python核心编程:学习笔记3--迭代器,列表解析

1. 迭代器和iter()函数

    迭代器是一组数据结构,迭代器有一个next()方法的对象,而不是通过索引来计数.通过next()将条目全部取出后,会引发一个StopIteration异常,这并不是表示错误发生,只是告诉外部调用者,迭代完成.

1. 使用迭代器

1. 序列

>>> myTuple = (123, "xyz", 45.67)
>>> i = iter(myTuple)
>>> i.__next__()
123
>>> i.__next__()
'xyz'
>>> i.__next__()
45.67
>>> i.__next__()
Traceback (most recent call last):
  File "<pyshell#6>", line 1, in <module>
    i.__next__()
StopIteration

    所以,对于for循环:

for i in seq:
    do_something_to(i)

实际上等价于:

fetch = iter(seq)
while True:
    try:
        i = fetch.__next__()
    except StopIteration:
        break
    do_something_to(i)

2. 字典

    字典的迭代器会遍历它的键(key):

>>> dict1 = {"x" : 1, "y" : 2}
>>> for key in dict1:
	print("key:%s, value:%s" % (key, dict1[key]))

	
key:y, value:2
key:x, value:1

3. 文件

    文件对象生成的迭代器会自动调用readline()方法.这样,循环就可以访问文本文件的所有行:

>>> with open("test.txt") as fobj:
	for eachLine in fobj:
		print(eachLine)

		
hello world
i love this world
and i love python too

2. 列表解析

    列表解析的语法如下:

[expr for iter_var in iterable]

    这个语句的核心是for循环,它迭代iterable对象的所有条目.前边的expr应用于序列的每个成员,最后的结果是该表达式产生的列表:

>>> [x ** 2 for x in range(6)]
[0, 1, 4, 9, 16, 25]

    而我们甚至可以加上if判断:

>>> [x ** 2 for x in range(6) if x > 3]
[16, 25]

    而列表中还可以存在双层for循环:

>>> [(x + 1, y + 1) for x in range(3) for y in range(5)]
[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5)]

    等价于:

>>> num = []
>>> for x in range(3):
	for y in range(5):
		num.append((x + 1, y + 1))

		
>>> num
[(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5)]