>>> def my_range(start,stop,step=1):
...     print('start...')
...     while start < stop:
...         yield start
...         start+=step
...     print('end...')
... 
>>> g=my_range(0,3)
>>> g
<generator object my_range at 0x104105678>

>>> g.__iter__
<method-wrapper '__iter__' of generator object at 0x1037d2af0>
>>> g.__next__
<method-wrapper '__next__' of generator object at 0x1037d2af0>

>>> next(g) # 触发函数执行直到遇到yield则停止,将yield后的值返回，并在当前位置挂起函数
start...
0
>>> next(g) # 再次调用next(g)，函数从上次暂停的位置继续执行，直到重新遇到yield...
1
>>> next(g) # 周而复始...
2
>>> next(g) # 触发函数执行没有遇到yield则无值返回，即取值完毕抛出异常结束迭代
end...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

>>> for i in countdown(3):
...     print(i)
... 
countdown start
3
2
1
Done!