`foo()的yield和foo()中的x的区别:yield x` [英] Difference between `yield from foo()` and `for x in foo(): yield x`

问题描述

在Python中，大多数产生收益的示例都说

In Python most examples of yield from explain it with saying that

yield from foo()

与

for x in foo(): yield x

另一方面，它似乎并不完全相同，并且抛出了一些魔术.对于使用执行我不了解的魔术的函数，我感到有些不安.关于yield from的魔术，我必须了解些什么，以避免陷入魔术无法完成的事情?我应该知道魔术提供了哪些优势?

On the other hand it doesn't seem to be exactly the same and there's some magic thrown in. I feel a bit uneasy about using a function that does magic that I don't understand. What do I have to know about the magic of yield from to avoid getting into a situation where the magic does something I don't expect? What advantages does the magic provide, that I should be aware of?

推荐答案

当foo()返回常规可迭代对象时，两者是等效的.当foo()也是 generator 时，魔术"开始起作用.那时，yield from foo()和for x in foo(): yield x情况大不相同.

When foo() returns a regular iterable, the two are equivalent. The 'magic' comes into play when foo() is a generator too. At that moment, the yield from foo() and for x in foo(): yield x cases differ materially.

也可以使用foo()生成器将永远不会看到此消息.但是，当您使用yield from时，发送的数据会直接转到委托生成器当前暂停的任何yield表达式.换句话说，yield from传递已发送的数据，以便委托生成器可以代替接收它.

A generator can be sent data too, using the generator.send() method. When you use the for loop, the yield x expression 'receives' the sent data; the foo() generator will never see this. But when you use yield from the sent data goes straight to whatever yield expression the delegated-to generator is currently paused at. In other words, yield from passes on the sent data so the delegated-to generator can receive it instead.

您还可以在生成器中使用

You can also raise exceptions in a generator, with generator.throw(); with the for loop case, the exception is raised from the yield x line, while with yield from the exception is passed on again; the exception is raised inside foo() instead.

在一起，这意味着yield from 本质上在委派迭代的持续时间内替换了当前生成器.

Together, this means that yield from in essence replaces the current generator for the duration of the delegated iteration.

受委托生成器还可以与父生成器进行通信，完成后，将引发StopIteration异常的.value属性作为yield from表达式的值返回.您可以使用委托给foo()生成器中的return <expression>来设置该异常的值，也可以显式使用raise StopIteration(<expression>).

The delegated-to generator also gets to communicate with the parent generator, when done the .value attribute of the StopIteration exception raised is returned as the value of the yield from expression. You can set the value of that exception by using return <expression> in the delegated-to foo() generator, or you can use raise StopIteration(<expression>) explicitly.

yield from通过 PEP 380:语法用于委托给子生成器 .

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