一、反向迭代：reversed()
>>> a [1, 2, 3, 4] >>> for x in reversed(a): ... print(x, end=' ') ... 4 3 2 1
#反向迭代只有在待处理的对象具有确定的大小或者对象实现了__reversed()__特殊方法时才能奏效，否则必须先将对象转化为列表（可能消耗大量内存）
>>> with open('/etc/passwd', 'rt') as file: ... for x in reversed(file): #要用list(file) ... print(x) ... Traceback (most recent call last): File "<stdin>", line 2, in <module> TypeError: argument to reversed() must be a sequence
二、迭代器切片:itertools.islice
import itertools >>> def count(n): ... while True: ... yield n ... n += 1
... >>> for x in itertools.islice(count(0), 2, 10): #相当于列表切片取[2:10] ... print(x, end=' ') ... 2 3 4 5 6 7 8 9
>>>for x in itertools.islice(count(0), 5, None): #相当于列表切片取[5:] ... print(x, end=' ') ... if x >10: ... break ... 5 6 7 8 9 10 >>> for x in itertools.islice(count(0), 5): #相当于列表切片取[:5] ... print(x, end=' ') ... 0 1 2 3 4
#迭代器和生成器无法进行普通的切片操作（其长度不确定且没有实现索引），islice会产生一个新迭代器，消耗掉初始迭代序列中的所有数据
三、以索引－值对的形式迭代序列：enumerate
>>> a [1, 2, 3, 4] >>> for index, value in enumerate(a, 1): #从1开始计数，语法：enumerate(iterable[, start]) ... print(index, value) ... 1 1 2 2 3 3 4 4
#enumerate的返回值是一个迭代器，元素是元组
四、同时迭代多个序列
并行成对迭代：zip()、itertools.zip_longest()
>>> a [1, 2, 3, 4] >>> b [1, 2, 3, 4, 8, 9] >>> for x, y in zip(a, b): ... print(x, y) ... 1 1 2 2 3 3 4 4 >>> for x, y in itertools.zip_longest(a, b): ... print(x, y) ... 1 1 2 2 3 3 4 4 None 8 None 9 >>> for x, y in itertools.zip_longest(a, b, fillvalue=0): ... print(x, y) ... 1 1 2 2 3 3 4 4 0 8 0 9
串行顺序迭代：itertools.chain()
>>> for x in itertools.chain(a, b): ... print(x) ... 1 2 3 4 1 2 3 4 8 9
串行交叉迭代：heapq.merge()
>>> import heapq >>> for x in heapq.merge(a, b): ... print(x) ... 1 1 2 2 3 3 4 4 8 9