第一次进入luigi（和python！）还有一些问题。相关代码为：

from Database import Database
import luigi
class bbSanityCheck(luigi.Task):
  conn = luigi.Parameter()
  date = luigi.Parameter()
  def __init__(self, *args,**kwargs):
    super(bbSanityCheck, self).__init__(*args,**kwargs)
    self.has_run = False
  def run(self):
    print "Entering run of bb sanity check"
    # DB STUFF HERE THAT DOESN"T MATTER
   print "Are we in la-la land?"
  def complete(self):
    print "BB Sanity check being asked for completeness: " , self.has_run
    return self.has_run
class Pipeline(luigi.Task):
  date = luigi.DateParameter()
  def requires(self):
    db = Database('cbs')
    self.conn = db.connect()
    print "I'm about to yield!"
    return bbSanityCheck(conn = self.conn, date = self.date)
  def run(self):
    print "Hello World"
    self.conn.query("""SELECT * 
              FROM log_blackbook""")
    result = conn.store_result()
    print result.fetch_row()
  def complete(self):
    return False
if __name__=='__main__':
  luigi.run()

输出在这里（删除了相关的db返回，原因是）：

DEBUG: Checking if Pipeline(date=2013-03-03) is complete
I'm about to yield!
INFO: Scheduled Pipeline(date=2013-03-03)
I'm about to yield!
DEBUG: Checking if bbSanityCheck(conn=<_mysql.connection open to 'sas1.rad.wc.truecarcorp.com' at 223f050>, date=2013-03-03) is complete
BB Sanity check being asked for completeness:  False
INFO: Scheduled bbSanityCheck(conn=<_mysql.connection open to 'sas1.rad.wc.truecarcorp.com' at 223f050>, date=2013-03-03)
INFO: Done scheduling tasks
DEBUG: Asking scheduler for work...
DEBUG: Pending tasks: 2
INFO: [pid 5150] Running   bbSanityCheck(conn=<_mysql.connection open to 'sas1.rad.wc.truecarcorp.com' at 223f050>, date=2013-03-03)
Entering run of bb sanity check
Are we in la-la land?
INFO: [pid 5150] Done      bbSanityCheck(conn=<_mysql.connection open to 'sas1.rad.wc.truecarcorp.com' at 223f050>, date=2013-03-03)
DEBUG: Asking scheduler for work...
INFO: Done
INFO: There are no more tasks to run at this time
INFO: There are 1 pending tasks possibly being run by other workers
INFO: Worker was stopped. Shutting down Keep-Alive thread

所以问题是：
1.）为什么“我要屈服”会被打印两次？
2.）为什么“你好世界”从未被印刷？
3.）什么是“1个可能由其他工作人员运行的挂起任务”？
我更喜欢超净输出，因为它更容易维护。我希望我能解决这些问题。
我还注意到需要“yield”或“returnitem，item2，item3”。我读过关于屈服的书，并且理解它。我不明白的是哪种习惯在这里被认为是优越的，或者他们之间的细微差别是我对这门语言还不熟悉的。