Python3 CookBook | 迭代器与生成器

In [1]: a = [1, 2, 3, 4]
In [2]: for x in reversed(a):
   ...:     print(x)
   ...:
4
3
2
1
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# Print a file backwards
f = open('somefile')
for line in reversed(list(f)):
    print(line, end='')
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#!/usr/bin/env python
#-*- encoding: utf-8 -*-
def reverse_iterate(): 
  for rr in reversed(Countdown(30)):
    print(rr)
  for rr in Countdown(30):
    print(rr)
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 当使用reversed函数翻转对象时调用
  def __reversed__(self):
    n = 1
    while n <= self.start:
      yield n
      n += 1
if __name__ == '__main__':
  reverse_iterate()
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In [3]: def count(n):
   ...:     while True:
   ...:         yield n
   ...:         n += 1
   ...:
In [4]: c = count(0)
In [5]: c[10: 20]
-----------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-60489cd5ce42> in <module>()
> 1 c[10: 20]
TypeError: 'generator' object is not subscriptable
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In [6]: import itertools
In [7]: for x in itertools.islice(c, 10, 20):
   ...:     print(x)
   ...:
10
11
12
13
14
15
16
17
18
19
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In [8]: for x in itertools.islice(c, 10, 20):
   ...:     print(x)
   ...:
   ...:
30
31
32
33
34
35
36
37
38
39
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In [9]: import heapq
In [10]: a = [1, 4, 7, 10]
In [11]: b = [2, 5, 6, 11]
In [12]: heapq.merge(a, b)
Out[12]: <generator object merge at 0x1087ab570>
In [13]: for x in heapq.merge(a, b):
  ...:     print(x)
  ...:
1
2
4
5
6
7
10
11
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