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：

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：

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：

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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