迭代器和解析
可迭代对象包括实际序列和按照需求计算的虚拟序列
手动迭代：iter和next
python3.0提供了一个内置函数next，它会自动调用一个对象的__next__方法，给定一个
可迭代对象x。调用next(x)等同于x,next()
列表以及其他对象不是自身的迭代器，他们可以调用iter来启动迭代
l = [1, 2, 3]
print(iter(l) is l)
False
print(l.next())
AttributeError: 'list' object has no attribute 'next'
l = [1, 2, 3]
i = iter(l)
print(next(i))
1
print(i.next())
2
l = [1, 2, 3]
for x in l:
print(x**2, end=' ')
1 4 9
l = [1, 2, 3]
i = iter(l)
while True:
try:
j = next(i)
except StopIteration:
break
else:
print(j ** 2, end=' ')
1 4 9
python2 中迭代方法是x.next, python3是x.next,但next函数确是都可用的。
其他迭代器
d = {'a':1, 'b':2, 'c':3}
for k in d.keys():
print(k, d[k])
a 1
b 2
c 3
python3中，字典自己有个迭代器
i = iter(d)
print(next(i))
print(next(i))
print(next(i))
a
b
c
因此我们不需要调用keys方法来遍历字典，for将使用迭代协议获取键。
for k in d:
print(k, d[k])
a 1
b 2
c 3
shelves和os.popen的结果也是可迭代的
import os
p = os.popen('dir')
print(p.next())
驱动器 F 中的卷没有标签。
注意py2中popen对象支持p.next方法，py3中支持p.next()方法。
迭代协议也是需要将结果包装到一个list中才能看到值。可迭代对象一次返回一
个结果，而不是一个列表
a = range(5)
print(a)
range(0, 5)
i = iter(a)
print(next(i))
print(next(i))
0
1
print(list(range(5)))
[0, 1, 2, 3, 4]
列表解析
l = []
for i in [1, 2, 3]:
l.append(i+1)
print(l)
[2, 3, 4]
相比普通的循环，列表解析也可以达到相同的目的，并且更加简洁，运行速度也比
普通的循环快
print([i+1 for i in [1, 2, 3]])
[2, 3, 4]
文件中的列表解析
with open('some.py', 'w') as f:
f.write('import os\n')
f.write('a = 1\n')
f.write('b = 2\n')
with open('some.py') as f:
print(f.read())
import os
a = 1
b = 2
f = open('some.py')
line = f.readlines()
print(line)
['import os\n', 'a = 1\n', 'b = 2\n']
如果要去掉行末的换行符的话。
print([l.rstrip() for l in line])
['import os', 'a = 1', 'b = 2']
因为列表解析像for循环语句是一个迭代环境， 我们甚至不用提前打开文件。
l = [line.rstrip() for line in open('some.py')]
print(l)
['import os', 'a = 1', 'b = 2']
除此之外。列表解析的表现力也很强，我们可以在迭代时在一个文件上运行任何的
字符串操作
print([line.upper() for line in open('some.py')])
['IMPORT OS\n', 'A = 1\n', 'B = 2\n']
列表解析也可以嵌套for循环和if
lines = [line.rstrip() for line in open('some.py') if line[0] == 'a']
print(lines)
['a = 1']
print([x+y for x in 'abc' for y in 'lmn'])
['al', 'am', 'an', 'bl', 'bm', 'bn', 'cl', 'cm', 'cn']
列表解析，in成员测试，map内置函数，sorted和zip都使用了迭代协议
当应用文件时，会自动扫描
upper = [line.upper() for line in open('some.py')]
print(upper)
['IMPORT OS\n', 'A = 1\n', 'B = 2\n']
print(list(map(str.upper, open('some.py'))))
['IMPORT OS\n', 'A = 1\n', 'B = 2\n']
print('a = 1\n' in open('some.py'))
True
print(sorted(open('some.py')))
['a = 1\n', 'b = 2\n', 'import os\n']
print(list(zip(open('some.py'), open('some.py'))))
[('import os\n', 'import os\n'), ('a = 1\n', 'a = 1\n'), ('b = 2\n', 'b = 2\n')]
print(list(enumerate(open('some.py'))))
[(0, 'import os\n'), (1, 'a = 1\n'), (2, 'b = 2\n')]
print(list(filter(bool, open('some.py'))))
['import os\n', 'a = 1\n', 'b = 2\n']
import functools, operator
print(functools.reduce(operator.add, open('some.py')))
import os
a = 1
b = 2
print(sum([1, 2, 3, 4, 5]))
15
print(any(['spam', '', 'mi']))
True
print(all(['spam', '', 'mi']))
False
print(max([1, 2, 3, 4, 5]))
5
print(min([1, 2, 3, 4, 5]))
1
max 和min 也可以应用于文件
print(max(open('some.py')))
import os
print(min(open('some.py')))
a = 1
迭代协议也可直接作用于文件
print(list(open('some.py')))
['import os\n', 'a = 1\n', 'b = 2\n']
print(tuple(open('some.py')))
('import os\n', 'a = 1\n', 'b = 2\n')
print('&&'.join(open('some.py')))# (牛逼)
import os
&&a = 1
&&b = 2
a, *b = open('some.py')
print(a)
print(b)
import os
['a = 1\n', 'b = 2\n']
解析和集合
print(set(open('some.py')))
print({line for line in open('some.py')})
print({ix: line for ix, line in enumerate(open('some.py'))})
print({line for line in open('some.py') if line[0] == 'a'})
print({ix: line for ix, line in enumerate(open('some.py')) if line[0] == 'a'})
def f(a, b, c):
print(a, b, c, sep = '&')
f(1, 2, 3)
1-2-3
f(*open('some.py'))
import os
&a = 1
&b = 2
x , y = (1, 2), (3, 4)
print(list(zip(x, y)))
[(1, 3), (2, 4)]
a, b = zip(*zip(x, y))
print(a)
print(b)
(1, 2)
(3, 4)
range和其他迭代器不同，rang它不是自己的迭代器，并且他支持在结果上的多个迭代器
他们会记住自己的位置
r = range(3)
i1 = iter(r)
print(next(i1))
0
print(next(i1))
1
i2 = iter(r)
print(next(i2))
0
print(next(i1))
2
而zip，map,filter不支持相同结果上的活跃迭代器
z = zip((1, 2, 3), (10, 11, 12))
i1 = iter(z)
i2 = iter(z)
print(next(i1))
(1, 10)
print(next(i1))
(2, 11)
print(next(i2))
(3, 12)
m = map(abs,(-1, 0, 1))
i1 = iter(m)
i2 = iter(m)
print(next(i1)) # 1
print(next(i1)) # 0
print(next(i1)) # 1
print(next(i2)) # StopIteration
r = range(3)
r1, r2 = iter(r), iter(r)
print(next(r1)) # 0
print(next(r1)) # 1
print(next(r1)) # 2
print(next(r2)) # 0
通过iter返回一个新的对象来支持多个迭代器，单个迭代器返回自身。
py3中字典中由于keys不在返回一个列表，所以应该首先用list来展示他
或者使用sorted
d = {'a':1, 'b':2, 'c':3}
for k in sorted(d.keys()):
print(k, d[k], end=' ')
a 1 b 2 c 3
for k in sorted(d):
print(k, d[k], end = ' ')
