[BUG]add_filter(order=0) not work

[gundam78]

  # xmlstream.py:  class XMLStream
  1 def add_filter(self, mode, handler, order=None):
  2     if order:
  3         self.__filters[mode].insert(order, handler)
  4     else:
  5         self.__filters[mode].append(handler)

I want my filter to be called before all other filters. So, I use order=0, in order to make my filter insert into the head of the list. But because of line 2, if order (should be if order is None), it will just call line 5...

My tricky solution is give order a big negative number, like -1000.