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(gè)基本方法：
1）next方法
返回容器的下一個(gè)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(gè)復(fù)雜的一次只計(jì)算出一個(gè)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