Python 元组、集合和字典 实例

def main():    # Prompt the user to enter a file    filename = raw_input("Enter a filename: ").strip()    infile = open(filename, "r") # Open the file    wordCounts = {} # Create an empty dictionary to count words    for line in infile:        processLine(line.lower(), wordCounts)    pairs = list(wordCounts.items()) # Get pairs from the dictionary    items = [[x, y] for (y, x) in pairs] # Reverse pairs in the list    items.sort() # sort pairs in items    for i in range(len(items) - 1, len(items) - 11, -1):        print(items[i][1] + "\t" + str(items[i][0]))# Count each word in the linedef processLine(line, wordCounts):    line = replacePunctuations(line) # Replace punctuation with space    words = line.split()    for word in words:        if word in wordCounts:            wordCounts[word] += 1        else:            wordCounts[word] =1# Replace punctuation in the line with spacedef replacePunctuations(line):    for ch in line:        if ch in "~@#$%^&*()_-+=~<>?/,.;:!{}[]|'\"":            line = line.replace(ch, " ")    return linemain()

编写程序统计一个文本文件中单词的出现次数，并将出现次数最多的单词和它们的出现次数按降序显示。字典对象没有sort方法，那么如何对它进行排序呢？将字典的每一对放入一个列表中，然后对这个列表排序。如果使用sort方法对这个列表排序，程序将按每对的第一个元素进行排序，但是我们对出现次数(每对的第二个元素)进行排序。因此，需要利用倒置每一对来创建一个新的列表，然后利用sort方法。
—摘自《Python 程序语言设计》 李娜译