「 yield」關鍵字有什麼做用？ - What does the 「yield」 keyword do?

問題：

What is the use of the yield keyword in Python? Python中yield關鍵字的用途是什麼？ What does it do? 它有什麼做用？

For example, I'm trying to understand this code ¹ : 例如，我試圖理解這段代碼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: 這是呼叫者：

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? 調用_get_child_candidates方法時會發生什麼？ Is a list returned? 是否返回列表？ A single element? 一個元素？ Is it called again? 再叫一次嗎？ When will subsequent calls stop? 後續通話什麼時候中止？

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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.這段代碼是由Jochen Schulz（jrschulz）編寫的，Jochen Schulz是一個很好的用於度量空間的Python庫。 This is the link to the complete source: Module mspace . 這是完整源代碼的連接： Module mspace 。}

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解決方案：

參考一： 「 yield」關鍵字有什麼做用？
參考二： What does the 「yield」 keyword do?