树型结构的Python问题

Question

我正在创建一个有限状态自动机（FSA），它基本上是一棵树。单词中的每个字母构成一个节点（State）。一串字母构成FSA的路径，路径可以重叠。但是，我似乎遇到了一个问题，即节点的next_states或子节点。

输入文件vocab.small如下所示，每行末尾都有\n：

A 
A A A 
A A B E R G 
A A C H E N

现在，我构建的FSA非常类似于树，但我创建了State start_state和State end_state并将其链接起来：

class State (object):
    __slots__ = "chars","next_states"
    def __init__(self,chars,next_states=[]):
        self.chars = chars
        self.next_states = next_states

class FSA (object):
    __slots__ = "vocab","start_state","end_state"
    def __init__(self,vocab):
        self.vocab = vocab
        self.start_state = State("0")
        self.end_state = State("1")
        self.end_state.next_states.append(self.start_state)
        self.start_state.next_states.append(self.end_state)

FSA中的其他方法是：

    def create_fsa(self):
        vocab_file = open(self.vocab, 'r')
        for word in vocab_file:
            self.add_word(word)
        vocab_file.close()

    def add_word(self,word,current_state=None):
        next_char = word[0:2]
        if current_state == None:
            current_state = self.start_state
        ## next_char found in current_state.next_states
        if next((x for x in current_state.next_states if x.chars == next_char), None):
            if len(word) > 3:
                print "next_char found"
                current_state = next((x for x in current_state.next_states if x.chars == next_char), None)
                self.add_word(word[2:],current_state)
            else:
                print "next_char found + add end"
                current_state = next((x for x in current_state.next_states if x.chars == next_char), None)
                current_state.next_states.append(self.end_state)
##                current_state.children[current_state.children.index(next_char)].append(self.end_state)
        ## next_char NOT found in current_state.next_states
        else:
            current_state.next_states.append(State(next_char))
            current_state = next((x for x in current_state.next_states if x.chars == next_char), None)
            if len(word) > 3:
                print "next_char added"
                self.add_word(word[2:],current_state)
            else:
                print "next_char added + end"
                current_state.next_states.append(self.end_state)
        print

问题似乎与current_state.next_states[]一致，因为它会继续产生对象，而不是在每个新State上创建新对象。问题是current_state.next_states.append(State(next_char))这样的行，我是如何创建新的State对象的？

感谢您的帮助。