第二十五章：collections与itertools

完成本章学习后，你将能够：

使用collections的高级数据结构
掌握itertools的迭代工具
编写高效的迭代代码

from collections import Counter, defaultdict, OrderedDict, deque, namedtuple
 
# Counter
words = ['apple', 'banana', 'apple', 'cherry', 'banana', 'apple']
count = Counter(words)
print(count.most_common(2))  # [('apple', 3), ('banana', 2)]
 
# defaultdict
groups = defaultdict(list)
for word in words:
    groups[len(word)].append(word)
 
# namedtuple
Point = namedtuple('Point', ['x', 'y'])
p = Point(3, 4)
print(p.x, p.y)
 
# OrderedDict（Python 3.7+ dict也是有序的）
od = OrderedDict()
od['a'] = 1
od['b'] = 2

import itertools
 
# 无限迭代器
count = itertools.count(10, 2)  # 10, 12, 14, ...
cycle = itertools.cycle('ABC')  # A, B, C, A, B, C, ...
repeat = itertools.repeat(10, 3)  # 10, 10, 10
 
# 组合
print(list(itertools.combinations('ABC', 2)))   # [('A','B'), ('A','C'), ('B','C')]
print(list(itertools.permutations('ABC', 2)))   # 所有排列
print(list(itertools.product('AB', '12')))      # 笛卡尔积
 
# 过滤和分组
data = [1, 2, 3, 4, 5, 6]
print(list(itertools.filterfalse(lambda x: x % 2, data)))
 
# 分组
for key, group in itertools.groupby('AAAABBBCCD'):
    print(key, list(group))
 
# 拉链
print(list(itertools.zip_longest([1,2], ['a','b','c'], fillvalue='-')))