一、迭代器Iterators
迭代器僅是一容器對象，它實現了迭代器協議。它有兩個基本方法：
1）next方法
返回容器的下一個元素
2）__iter__方法
返回迭代器自身
迭代器可使用內建的iter方法創建，見例子：
>>> 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
--------------------
結果：
3
2
1
0
二、生成器Generators
從Python2.2起，生成器提供了一種簡潔的方式幫助返回列表元素的函數來完成簡單和有效的代碼。
它基于yield指令，允許停止函數并立即返回結果。
此函數保存其執行上下文，如果需要，可立即繼續執行。
例如Fibonacci函數：
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)]
--------------------
結果：
1
1
2
[3, 5, 8, 13, 21, 34, 55, 89, 144, 233]
PEP Python Enhancement Proposal Python增強建議
tokenize模塊
>>> 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()
--------------------
結果：
powering 1
adding to 1
3
powering 4
adding to 4
7
保持代碼簡單，而不是數據。
注意：寧可有大量簡單的可迭代函數，也不要一個復雜的一次只計算出一個值的函數。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")
--------------------
結果：
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
