看起来dir（）的默认工作方式是：
dir(obj) == sorted(obj.__dict__.keys() + dir(obj.__class__))
（嗯，无论如何要删除重复项）
因此，一种方法是：
class Wizard(object):
def __init__(self):
self.mana = 0
def __dir__(self):
natdir = set(self.__dict__.keys() + dir(self.__class__))
if self.mana <= 0:
natdir.remove("domagic")
return list(natdir)
def addmana(self):
self.mana += 1
def domagic(self):
if self.mana <= 0:
raise NotEnoughMana()
print "Abracadabra!"
self.mana -= 1
在Py2.6中的行为是：
>>> wiz = Wizard()
>>> [x for x in dir(wiz) if not x.startswith("_")]
['addmana', 'mana']
>>> wiz.addmana()
>>> [x for x in dir(wiz) if not x.startswith("_")]
['addmana', 'domagic', 'mana']
>>> wiz.domagic()
Abracadabra!
>>> [x for x in dir(wiz) if not x.startswith("_")]
['addmana', 'mana']
>>> wiz.domagic()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 13, in domagic
__main__.NotEnoughMana
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