python cookbook 迭代器与生成器

代理迭代

a = [1, 2, 3]
for i in iter(a):
print(i)
for i in a.__iter__():
print(i)

这里的两个方法是一样的，调用iter()其实就是简单的调用了对象的__iter__()方法。

使用生成器创建新的迭代器

def frange(start, stop, increment):
x = start
while x < stop:
yield x
x += increment
for n in frange(0, 4, 0.5):
print(n)
print(list(frange(0, 4, 0.5)))

看下面这个

>>> def countdown (n):
...     print ('Starting to count from', n)
...     while n > 0:
...        yield n
...        n -= 1
...     print ('Done!')
...
>>> # Create the generator, notice no output appears
>>> c = countdown(3)
>>> c
<generator object countdown at 0x1006a0af0>>>> next(c)
Starting to count from 3
3>>> next(c)
2>>> next(c)
1>>> next(c)
Done!
Traceback (most recent call last):
File "<stdin>", line 1, in < module >
StopIteration

一个生成器函数主要特征是它只会回应在迭代中使用到的 next 操作。 一旦生成器函数返回退出,迭代终止。我们在迭代中通常使用的for语句会自动处理这些细节,所以你无需担心。

def frange(start, stop, increment):
x = start
while x < stop:
yield x
x += increment
for n in frange(0, 4, 0.5):
print(n)
print(list(frange(0, 4, 0.5)))

反向迭代

a = [1, 2, 3, 4]
for x in reversed(a):
print(x, end=' ')

主要想说的是，对象重写__reversed__()方法即可调用reversed()进行反向迭代

itertools的一些使用

def count(n):
while True:
yield n
n += 1
c = count(0)
import itertools
for x in itertools.islice(c, 10, 20):
print(x, end=" ")

with open('test.txt') as f:
for line in f:
print(line, end='')
print(format('开始部分的注释不显示', '*>30'))
with open('test.txt') as f:
for line in itertools.dropwhile(lambda line: line.startswith('#'), f):
print(line, end='')

items = ['a', 'b', 'c']
#items的所有可能组合，不包含相同元素
for p in itertools.permutations(items):
print(p, end=' ')
print()
#指定长度的所有排序
for p in itertools.permutations(items, 2):
print(p, end=' ')

如果遇到一些复杂的迭代器的使用，可以先看看itertools里有没有可用的方法。

同时迭代多个序列

a = [1, 2, 3]
b = ['w', 'x', 'y', 'z']
for i in zip(a,b):
print(i)
输出：
(1, 'w')
(2, 'x')
(3, 'y')
for i in itertools.zip_longest(a, b):
print(i)
输出：
(1, 'w')
(2, 'x')
(3, 'y')
(None, 'z')

不同序列上的迭代

import itertools
a = [1, 2, 3, 4]
b = ['x', 'y', 'z']
for x in itertools.chain(a, b):
print(x)

这样比 a + b 在进行迭代要好很多

展开嵌套的序列

from collections import Iterable
def flattern(items, ignore_types=(str, bytes)):
for x in items:
if isinstance(x, Iterable) and not isinstance(x, ignore_types):
yield from flattern(x)
else:
yield x
items = [1, 2, [3, 4, [5, 'hello world'], 7], 8]
for x in flattern(items):
print(x, end=' ')

def count(n): while True: yield n n += 1 c = count(0) import itertools for x in itertools.islice(c, 10, 20): print(x, end=" ") with open('test.txt') as f: for line in f: print(line, end='') print(format('开始部分的注释不显示', '*>30')) with open('test.txt') as f: for line in itertools.dropwhile(lambda line: line.startswith('#'), f): print(line, end='') items = ['a', 'b', 'c'] #items的所有可能组合，不包含相同元素 for p in itertools.permutations(items): print(p, end=' ') print() #指定长度的所有排序 for p in itertools.permutations(items, 2): print(p, end=' ')