yield

通過yield返回的是一個生成器，yield既可以產出值又可以生成值，yield可以用next()來啟動生成器，同時可以用send向生成器傳遞值；在初次啟動生成器時，需調用next()或send(None)來激活生成器，一個yield對應一個send或next(),當send或next()個數超過yield個數時，會拋出StopIteration

 1 def gen_yield():
 2     value = yield 1
 3     print(value)
 4     value = yield 2
 5     print(value)
 6     value = yield
 
 3
 7     print(value)
 8     return "end"
 9 
10 
11 gen = gen_yield()
12 
13 print(next(gen))
14 print(gen.send("a"))
15 print(gen.send("b"))
16 print(gen.send("c"))

next(gen)激活生成器yield 1，返回1，並將生成器掛起；gen.send("a")會將"a"傳遞給value = yield 1，此時value="a"，打印出a，返回2，將生成器掛起；gen.send("b")將"b"傳遞給value = yield 2，此時value="b"，打印"b"並返回3，將生成器掛起；gen.send("c")將"c"傳遞給value = yield 3，此時value="c"，打印"c"，由於後面沒有生成器了，會拋出StopIteration異常。

1
a
2
b
3
c
Traceback (most recent call last):
    print(gen.send("c"))
StopIteration: end

close()/throw() 關閉/暫停 生成器

 1 def gen_yield():
 2     try:
 3         yield "a"
 4     except StopIteration:
 5         pass
 6     yield "b"
 7     yield "c"
 8     return "end"
 9 
10 
11 gen = gen_yield()
 
12 
13 print(next(gen))
14 gen.close()
15 print(111)
16 print(next(gen))

close()是關閉生成器，當生成器執行了close()方法後，即使做了異常處理，與yield相關的語句不會再執行。

throw()相當於中斷、暫停生成器，通過throw()拋出異常後，當異常處理後生成器會執行下一個yield，下一個yield之後的上長期不再受throw()影響。

 1 def gen_yield():
 2     try:
 3         yield "a"
 4     except Exception:
 5         pass
 6     yield "b"
 7     yield "c"
 8     return "end"
 9 
10 
11 gen = gen_yield()
12 
13 print(next(gen))
14 print(gen.throw(Exception))
15 print(111)
16 print(next(gen))

result：

a
b
111
c

yield from

yield from類似於yield，但yield from後面必須接一個可叠代的對象，返回該叠代對象的iter，yield則直接返回接受的對象

def gen_yield(iterable):
    yield iterable


def gen_yield_from(iterable):
    yield from iterable


for value in gen_yield([1, 2, 3]):
    print("yield:{}".format(value))
for value in gen_yield_from([1, 2, 3]):
    print("yield from:{}".format(value))

result：
yield:[1, 2, 3]
yield from:1
yield from:2
yield from:3

yield from會在調用方與子生成器之間建立雙向通道，即yield from後可以接受一個生成器函數，調用方通過send可以直接傳遞數據給子生成器，同時yield from可以接受子生成器返回的數據

 1 def gen_yield():
 2     """
 3     子生成器
 4     :return: 
 5     """
 6     nums = []
 7     i = 1
 8     while i < 4:
 9         n = yield i
10         print("receive {}".format(n))
11         i += 1
12         nums.append(n)
13     return nums
14 
15 
16 def gen_yield_from():
17     val = yield from gen_yield()
18     return val
19 
20 
21 gen_value_list = [100, 200, 300]
22 # 調用方
23 gen = gen_yield_from()
24 # send(None) 激活生成器
25 print("yield return {}".format(gen.send(None)))
26 j = 0
27 while j < 3:
28     try:
29         print("yield return {}".format(gen.send(gen_value_list[j])))
30     except StopIteration as e:
31         print(e)
32     j += 1

gen激活生成器後會通過send發送數據，數據會直接傳遞給子生成器gen_yield
執行結果

yield return 1
receive 100
yield return 2
receive 200
yield return 3
receive 300
[100, 200, 300]

yield與yield from