Python內置函數(36)——iter

英文文檔：

iter(object[, sentinel])

Return an iterator 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 __iter__() method), or it must support the sequence protocol (the __getitem__() method with integer arguments starting at 0). If it does not support either of those protocols, TypeError 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 __next__() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.

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

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

 

說明：

　　1. 函數功能返回一個迭代器對象。

　　2. 當第二個參數不提供時，第一個參數必須是一個支持可迭代協議(即實現了__iter__()方法)的集合(字典、集合、不可變集合)，或者支持序列協議(即實現了__getitem__()方法，方法接收一個從0開始的整數參數)的序列(元組、列表、字符串)，否則將報錯。

>>> a = iter({'A':1,'B':2}) #字典集合
>>> a
<dict_keyiterator object at 0x03FB8A50>
>>> next(a)
'A'
>>> next(a)
'B'
>>> next(a)
Traceback (most recent call last):
  File "<pyshell#36>", line 1, in <module>
    next(a)
StopIteration
 
>>> a = iter('abcd') #字符串序列
>>> a
<str_iterator object at 0x03FB4FB0>
>>> next(a)
'a'
>>> next(a)
'b'
>>> next(a)
'c'
>>> next(a)
'd'
>>> next(a)
Traceback (most recent call last):
  File "<pyshell#29>", line 1, in <module>
    next(a)
StopIteration

 

　　3. 當第二個參數sentinel提供時，第一個參數必須是一個可被調用對象。創建的迭代對象，在調用__next__方法的時候會調用這個可被調用對象，當返回值和sentinel值相等時，將拋出StopIteration異常， 終止迭代。

# 定義類
>>> 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() #實例化類
>>> a = iter(iterTest.get_next_value,4) # iterTest.get_next_value為可調用對象，sentinel值為4
>>> a
<callable_iterator object at 0x03078D30>
>>> next(a)
0
>>> next(a)
1
>>> next(a)
2
>>> next(a)
3
>>> next(a) #迭代到4終止
Traceback (most recent call last):
  File "<pyshell#22>", line 1, in <module>
    next(a)
StopIteration