python - 使用 "yield from"表达式时 return 有什么作用？

Question

我无法从yield from expression 中找到返回值的任何示例.我试过这个简单的代码，但没有成功:

def return4():
    return 4


def yield_from():
    res = yield from range(4)
    res = yield from return4()


def test_yield_from():
    for x in yield_from():
        print(x)


test_yield_from()

产生:

» python test.py 
0
1
2
3
Traceback (most recent call last):
  File "test.py", line 52, in <module>
    test_yield_from()
  File "test.py", line 48, in test_yield_from
    for x in yield_from():
  File "test.py", line 44, in yield_from
    res = yield from return4()
TypeError: 'int' object is not iterable

但我期待:

» python test.py 
0
1
2
3
4

因为，如 PEP 中所述:

Furthermore, when the iterator is another generator, the subgenerator is allowed to execute a return statement with a value, and that value becomes the value of the yield from expression.

显然，我没有得到这个解释。 “子生成器”中的 return 如何处理 yield from？

Answer 1

生成器可以在耗尽时返回一个值:

def my_gen():
    yield 0
    return "done"

g = my_gen()
next(g)
next(g) # raises StopIteration: "done"

yield from 语句中的返回值将是这个值。例如。

def yield_from():
    res = yield from my_gen()
    assert res == "done"

默认情况下，此值为 None。即 res = yield from range(4) 会将 res 设置为 None。