英文文档：

iter(object[, sentinel])

Return an object. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument, object must be a collection object which supports the iteration protocol (the method), or it must support the sequence protocol (the method with integer arguments starting at 0). If it does not support either of those protocols, is raised. If the second argument, sentinel, is given, then object must be a callable object. The iterator created in this case will call object with no arguments for each call to its method; if the value returned is equal to sentinel, will be raised, otherwise the value will be returned.

One useful application of the second form of is to read lines of a file until a certain line is reached. The following example reads a file until the method returns an empty string:

with open('mydata.txt') as fp:    for line in iter(fp.readline, ''):        process_line(line)

 

说明：

　　1. 函数功能返回一个迭代器对象。

　　2. 当第二个参数不提供时，第一个参数必须是一个支持可迭代协议(即实现了__iter__()方法)的集合(字典、集合、不可变集合)，或者支持序列协议(即实现了__getitem__()方法，方法接收一个从0开始的整数参数)的序列(元组、列表、字符串)，否则将报错。

>>> a = iter({
   'A':1,'B':2}) #字典集合>>> a
     
      >>> next(a)'A'>>> next(a)'B'>>> next(a)Traceback (most recent call last):  File "
      
       ", line 1, in 
       
            next(a)StopIteration >>> a = iter('abcd') #字符串序列>>> a
        
         >>> next(a)'a'>>> next(a)'b'>>> next(a)'c'>>> next(a)'d'>>> next(a)Traceback (most recent call last):  File "
         
          ", line 1, in 
          
            next(a)StopIteration
          
         
        
       
      
     

 

　　3. 当第二个参数sentinel提供时，第一个参数必须是一个可被调用对象。创建的迭代对象，在调用__next__方法的时候会调用这个可被调用对象，当返回值和sentinel值相等时，将抛出

# 定义类>>> class IterTest:     def __init__(self):        self.start = 0        self.end = 10    def get_next_value(self):        current = self.start        if current < self.end:            self.start += 1        else:            raise StopIteration        return current>>> iterTest = IterTest() #实例化类>>> a = iter(iterTest.get_next_value,4) # iterTest.get_next_value为可调用对象，sentinel值为4>>> a
     
      >>> next(a)0>>> next(a)1>>> next(a)2>>> next(a)3>>> next(a) #迭代到4终止Traceback (most recent call last):  File "
      
       ", line 1, in 
       
            next(a)StopIteration