Exercise 49 创建句子

peek函数的功能：查看元组列表中的下一个成员，做匹配以后再对它做下一步动作

def peek(word_list):
    if word_list:
        word = word_list[0]
        return word[0]
    else:
        return None

match函数：

def match(word_list, expecting):
    if word_list:
        word = word_list[0]
    
        if word[0] = expecting:
            return word
        else:
            return None
    else:
        return None

skip函数：

def skip(word_list, word_type):
    while peek(word_list) == word_type:
        match(word_list, word_type)

有了工具，我们现在可以从元组列表来构建句子(Sentence)对象了。我们的处理流程如下：

使用 peek 识别下一个单词。

如果这个单词和我们的语法匹配，我们就调用一个函数来处理这部分语法。假设函数的名字叫parse_subject 好了。

如果语法不匹配，我们就 raise 一个错误，接下来你会学到这方面的内容。

全部分析完以后，我们应该能得到一个 Sentence 对象，然后可以将其应用在我们的游戏中。

作者写的程序如下，我们需要编写测试这部分的程序。

class ParserError(Exception):
    pass 

    
class Sentence(object):
    
    def __init__(self, object, verb, object):
        #remember we take ('noun','princess') tuples and convert them
        self.subject = subject[1]
        self.verb = verb[1]
        self.object = object[1]

        
def peek(word_list):
    if word_list:
        word = word_list[0]
        return word[0]
    else:
        return None

            
def match(word_list, expecting):
    if word_list:
        word = word_list[0]
    
        if word[0] = expecting:
            return word
        else:
            return None
    else:
        return None

        
def skip(word_list, word_type):
    while peek(word_list) == word_type:
        match(word_list, word_type)

        
def parse_verb(word_list):
    skip(word_list, 'stop')
    
    if peek(word_list) == 'verb':
        return match(word_list, 'verb')
    else:
        raise ParserError("Expected a verb next.")
        

def parse_object(word_list):
    skip(word_list, 'noun')
    next = peek(word_list)

    if next == 'noun':
        return match(word_list, 'noun')
    if next == 'direction':
        return match(word_list, 'direction')
    else:
        raise ParserError("Expected a noun or a direction next.")
        
        
def parse_subject(word_list, subj):
    verb = parse_verb(word_list)
    obj = parse_object(word_list)
    
    return Sentence(subj, verb, obj)
    
    
def parse_sentence(word_list):
    skip(word_list, 'stop')
    
    start = peek(word_list)
    
    if start == 'noun':
        subj = match(word_list, 'noun')
        return parse_subject(word_list, subj)
    elif start == 'verb':
        #assume the subject is the player then
        return parse_subject(word_list, ('noun', 'player'))
    else:
        raise ParseError("Must start with subject, object, or verb not : %s" %start)

没搞懂一开始三个函数的功能。。。编不出来：（先跳过吧