[Twisted-Python] Why classes in twisted.protocols.basic use "static member" to store states?

[甜瓜]

Hi,
    I'm a newbie. After went through the source code in
twisted.protocols.basic, I found it's really strange that all classes
in twisted.protocols.basic use "static member" to store states.
    Take class StatefulStringProtocol as the example. Some Factory
will creates a group of StatefulStringProtocol instances which share
the static member "state". But why all instances MUST have the same
state? In my opinion, instance A may be in the "handshaking" state,
while instance B may be in the "data sending" state. The static member
can not represent this case.
    Therefore, I think protocols should use "member variable" rather
that "static member":

class StatefulStringProtocol:
	def __init__(self):
		self.state = 'init'
...

    Could you plz give me some clue? I am a little confused. Most
possibly, I misunderstanding the design about protocols-factory.

   Thank you.

ShenLei