute(self, identifier):
 """Retrieves a specific attribute. Args: identifier (str):
 identifier of the attribute within the volume. Returns: VolumeAttribute: volume attribute or None if not available. """

    if not self._is_parsed:
      self._Parse()
      self._is_parsed = True

    if identifier not in self._attributes:
      return None

    return self._attributes[identifier]