Python中一个可迭代对象(iterable object)是一个实现了__iter__方法的对象，它应该返回一个迭代器对象(iterator object)。迭代器是一个实现__next__方法的对象，它应该返回它的可迭代对象的下一个元素，并在没有可用元素时触发StopIteration异常。

      当我们使用for循环遍历任何可迭代对象时，它在内部使用iter()方法获取迭代器对象，该迭代器对象进一步使用next()方法进行迭代。此方法触发StopIteration以表示迭代结束。

      Python中大多数内置容器，如列表、元组、字符串等都是可迭代的。它们是iterable但不是iterator，把它们从iterable变成iterator可以使用iter()函数。

      测试代码如下：

# reference: https:///python-programming/iterator
my_list = [1, 3, 5] # define a list
if 0:
    my_iter = iter(my_list) # get an iterator using iter()
    print(next(my_iter)) # iterate through it using next()
    print(my_iter.__next__()) # next(obj) is name as obj.__next__()
    print(my_iter.__next__()) # 5
    #print(my_iter.__next__()) # this will raise error(StopIteration), no items left
else: # use the for loop
    for element in iter(my_list): # 迭代器对象可以使用for语句进行遍历
        print(element, end=" ")

'''
for element in iterable:
    # do something with element

Is actually implemented as:
# create an iterator object from that iterable
iter_obj = iter(iterable)
while True: # infinite loop
    try:
        # get the next item
        element = next(iter_obj)
        # do something with element
    except StopIteration:
        # if StopIteration is raised, break from loop
        break
'''

# reference: https:///iterators-in-python/
class Test:
    def __init__(self, limit):
        self.limit = limit

    # Creates iterator object, Called when iteration is initialized
    def __iter__(self):
        self.x = 10
        return self

    # To move to next element. In Python 3, we should replace next with __next__
    def __next__(self):
        # Store current value ofx
        x = self.x

        # Stop iteration if limit is reached
        if x > self.limit:
            raise StopIteration

        # Else increment and return old value
        self.x = x + 1
        return x

# Prints numbers from 10 to 15
print("\n")
for i in Test(15):
    print(i, end=" ")

print("\n")
value = iter(Test(11))
print(next(value))
print(next(value))
#print(next(value)) # raise StopIteration

print("test finish")