小弟正在学习PYTHON，看到generator时，下面这支是看得懂...
gen.send.preparation.stop.py
stop = False
def counter(start=0):
n = start
while not stop:
yield n
n += 1
c = counter()
print(next(c)) # prints: 0
print(next(c)) # prints: 1
stop = True
print(next(c)) # raises StopIteration
不过到了下面这支后就卡关了...
我卡关的点在于... 为什么执行# C步骤时，只有印出0；
而执行# D时却是印出：
<class 'str'> Wow!
1
为什么不是先印出1呢? 明明result = yield n 就是在前面啊...
如果按照这逻辑的话，步骤# F应该也要先印出3才对...
可是却先执行# B 印出<class 'str'> Q 后产生StopIteration
麻烦各位大大赐教了，感谢!!
gen.send.py
def counter(start=0):
n = start
while True:
result = yield n # A
print(type(result), result) # B
if result == 'Q':
break
n += 1
c = counter()
print(next(c)) # C
print(c.send('Wow!')) # D
print(next(c)) # E
print(c.send('Q')) # F
执行结果：
$ python gen.send.py
0
<class 'str'> Wow!
1
<class 'NoneType'> None
2
<class 'str'> Q
Traceback (most recent call last):
File "gen.send.py", line 14, in <module>
print(c.send('Q')) # F
StopIteration

C->A->C(0)->D->A->B(Wow)->A->D(1)->....another example: https://goo.gl/5VVhSZ

next(g) == g.send(None)yield 右边的值会先出来，左值会在下次 send 的时候 assign& 这个其实是 coroutine 了，见 PEP342