Python进阶——如何正确使用yield？

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"阅读本文大约需要 10 分钟。","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在 Python 开发中，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 关键字的使用其实较为频繁，例如大集合的生成，简化代码结构、协程与并发都会用到它。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是，你是否真正了解 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 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关键字，那么这个函数就是一个「生成器」。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"生成器其实就是一个特殊的迭代器，它可以像迭代器那样，迭代输出方法内的每个元素。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果你还不清楚「迭代器」是什么，可以参考我写的这篇文章：","attrs":{}},{"type":"link","attrs":{"href":"https://xie.infoq.cn/article/21c4bdc50e084b08dff0e40dc","title":""},"content":[{"type":"text","text":"Python技术进阶——迭代器、可迭代对象、生成器","attrs":{}}]},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们来看一个包含 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中的代码其实并没有执行，此时我们只是创建了一个「生成器对象」，它的类型是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"generator","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"然后，当我们执行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for i in g","attrs":{}}],"attrs":{}},{"type":"text","text":"，每执行一次循环，就会执行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 处，返回一次 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 后面的值。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个迭代过程是和迭代器最大的区别。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"换句话说，如果我们想输出 5 个元素，在创建生成器时，这个 5 个元素其实还并没有产生，什么时候产生呢？只有在执行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 循环遇到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 时，才会依次生成每个元素。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"此外，生成器除了和迭代器一样实现迭代数据之外，还包含了其他方法：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"generator.__next__()","attrs":{}}],"attrs":{}},{"type":"text","text":"：执行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 时调用此方法，每次执行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 就会停止，然后返回 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就返回结果，下一次执行只能再次重复上一次的流程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"生成器除了能保存状态之外，我们还可以通过其他方式，改变其内部的状态，这就是下面要讲的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"send","attrs":{}}],"attrs":{}},{"type":"text","text":" 和 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"throw","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"send","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的例子中，我们只展示了在 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出来，如果我们想让代码执行到这个地方，如何做呢？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里就要用到生成器的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"send","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法了，","attrs":{}},{"type":"codeinline","content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"send","attrs":{}}],"attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":" 方法可以把外部的值传入生成器内部，从而改变生成器的状态。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代码可以像下面这样写：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"g = gen() # 创建一个生成器\nprint(g.__next__()) # 1\nprint(g.__next__()) # 2\nprint(g.__next__()) # 4\n# send 把 -1 传入生成器内部 走到了 j = -1 这个分支\nprint(g.send(-1)) # StopIteration 迭代停止","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当我们执行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.send(-1)","attrs":{}}],"attrs":{}},{"type":"text","text":" 时，相当于把 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-1","attrs":{}}],"attrs":{}},{"type":"text","text":" 传入到了生成器内部，然后赋值给了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 前面的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"j","attrs":{}}],"attrs":{}},{"type":"text","text":"，此时 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"j = 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的方式，向生成器内部传入了一个异常，走到了生成器异常处理的分支逻辑。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"close","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"生成器的 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"close","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法也比较简单，就是手动关闭这个生成器，关闭后的生成器无法再进行操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":">>> g = gen()\n>>> g.close() # 关闭生成器\n>>> g.__next__() # 无法迭代数据\nTraceback (most recent call last):\n File \"\", line 1, in 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