'Called when the object is about to be deleted.
NOTE: If the implementation of this method has to list children or delete
other dependent objects, make sure to use DeletionPool\'s API instead of a
generic aff4.FACTORY one. DeletionPool is optimized for deleting large
amounts of objects - it minimizes number of expensive data store calls,
trying to group as many of them as possible into a single batch, and caches
results of these calls.
Args:
deletion_pool: DeletionPool object used for this deletion operation.
Raises:
ValueError: if deletion pool is None.'

def OnDelete(self, deletion_pool=None):

    if (deletion_pool is None):
        raise ValueError("deletion_pool    can't    be    None")
