[Python知识库]Python高效编程2

1：如何让字典保持有序

from collections import OrderedDict

d=OrderedDict()

2:用户历视记录功能

from collections import deque
q=deque([],5)
pickle.dump(1,open('history','w'))
q2=pickle.load(open('history'))

3：实现可迭代对象和迭代器对象

__iter__实现了此方法可迭代 或者 __getitem__

t=iter(l)
t.next()

实现迭代

from collections import Iterable,Iterator

#气温的迭代器
class WeatherIterator(Iterator):
    def __init__(self,cities):
        self.cities=cities
        self.index=0
    
    def getWeather(self,city)"
        ....

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


#可迭代对象
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'])
    print(x)

这一块可以参看：(67条消息) 理解python中的迭代器对象、可迭代对象、生成器对象_helloworld987456的博客-CSDN博客https://blog.csdn.net/helloworld987456/article/details/117407478

4：使用生成器函数实现可迭代对象

生成器：yield

calss  P:
    def __init__(self,start,end)
        self.start=start
        self.end=end
    
    def isP(self,k)
        if k<2:
            return False
        for i in xrange(2,k):
            if k%i==0:        
                   return False
        rerturn True
    
    def __iter__(self):
        
        for k in xrange(self.start,self.end+1)
            if self.isP(k):
                yield k



for x in P(1,100):
    print(x)

5:进行反向迭代以及如何实现反向迭代

l=[1,2,3,4,5]
l.reverse()
l[::-1]

上面两种消耗资源。
iter(l)  得到正向迭代器
reversed(l)  得到反向迭代器


for x in reversed(l):
    print(x)

iter需要__iter__
reversed需要__reversed__方法

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

6: 对迭代器做切片操作

from itertools improt islice

islice(f,100,300)

for line islice(f,100,300)

islice(f,500)
islice(f,None)