理解列表理解

Question

我对编程有点陌生。我创建了一个在其初始化程序中使用列表理解的类。如下：

class Collection_of_word_counts():
 '''this class has one instance variable, called counts which stores a 
dictionary where the keys are words and the values are their occurences'''

def __init__(self:'Collection_of_words', file_name: str) -> None:
    '''  this initializer will read in the words from the file,
    and store them in self.counts'''
    l_words = open(file_name).read().split()
    s_words = set(l_words)

    self.counts = dict([ [word, l_words.count(word)] 
                        for word 
                        in s_words])

我认为对于新手来说我做得还不错。有用！但我不完全理解这将如何在 for 循环中表示。我的猜测大错特错：

self.counts =[] 
for word in s_words:
    self.counts = [word, l_words.count(word)]
dict(self.counts)

Answer 1

这就是你对 for 循环的理解：

dictlist = []
for word in s_words:
    dictlist.append([word, l_words.count(word)])
self.counts = dict(dictlist)

Answer 2

你的猜测完全没有错；您只是忘记附加并分配回self.counts：

counts = [] 
for word in s_words:
    counts.append([word, l_words.count(word)])
self.counts = dict(counts)

本质上，这就是列表推导所做的；从循环表达式构建一个列表。

您也可以将其转换为字典理解：

self.counts = {word: l_words.count(word) for word in s_words}

或者更好的是，使用collections.Counter() object 并省去所有工作：

from collections import Counter

def __init__(self:'Collection_of_words', file_name: str) -> None:
    '''  this initializer will read in the words from the file,
    and store them in self.counts'''
    with open(file_name) as infile:
        self.counts = Counter(infile.read().split())

Counter() 对象可以更有效地计算您的单词，并为您提供其他有用的功能，例如列出前 N 个计数和合并计数的能力。

Answer 3

您实际上是在创建一个字典，其中字典的键是单词，与该键对应的值是单词出现的次数。

self.counts ={}

for word in s_words:
   self.counts[word] = l_words.count(word)