Python3_CookBook_之迭代器与生成器

#iter and generator
with open("somefile.txt") as f :
     try :
             while True :
                     line = next(f)
                     print (line)
     except StopIteration:
             print ("all is ok !")
############################
with open("somefile.txt") as f :
    while True :
        line = next(f,"GuoqiLiu")###default 
        print (line,end="")
        if line is "GuoqiLiu" :
            break
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class Node :
    def __init__(self,value):
        self._value = value
        self._children = []
    def __repr__(self) :
        return 'Node({!r})'.format(self._value)
    def add_child(self,node) :
        self._children.append(node)
    def __iter__(self) :
        return iter(self._children)
    def depth_first(self) :
        print ("t++",self)
        yield self
        for c in self :
            yield from c.depth_first()####说明：yield from iterable本质上等于for item in iterable: yield item的缩写版
#Example
if __name__ == "__main__" :
    root = Node(0)
    child1 = Node(1)
    child2 = Node(2)
    root.add_child(child1)
    root.add_child(child2)
    child1.add_child(Node(3))
    child1.add_child(Node(4))
    child2.add_child(Node(5))
    #child1.depth_first()
    for ch in root.depth_first():
        print (ch)
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a = [1,2,3,4]
r = reversed(a)
#list(r)
#__reversed__() 实现自定义类的反向迭代
class Countdown :
    def __init__(self,start):
        self.start = start
#Forward iterator 
    def __iter__(self) :
        n = self.start
        while n>0:
            yield n 
            n -= 1
#Reverse iterator 
    def __reversed__(self) :
        n = 1
        while n<= self.start :
            yield n
            n += 1
a = Countdown(5)
for i in iter(a) :
    print ("iter:",i)
####$$$$$$$$$$$$$$$$$$$$$$###################
for i in reversed(a) :
    print ("reversed:",i)
###切片
import itertools 
for x in itertools.islice(iter(a),2,4) :######### itertools.islice(iter(a),2,None)##余下所有的元素
    print ("slice",x)
##output:
iter: 5
iter: 4
iter: 3
iter: 2
iter: 1
reversed: 1
reversed: 2
reversed: 3
reversed: 4
reversed: 5
slice 3
slice 2
#######################################

#跳过所有的注释行
"""
from itertools import dropwhile
with open ('/etc/passwd') as f :
    for line in dropwhile(lambda line: line.startswith('#'),f):
        print (line)
"""
#zip
data = [(1,2),(3,4)]
for n,(x,y) in enumerate(data) :
    print (n,"++>>>",(x,y))
a = [1,2,3]
b = ['x','y','z','t']
for i in zip(a,b) :
    print(i)
#zip则是以最短的那个为准，而itertools.zip_longest()则是以最长的为准##Note:
from itertools import zip_longest
for i in zip_longest(a,b) :
    print (i)
for i in zip_longest(a,b,fillvalue=0) :
    print (i)
#扁平化处理嵌套型序列
from collections import Iterable

def flatten(items, ignore_types=(str, bytes)):
    for x in items:
        if isinstance(x, Iterable) and not isinstance(x, ignore_types):
            for i in flatten(x) :
                yield i
            #yield from flatten(x)##如果想编写生成器来把其他生成器当子例程调用yield from 是一个不错的选择。如果不这么用，就需要编写有额外for循环的代码
        else:
            yield x

items = [1, 2, [3, 4, [5, 6], 7], 8]

# Produces 1 2 3 4 5 6 7 8
for x in flatten(items):
    print(x)

items = ['Dave', 'Paula', ['Thomas', 'Lewis']]
for x in flatten(items):
    print(x)