<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _AddAttribute(self, attribute):
 """Adds an attribute. Args: attribute (VolumeAttribute):
 a volume attribute. Raises: KeyError: if volume attribute is already set for the corresponding volume attribute identifier. """

    if attribute.identifier in self._attributes:
      raise KeyError((
          'Volume attribute object already set for volume attribute '
          'identifier: {0:s}.').format(attribute.identifier))

    self._attributes[attribute.identifier] = attribute