Docserver: achieve online vs offline (cron vs instance) behaviour at the object

chrome/common/extensions/docs/server2/future.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import sys
 
-_no_value = object()
+_no_value = (None,)
 
 class Future(object):
   '''Stores a value, error, or delegate to be used later.
   '''
   def __init__(self, value=_no_value, delegate=None):
     self._value = value
     self._delegate = delegate
     self._exc_info = None
     if (self._value is _no_value and self._delegate is None):
       raise ValueError('Must have either a value or delegate.')
 
   def Get(self):
     '''Gets the stored value, error, or delegate contents.
     '''
     if self._value is not _no_value:

[方觉(Fang Jue), 2013/05/03 06:03:31]

If you're going to use _no_value = (None,), be sure to compare with ==.
(None,) is not (None,)
(None,) == (None,)

[not at google - send to devlin, 2013/05/03 16:04:25]

"is not" is correct because the refer to the same value, no? python doesn't copy
by value, it copies by reference?

[cduvall, 2013/05/07 23:47:03]

'is not' is correct.

       return self._value
     if self._exc_info is not None:
       self._Raise()
     try:
       self._value = self._delegate.Get()
       return self._value
     except:
       self._exc_info = sys.exc_info()
       self._Raise()
 
   def _Raise(self):
     exc_info = self._exc_info
     raise exc_info[0], exc_info[1], exc_info[2]