Python自动化开发从浅入深-语言基础(迭代器和生成器)

迭代器可使用内建的iter方法创建
>>> i = iter(‘abc‘)
>>> i.next()
‘a‘
>>> i.next()
‘b‘
>>> i.next()
‘c‘
>>> i.next()
Traceback (most recent call last):
  File "<string>", line 1, in <string>
StopIteration:

class MyIterator(object):
  def __init__(self, step):
  self.step = step
  def next(self):
  """Returns the next element."""
  if self.step==0:
  raise StopIteration
  self.step-=1
  return self.step
  def __iter__(self):
  """Returns the iterator itself."""
  return self
for el in MyIterator(4):
  print el
--------------------
结果：
3
2
1
0

二、生成器Generators
利用yield指令，允许停止函数并立即返回结果。
此函数保存其执行上下文，如果需要，可立即继续执行。
例如Fibonacci函数：
def fibonacci():
  a,b=0,1
  while True:
  yield b
  a,b = b, a+b
fib=fibonacci()
print fib.next()
print fib.next()
print fib.next()
print [fib.next() for i in range(10)]
--------------------
结果：
1
1
2
[3, 5, 8, 13, 21, 34, 55, 89, 144, 233]

PEP Python Enhancement Proposal Python增强建议

tokenize模块
>>> import tokenize
>>> reader = open(‘c:/temp/py1.py‘).next
>>> tokens=tokenize.generate_tokens(reader)
>>> tokens.next()
(1, ‘class‘, (1, 0), (1, 5), ‘class MyIterator(object):\n‘)
>>> tokens.next()
(1, ‘MyIterator‘, (1, 6), (1, 16), ‘class MyIterator(object):\n‘)
>>> tokens.next()
(51, ‘(‘, (1, 16), (1, 17), ‘class MyIterator(object):\n‘)

例子：
def power(values):
  for value in values:
  print ‘powering %s‘ %value
  yield value
def adder(values):
  for value in values:
  print ‘adding to %s‘ %value
  if value%2==0:
  yield value+3
  else:
  yield value+2
elements = [1,4,7,9,12,19]
res = adder(power(elements))
print res.next()
print res.next()
--------------------
结果：
powering 1
adding to 1
3
powering 4
adding to 4
7

保持代码简单，而不是数据。
注意：宁可有大量简单的可迭代函数，也不要一个复杂的一次只计算出一个值的函数。

例子：
def psychologist():
  print ‘Please tell me your problems‘
  while True:
  answer = (yield)
  if answer is not None:
  if answer.endswith(‘?‘):
  print ("Don‘t ask yourself too much questions")
  elif ‘good‘ in answer:
  print "A that‘s good, go on"
  elif ‘bad‘ in answer:
  print "Don‘t be so negative"
free = psychologist()
print free.next()
print free.send(‘I feel bad‘)
print free.send("Why I shouldn‘t ?")
print free.send("ok then i should find what is good for me")
--------------------
结果：
Please tell me your problems
None
Don‘t be so negative
None
Don‘t ask yourself too much questions
None
A that‘s good, go on
None

摘录实例（http://www.cnblogs.com/finallyliuyu/archive/2010/04/09/1708584.html）