1. iter函式

iter 函式只是呼叫物件的__iter__方法，這個方法按照協議會返回一個迭代器，使得for迴圈這樣的語法結構能夠進行下去。

2. 可迭代物件

一個物件實現了__iter__方法，就是可迭代的，可以使用for迴圈訪問物件中的每個元素，其實質是呼叫iter函式使物件返回一個迭代器，再使用迭代器進行迴圈。

3. 迭代器：實現了__next__()方法，呼叫該方法時，直到物件沒有可呼叫的元素就返回異常StopIteration

>>> st = 'Hello'
>>> for i in st:
...     print(i)
... 
H
e
l
l
o
>>> next(st)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object is not an iterator
>>> st = iter(st)
>>> next(st)
'H'
>>> next(st)
'e'
>>> next(st)
'l'
>>> next(st)
'l'
>>> next(st)
'o'
>>> next(st)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
 

>>> class Fibs:
...     def __init__(self,n=10):
...         self.a = 0
...         self.b = 1
...         self.n = n
...     def __iter__(self):
...         return self
...     def __next__(self):
...         self.a,self.b = self.b,self.a+self.b
...         if self.a > self.n:
...             raise StopIteration
...         else:
...             return self.a
... 
>>> f = Fibs(20)
>>> for i in f:
...     print(i)
... 
1
1
2
3
5
8
13