def manual_iter():
  with open('/etc/passwd') as f:
    try:
      while True:
        line=next(f)
        if line is None:
          break
        print(line,end='')
      except StopIteration:
        pass

#test case
items=[1,2,3]
it=iter(items)
next(it)
next(it)
next(it)

class Node:
  def __init__(self,value):
    self._value=value
    self._children=[]
  def __repr__(self):
    return 'Node({!r})'.fromat(self._value)
  def add_child(self,node):
    self._children.append(node)
  def __iter__(self):
    #将迭代请求传递给内部的_children属性
    return iter(self._children)

#test case
if __name='__main__':
  root=Node(0)
  child1=Node(1)
  child2=Nide(2)
  root.add_child(child1)
  root.add_child(child2)
  for ch in root:
    print(ch)

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


f=open('somefile')
for line in reversed(list(f)):
  print(line,end='')
#test case
for rr in reversed(Countdown(30)):
  print(rr)

for rr in Countdown(30):
  print(rr)

class Countdown:
  def __init__(self,start):
    self.start=start
  #常规迭代
  def __iter__(self):
    n=self.start
    while n > 0:
      yield n
      n -= 1
  #反向迭代
  def __reversed__(self):
    n=1
    while n <= self.start:
      yield n
      n +=1

with open('/etc/passwd') as f:
  for line in f:
    print(line,end='')

from itertools import dropwhile
with open('/etc/passwd') as f:
  for line in dropwhile(lambda line:line.startwith('#'),f):
    print(line,end='')

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):
      yield from flatten(x)
    else:
      yield x

#test case
items=[1,2,[3,4,[5,6],7],8]
for x in flatten(items):
  print(x)