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一、迭代器Iterators

迭代器僅是一容器對象，它實(shí)現(xiàn)了迭代器協(xié)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(nèi)建的iter方法創(chuàng)建，見例子：

代碼如下:

>>> 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(jié)果：

代碼如下:

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(shù)來完成簡單和有效的代碼。

它基于yield指令，允許停止函數(shù)并立即返回結(jié)果。

此函數(shù)保存其執(zhí)行上下文，如果需要，可立即繼續(xù)執(zhí)行。

例如Fibonacci函數(shù)：

代碼如下:

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(jié)果：

代碼如下:

1

1

2

[3, 5, 8, 13, 21, 34, 55, 89, 144, 233]

PEP Python Enhancement Proposal Python增強(qiáng)建議

tokenize模塊

代碼如下:

>>> 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：

代碼如下:

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(jié)果：

代碼如下:

powering 1

adding to 1

3

powering 4

adding to 4

7

保持代碼簡單，而不是數(shù)據(jù)。

注意：寧可有大量簡單的可迭代函數(shù)，也不要一個復(fù)雜的一次只計(jì)算出一個值的函數(shù)。

例子：

代碼如下:

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(jié)果：

代碼如下:

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

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