详解Python迭代和迭代器
我们将要来学习python的重要概念迭代和迭代器，通过简单实用的例子如列表迭代器和xrange。
可迭代
一个对象，物理或者虚拟存储的序列。list，tuple，strins，dicttionary，set以及生成器对象都是可迭代的，整型数是不可迭代的。如果你不确定哪个可迭代哪个不可以，你需要用python内建的iter()来帮忙。
>>> iter([1,2,3])
<listiterator object at 0x026C8970>
>>> iter({1:2, 2:4})
<dictionary-keyiterator object at 0x026CC1B0>
>>> iter(1234)
Traceback (most recent call last):
File "<pyshell#145>", line 1, in <module>
iter(1234)
TypeError: 'int' object is not iterable
iter()为list返回了listiterator对象，为dictionary返回了dictionary-keyiterator对象。类似对其他可迭代类型也会返回迭代器对象。
iter()用在自定义的类型会怎样呢？我们先自己定义一个String类：
class String(object):
def __init__(self, val):
self.val = val
def __str__(self):
return self.val
st = String('sample string')
那么，st是可迭代的吗？
>>> iter(st) TypeError: 'String' object is not iterable
你可能会有几个问题要问：
怎么让自定义的类型可迭代？
iter()究竟做了些什么？
让我们补充String类来找找答案
class String(object):
def __init__(self, val):
self.val = val
def __str__(self):
return self.val
def __iter__(self):
print "This is __iter__ method of String class"
return iter(self.val) #self.val is python string so iter() will return it's iterator
>>> st = String('Sample String')
>>> iter(st)
This is __iter__ method of String class
<iterator object at 0x026C8150>
在String类中需要一个'__iter__'方法把String类型变成可迭代的，这就是说'iter'内部调用了'iterable.__iter__()'
别急，不是只有增加'__iter()'方法这一种途径
class String(object):
def __init__(self, val):
self.val = val
def __str__(self):
return self.val
def __getitem__(self, index):
return self.val[index]
>>> st = String('Sample String')
>>> iter(st)
<iterator object at 0x0273AC10>
‘itr'也会调用'iterable.__getitem__()'，所以我们用'__getitem__'方法让String类型可迭代。
如果在String类中同时使用'__iter__()'和'__getitem__()',就只有'__iter__'会起作用。
自动迭代
for循环会自动迭代
for x in iterable: print x
我们可以不用for循环来实现吗？
def iterate_while(iterable):
index = 0
while(i< len(iterable)):
print iterable[i]
i +=1
这样做对list和string是管用的，但对dictionary不会奏效，所以这绝对不是python式的迭代，也肯定不能模拟for循环的功能。我们先看迭代器，等下回再过头来。
迭代器
关于迭代器先说几条………..
1. 迭代器对象在迭代过程中会会产生可迭代的值，`next()`或者`__next()__`是迭代器用来产生下一个值的方法。
2. 它会在迭代结束后发出StopIteration异常。
3. `iter()`函数返回迭代器对象
4. 如果`iter()`函数被用在迭代器对象，它会返回对象本身
我们试一试模仿for循环
def simulate_for_loop(iterable): it = iter(iterable) while(True): try: print next(it) except StopIteration: break >>> simulate_for_loop([23,12,34,56]) 23 12 34 56
前面我们看过了iterable类，我们知道iter会返回迭代器对象。
现在我们试着理解迭代器类的设计。
class Iterator:
def __init__(self, iterable)
self.iterable = iterable
.
.
def __iter__(self): #iter should return self if called on iterator
return self
def next(self): #Use __next__() in python 3.x
if condition: #it should raise StopIteration exception if no next element is left to return
raise StopIteration
我们学了够多的迭代和迭代器，在python程序中不会用到比这更深的了。
但是为了学习的目的我们就到这儿。。。。
列表迭代器
你可能会在面试中写这个，所以打起精神来注意了
class list_iter(object):
def __init__(self, list_data):
self.list_data = list_data
self.index = 0
def __iter__(self):
return self
def next(self): #Use __next__ in python 3.x
if self.index < len(self.list_data):
val = self.list_data[self.index]
self.index += 1
return val
else:
raise StopIteration()
我们来用`list_iter`自己定义一个列表迭代器
class List(object):
def __init__(self, val):
self.val = val
def __iter__(self):
return list_iter(self.val)
>>> ls = List([1,2,34])
>>> it = iter(ls)
>>> next(it)
1
>>> next(it)
2
>>> next(it)
34
>>> next(it)
Traceback (most recent call last):
File "<pyshell#254>", line 1, in <module>
next(it)
File "<pyshell#228>", line 13, in next
raise StopIteration()
StopIteration
xrange
从一个问题开始——xrange是迭代还是迭代器？
我们来看看
>>> x = xrange(10) >>> type(x) <type 'xrange'>
几个关键点：
1. `iter(xrange(num))`应该被支持
2. 如果`iter(xrange(num))`返回同样的对象（xrange类型）那xrange就是迭代器
3. 如果`iter(xrange(num))`返回一个迭代器对象那xrange就是迭代
>>> iter(xrange(10)) <rangeiterator object at 0x0264EFE0>
它返回了rangeiterator，所以我们完全可以叫它迭代器。
让我们用最少的xrange函数实现自己的xrange
xrange_iterator
class xrange_iter(object):
def __init__(self, num):
self.num = num
self.start = 0
def __iter__(self):
return self
def next(self):
if self.start < self.num:
val = self.start
self.start += 1
return val
else:
raise StopIteration()
my xrange
class my_xrange(object):
def __init__(self, num):
self.num = num
def __iter__(self):
return xrange_iter(self.num)
>>> for x in my_xrange(10):
print x,
0 1 2 3 4 5 6 7 8 9
以上就是本文的全部内容，希望对大家学习掌握Python迭代和迭代器有所帮助。
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