python - 从其中包含 return 语句的生成器中产生

Question

我有一个带有 return value 语句的生成器。如果我在它上面使用 next，我会按预期从中得到 Stopiteration: value。但是，当我使用 yield from 时，value 丢失了。

In [1]: def test():
   ...:     return 1
   ...:     yield 2
   ...:

In [2]: t = test()

In [3]: t
Out[3]: <generator object test at 0x000000000468F780>

In [4]: next(t)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-4-9494367a8bed> in <module>()
----> 1 next(t)

StopIteration: 1

In [5]: def new():
   ...:     yield from test()
   ...:

In [6]: n = new()

In [7]: n
Out[7]: <generator object new at 0x00000000050F23B8>

In [8]: next(n)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-8-1c47c7af397e> in <module>()
----> 1 next(n)

StopIteration:

有没有办法在使用 yield from 时保留 value ？这是按预期工作还是可能是错误？

Answer 1

通过接收子生成器在yield from中发送的值声明。

摘自 PEP 380 -- Syntax for Delegating to a Subgenerator:

The value of the yield from expression is the first argument to the StopIteration exception raised by the iterator when it terminates.

所以稍微调整一下，res在new生成器将包含 StopIteration 的值从 test 中提出子生成器:

def new():
   res = yield from test()
   return res

现在 next(n)执行后，您将获得异常消息中的值:

n = new()

next(n)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-39-1c47c7af397e> in <module>()
----> 1 next(n)

StopIteration: 1

---

哦，作为附录，您当然可以获得“返回”值，而无需将其封装在 StopIteration 中。使用 yield 对象再次:

def new():
    res = yield from test()
    yield res

现在打电话 next(new())将返回从 test() 返回的值:

next(new())
Out[20]: 1