Python 2.7 值错误：解包需要超过0个值（艰难学习Python第41章）

Question

我看了所有关于解包错误和LPTHW第41题的回答，还花了一个小时检查我的代码，但我还是搞不懂我的错误在哪里。

以下是我的代码：

import random
from urllib import urlopen 
import sys

WORD_URL = "http://learncodethehardway.org/words.txt"
WORDS = []

PHRASES = {
"class ###(###):":
    "Make a class named ### that is-a ###.",
"class ###(object):\n\tdef __init__(self, ***)" :
    "class ### has-a __init that takes self and *** parameters.",
"class ###(object):\n\tdef ***(self, @@@)":
    "class ### has-a function named *** that takes self and @@@ parameters.",
"*** = ###()":
    "Set *** to an instance of class ###.",
"***.***(@@@)": 
    "From *** get the *** function, and call it with parameters self, @@@.",
"***.*** = '***'":
    "From *** get the *** attribute and set it to '***'."
}

# do they want to drill phrases first
PHRASE_FIRST = False
if len(sys.argv) == 2 and sys.argv[1] == "english":
    PHRASE_FIRST = True

# load up the words from the website
for word in urlopen(WORD_URL).readlines():
    WORDS.append(word.strip())

def convert(snippet, phrase):
    class_names = [w.capitalize() for w in
                random.sample(WORDS, snippet.count("###"))]
    other_names = random.sample(WORDS, snippet.count("***"))
    results = []
    param_names = []

    for i in range(0, snippet.count("@@@")): 
        param_count = random.randint(1,3)
        param_names.append(', '.join(random.sample(WORDS, param_count)))

    for sentence in snippet, phrase:
        result = sentence[:]

    # fake class names
    for word in class_names:
        result = result.replace("###", word, 1)

    # fake other names
    for word in other_names:
        result = result.replace("***", word, 1)

    # fake parameter lists
    for word in param_names:
        result = result.replace("@@@", word, 1)

        results.append(result)

    return results

# keep going until they hit CTRL-D
try:
    while True:
        snippets = PHRASES.keys()
        random.shuffle(snippets)

        for snippet in snippets:
            phrase = PHRASES[snippet]
            question, answer = convert(snippet, phrase)
            if PHRASE_FIRST:
                question, answer = answer, question

            print question

            raw_input(">")
            print "ANSWER: %s\n\n" % answer #ERROR : after print - weird

except EOFError:
    print "\nBye"

具体的错误信息是：

Value Error: Need more than 0 vales to unpack

我刚开始学习，所以如果能解释一下这个错误为什么会发生，以及怎么修复它，我会非常感激。

Answer 1

在你的 convert 函数里，你多次修改了 result，但是没有把它加到 results 列表里。你在最后一次循环中做对了，但可能需要在之前的循环中也加上：

for sentence in snippet, phrase:
    result = sentence[:]
    results.append(result)                  # added this

# fake class names
for word in class_names:
    result = result.replace("###", word, 1)
    results.append(result)                  # added this

# fake other names
for word in other_names:
    result = result.replace("***", word, 1)
    results.append(result)                  # added this

# fake parameter lists
for word in param_names:
    result = result.replace("@@@", word, 1)
    results.append(result)  # this one was already here

否则，results 就会一直是一个空列表，这样 convert 返回的就是 []。这样在这里就无法正确拆分：

question, answer = convert(snippet, phrase)

因为你需要的正好是两个元素：问题和答案。

Answer 2

你的代码有缩进问题，三个 for 循环必须放在第一个循环里面，也就是要嵌套在一起。

for sentence in snippet, phrase:
    result = sentence[:]

    # fake class names
    for word in class_names:
        result = result.replace("%%%", word, 1)


    # fake other names
    for word in other_names:
       result = result.replace("***", word, 1)


    # fake parameter lists
    for word in param_names:
       result = result.replace("@@@", word, 1)


    results.append(result)

Answer 3

你的代码基本上是对的，只是最后三个循环的缩进有问题，它们应该是嵌套在一起的。

把下面的代码缩进4个空格，你就可以正常运行了：

# fake class names
for word in class_names:
    result = result.replace("###", word, 1)

# fake other names
for word in other_names:
    result = result.replace("***", word, 1)

# fake parameter lists
for word in param_names:
    result = result.replace("@@@", word, 1)