<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def processing_blocks(self):
 """Return the a JSON dict encoding the PBs known to SDP."""

        pb_list = ProcessingBlockList()
        # TODO(BMo) realtime, offline etc.
        return json.dumps(dict(active=pb_list.active,
                               completed=pb_list.completed,
                               aborted=pb_list.aborted))