Python 迭代器与生成器实例详解

In [1]: l = [1,2,3,4]

In [2]: l.__iter__
Out[2]: <method-wrapper '__iter__' of list object at 0x000000000426C7C8>

In [3]: t = iter(l)

In [4]: t.next()
Out[4]: 1

In [5]: t.next()
Out[5]: 2

In [6]: t.next()
Out[6]: 3

In [7]: t.next()
Out[7]: 4

In [8]: t.next()
---------------------------------------------------------------------------
StopIteration               Traceback (most recent call last)
<ipython-input-8-3660e2a3d509> in <module>()
----> 1 t.next()

StopIteration:

for x in l：
  print x
for 循环的工作流程，就是先有iter(l)得到一个t，然后不停的调用t.nex(),到最后捕获到StopIteration，就结束迭代

def getWeather(city):
  r = requests.get(u'http://wthrcdn.etouch.cn/weather_mini?city='+city)
  data = r.json()['data']['forecast'][0]
  return '%s:%s,%s' %(city, data['low'], data['high'])
print getWeather(u'北京')
返回值：
北京:低温 13℃,高温 28℃

# -*- coding:utf-8 -*-
import requests
from collections import Iterable, Iterator

class WeatherIterator(Iterator):
  def __init__(self, cities):
    self.cities = cities
    self.index = 0

  def getWeather(self,city):
    r = requests.get(u'http://wthrcdn.etouch.cn/weather_mini?city='+city)
    data = r.json()['data']['forecast'][0]
    return '%s:%s,%s' %(city, data['low'], data['high'])

  def next(self):
    if self.index == len(self.cities):
      raise StopIteration
    city = self.cities[self.index]
    self.index += 1
    return self.getWeather(city)

class WeatherIterable(Iterable):
  def __init__(self, cities):
    self.cities = cities

  def __iter__(self):
    return WeatherIterator(self.cities)

for x in WeatherIterable([u'北京',u'上海',u'广州',u'深圳']):
  print x.encode('utf-8')

输出：
北京:低温 13℃,高温 28℃
上海:低温 14℃,高温 22℃
广州:低温 17℃,高温 23℃
深圳:低温 18℃,高温 24℃

class PrimeNumbers:
  def __init__(self, start, end):
    self.start = start
    self.end = end

  def isPrimeNum(self, k):
    if k < 2:
      return False
    for i in xrange(2, k):
      if k % i == 0:
        return False

    return True

  def __iter__(self):
    for k in xrange(self.start, self.end + 1):
      if self.isPrimeNum(k):
        yield k

for x in PrimeNumbers(1, 10):
  print x

输出：
2
3
5
7

class FloatRange:
  def __init__(self, start, end, step=0.1):
    self.start = start
    self.end = end
    self.step = step

  def __iter__(self):
    t = self.start
    while round(t,14) <= round(self.end, 14):
      yield t
      t = t + self.step

  def __reversed__(self):
    t = self.end
    while round(t, 14) >= round(self.start, 14):
      yield t
      t = t - self.step

for x in reversed(FloatRange(3.0, 4.0, 0.2)):
  print x
输出：
4.0
3.8
3.6
3.4
3.2
3.0
for x in FloatRange(3.0, 4.0, 0.2):
    print x
输出：
3.0
3.2
3.4
3.6
3.8
4.0

f = open('/var/log/dmesg')

from itertools import islice

# 对文件内容100到300行之间进行切片，返回的是个生成器对象，默认歩径是1
islice(f, 100, 300)

# 前500行内容
islice(f, 500)

# 100行到末尾结束内容
islice(f, 100, None)


ps: 每次使用islice要重新申请对象，它会消耗原来的迭代对象

from random import randint

chinese = [randint(60,100) for _ in xrange(40)]
math = [randint(60,100) for _ in xrange(40)]
english = [randint(60,100) for _ in xrange(40)]

total = []
for c,m,e in zip(chinese, math,english):
  total.append(c+m+e)

print total

输出：

[204, 227, 238, 201, 227, 205, 251, 274, 210, 242, 220, 239, 237, 207, 230, 267, 263, 240, 247, 249, 255, 268, 209, 270, 259, 251, 245, 262, 234, 221, 236, 250, 251, 249, 242, 255, 232, 272, 237, 253]

from random import randint
from itertools import chain

class1 = [randint(60,100) for _ in xrange(40)]
class2 = [randint(60,100) for _ in xrange(42)]
class3 = [randint(60,100) for _ in xrange(39)]
class4 = [randint(60,100) for _ in xrange(43)]

count = 0
for s in chain(class1, class2, class3, class4):
  if s > 90:
    count = count + 1

print count

输出：
38