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
