>>> L = [1,2,3]
>>> for X in L: # Automatic iteration
... print(X ** 2, end=' ') # Obtains iter, calls __next__, catches exceptions
...
1 4 9
>>> I = iter(L) # Manual iteration: what for loops usually do
>>> while Ture:
... try: # try statement catches exceptions
... X = next(I) # Or call I.__next()__
... except StopIteration:
... break
... print(X ** 2, end=' ')
...
1 4 9
=======================================================
enumerate()，用于获取index与对应位置的元素
>>> E = enumerate('spam')
>>> E
<enumerate object at 0x0382104F>
>>> I = iter(E)
>>> next(E)
(0,'s')
>>> next(E)
(1,'p')
>>> list(enumerate('spam'))
[(0,'s'),(1,'p'),(2,'a'),(3,'m')]
===================================================