python: learn yield and send[part 1]

Sample code

    def gen():
        for i in range(3):
            x = yield i
            print(x)

>>> g = gen()
>>> next(g)
0
>>> next(g)
None
1
>>> next(g)
None
2
>>> next(g)
None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

Rule 1: the generator resumed on the yield keyword expression/statement and went through the loop till it hit the yield expression/statement again. If there is no more yield, raise StopIteration

Rule 2: when the execution is resumed by calling by next or generator.send(), the function can proceed exactly as if the yield expression were just another external call.
name = yield [expression_list]
等號右邊邊是yield expression，每當執行到右邊，本地的函數執行狀態等信息被保存，然後返回。當使用next or generator.send()緊接着上文運行時，將執行接下來的賦值語句。可以將yield expression看作是一個函數，每次相當於調用函數的context switch。

Rule 3:If generator.send() is used, the variable is assigned with the value passed to generator.send()。

Ref:
https://docs.python.org/3/reference/expressions.html#grammar-token-yield_expression