迭代器就是重复地做一些事情,可以简单的理解为循环,在python中实现了__iter__方法的对象是可迭代的,实现了next()方法的对象是迭代器,这样说起来有
迭代器就是重复地做一些事情,可以简单的理解为循环,在python中实现了__iter__方法的对象是可迭代的,实现了next()方法的对象是迭代器,这样说起来有点拗口,实际上要想让一个迭代器工作,至少要实现__iter__方法和next方法。很多时候使用迭代器完成的工作使用列表也可以完成,但是如果有很多值列表就会占用太多的内存,而且使用迭代器也让我们的程序更加通用、优雅、pythonic。下边是一个例子,从里边你会感受到不用列表而用迭代器的原因。
#!/usr/bin/env python
#coding=utf-8
class Fib:
def __init__(self):
self.a,self.b = 0,1
def next(self):
self.a,self.b = self.b,self.a+self.b
return self.a
def __iter__(self):
return self
fibs = Fib()
for f in fibs:
if f < 10000:
print f
else:
break
迭代器是一个对象,而生成器是一个函数,迭代器和生成器是python中两个非常强大的特性,编写程序时你可以不使用生成器达到同样的效果,但是生成器让你的程序更加pythonic。创建生成器非常简单,只要在函数中加入yield语句即可。函数中每次使用yield产生一个值,函数就返回该值,然后停止执行,等待被激活,被激活后继续在原来的位置执行。下边的例子实现了同样的功能:
#!/usr/bin/env python
#coding=utf-8
def fib():
a,b = 0,1
while 1:
a,b = b,a+b
yield a
for f in fib():
if f < 10000:
print f
else:
break
生成器接收参数实例:
def counter(start_at):
count = start_at
while True:
print 'before count %s' % count
val = (yield count)
print 'after count %s, val %s' % (count, val)
if val is not None:
count = val
print 'sts a'
else:
count += 1
print 'sts b'
if __name__ == '__main__':
count = counter(5)
print 'calling1 count.next():', count.next()
print 'calling2 count.next():', count.next()
print 'calling3 count.next():', count.next()
print 'calling4 count.next():', count.next()
print 'calling5 count.next():', count.next()
print '-------start---------'
s = count.send(20)
print 's', s
print 'calling count.next():', count.next()
print 'calling count.close():', count.close()
print 'calling count.next():', count.next()
结果:
calling1 count.next(): before count 5
5
calling2 count.next(): after count 5, val None
sts b
before count 6
6
calling3 count.next(): after count 6, val None
sts b
before count 7
7
calling4 count.next(): after count 7, val None
sts b
before count 8
8
calling5 count.next(): after count 8, val None
sts b
before count 9
9
-------start---------
after count 9, val 20
sts a
before count 20
s 20
calling count.next(): after count 20, val None
sts b
before count 21
21
calling count.close(): None
calling count.next():
Traceback (most recent call last):
File "D:\Python27\counter.py", line 26, in <module>
print 'calling count.next():', count.next()
StopIteration