Python iter()函数用法实例分析

> l=[2,3,4]
> iterl=iter(l)
> iterl.next()
2
> iterl.next()
3
> iterl.next()
4
> iterl.next()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

> d={'one':1,'two':2,'three':3}
> d
{'three': 3, 'two': 2, 'one': 1}
> iterd=iter(d) #字典的迭代器会遍历字典的键（key）
> iterd.next()
'three'
> iterd.next()
'two'
> iterd.next()
'one'
> iterd.next()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

> help(iter)
Help on built-in function iter in module __builtin__:
iter(...)
  iter(collection) -> iterator
  iter(callable, sentinel) -> iterator
  Get an iterator from an object. In the first form, the argument must
  supply its own iterator, or be a sequence.
  In the second form, the callable is called until it returns the sentinel.

> s='abc' #s支持序列访问协议，它有__getitem__()方法
> help(str.__getitem__)
Help on wrapper_descriptor:
__getitem__(...)
  x.__getitem__(y) <==> x[y]
> s.__getitem__(1)
'b'
> s[1]
'b'
> iters=iter(s) #iters是一个iterator对象，它有next()和__iter__()方法
> iters1=iters.__iter__()
> iters2=iter(iters)
> iters
<iterator object at 0x030612D0>
> iters1
<iterator object at 0x030612D0>
> iters2
<iterator object at 0x030612D0>
iters iters1  iters2 是同一个迭代器！！
> iters.next()
'a'
> iters.next()
'b'
> iters.next()
'c'
> iters.next()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

> class test: # test 类支持迭代协议，因为它定义有__iter__()函数
...   def __iter__(self):
...     print '__iter__ is called!'
...     self.result=[1,2,3]
...     return iter(self.result)
...
> t=test() # t支持迭代协议
> for i in t:  #当执行for i in t 时，实际上是调用了t.__iter__()，也就是__iter__(t)，返回一个iterator对象
...   print i,
...
__iter__ is called!
1 2 3
> for i in t.__iter__():
        print i,
__iter__ is called!!
1 2 3
> for i in test.__iter__(t):
        print i,
__iter__ is called!!
1 2 3
> l=[1,2,3]
> for i in l:
...   print i,
...
1 2 3

#上述for循环实际上是这样工作的（for循环会自动调用迭代器的next()方法），如下：
> iterl=iter(l)
> while True:
...   try:
...     i=iterl.next()
...   except StopIteration:
...     break
...   print i,
...
1 2 3

> f=open(r'C:\Users\Administrator\Desktop\test.txt','w')
> f.writelines(['love python\n','hello python\n','love python\n'])
> f.close()
> f=open(r'C:\Users\Administrator\Desktop\test.txt','r')
> for line in f: # 文件对象生成的迭代器会自动调用readline()方法，这样循环遍历就可以访问文本文件的所有行
...   print line[:-1]
...
love python
hello python
love python

> while True:
...   line=f.readline()
...   if line!='':
...     print line[:-1]
...   else:
...     break
...
love python
hello python
love python

> f=open(r'C:\Users\91135\Desktop\test.txt','r')
> f.readlines()
['love python\n', 'hello python\n', '\n', 'love python\n']
> f.seek(0)
> f.next()
'love python\n'
> f.next()
'hello python\n'
> f.next()
'\n'
> f.next()
'love python\n'
> f.next()
Traceback (most recent call last):
 File "<pyshell#140>", line 1, in <module>
  f.next()
StopIteration
> f.seek(0)
> it1=iter(f)
> it2=f.__iter__()

> f
<open file 'C:\\Users\\91135\\Desktop\\test.txt', mode 'r' at 0x030E9A70>
> it1
<open file 'C:\\Users\\91135\\Desktop\\test.txt', mode 'r' at 0x030E9A70>
> it2
<open file 'C:\\Users\\91135\\Desktop\\test.txt', mode 'r' at 0x030E9A70>
> f.next()
'love python\n'
> it1.next()
'hello python\n'
> next(it2)
'\n'
> next(f)
'love python\n'
> next(f)
Traceback (most recent call last):
 File "<pyshell#247>", line 1, in <module>
  next(f)
StopIteration
> it1.next()
Traceback (most recent call last):
 File "<pyshell#248>", line 1, in <module>
  it1.next()
StopIteration
> it2.next()
Traceback (most recent call last):
 File "<pyshell#249>", line 1, in <module>
  it2.next()
StopIteration

iter(callable, sentinel) -> iterator

> class IT(object):
    def __init__(self):
        self.l=[1,2,3,4,5]
        self.i=iter(self.l)
    def __call__(self):  #定义了__call__方法的类的实例是可调用的
        item=next(self.i)
        print "__call__ is called,which would return",item
        return item
    def __iter__(self): #支持迭代协议(即定义有__iter__()函数)
        print "__iter__ is called!!"
        return iter(self.l)
> it=IT() #it是可调用的
> it1=iter(it,3) #it必须是callable的，否则无法返回callable_iterator
> callable(it)
True
> it1
<callable-iterator object at 0x0306DD90>
> for i in it1:
print i
__call__ is called,which would return 1
1
__call__ is called,which would return 2
2
__call__ is called,which would return 3

> it2=iter(it)
__iter__ is called!!
> it2
<listiterator object at 0x030A1FD0>
> for i in it2:
print i,
1 2 3 4 5

> class Next():
        def __init__(self,data=825):
              self.data=data
        def __iter__(self):
              return self
        def next(self):
              print "next is called!!"
              if self.data>828:
                  raise StopIteration
              else:
                  self.data+=1
                  return self.data
> for i in Next():
print i
next is called!!
826
next is called!!
827
next is called!!
828
next is called!!
829
next is called!!
> for i in Next(826):
print i
next is called!!
827
next is called!!
828
next is called!!
829
next is called!!
>

> it=Next()
> it.__iter__()
<__main__.Next instance at 0x02E75F80>
> Next.__iter__(it)
<__main__.Next instance at 0x02E75F80>
> iter(it)
<__main__.Next instance at 0x02E75F80>
> it
<__main__.Next instance at 0x02E75F80>
> it=Next()
> it.next()
next is called!!
826
> next(it)
next is called!!
827
> Next.next(it)
next is called!!
828
> next(it)
next is called!!
829
> it.next()
next is called!!
Traceback (most recent call last):
 File "<pyshell#68>", line 1, in <module>
  it.next()
 File "<pyshell#1>", line 9, in next
  raise StopIteration
StopIteration