“ yield”关键字有什么做用？ - What does the “yield” keyword do?

问题：

What is the use of the yield keyword in Python? Python中yield关键字的用途是什么？ What does it do? 它有什么做用？

For example, I'm trying to understand this code ¹ : 例如，我试图理解这段代码¹ ：

def _get_child_candidates(self, distance, min_dist, max_dist):
    if self._leftchild and distance - max_dist < self._median:
        yield self._leftchild
    if self._rightchild and distance + max_dist >= self._median:
        yield self._rightchild

And this is the caller: 这是呼叫者：

result, candidates = [], [self]
while candidates:
    node = candidates.pop()
    distance = node._get_dist(obj)
    if distance <= max_dist and distance >= min_dist:
        result.extend(node._values)
    candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
return result

What happens when the method _get_child_candidates is called? 调用_get_child_candidates方法时会发生什么？ Is a list returned? 是否返回列表？ A single element? 一个元素？ Is it called again? 再叫一次吗？ When will subsequent calls stop? 后续通话什么时候中止？

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_{1. This piece of code was written by Jochen Schulz (jrschulz), who made a great Python library for metric spaces. 1.这段代码是由Jochen Schulz（jrschulz）编写的，Jochen Schulz是一个很好的用于度量空间的Python库。 This is the link to the complete source: Module mspace . 这是完整源代码的连接： Module mspace 。}

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解决方案：

参考一： “ yield”关键字有什么做用？
参考二： What does the “yield” keyword do?