Python進階——如何正確使用yield？

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"閱讀本文大約需要 10 分鐘。","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 開發中，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 關鍵字的使用其實較爲頻繁，例如大集合的生成，簡化代碼結構、協程與併發都會用到它。","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，你是否真正瞭解 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 的運行過程呢？","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":"這篇文章，我們就來看一下 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 的運行流程，以及在開發中哪些場景適合使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"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":"如果在一個方法內，包含了 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關鍵字，那麼這個函數就是一個「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":"生成器其實就是一個特殊的迭代器，它可以像迭代器那樣，迭代輸出方法內的每個元素。","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":"如果你還不清楚「迭代器」是什麼，可以參考我寫的這篇文章：","attrs":{}},{"type":"link","attrs":{"href":"https://xie.infoq.cn/article/21c4bdc50e084b08dff0e40dc","title":""},"content":[{"type":"text","text":"Python技術進階——迭代器、可迭代對象、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":"我們來看一個包含 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中的代碼其實並沒有執行，此時我們只是創建了一個「生成器對象」，它的類型是 ","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":"然後，當我們執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for i in g","attrs":{}}],"attrs":{}},{"type":"text","text":"，每執行一次循環，就會執行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 處，返回一次 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 後面的值。","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":"這個迭代過程是和迭代器最大的區別。","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":"換句話說，如果我們想輸出 5 個元素，在創建生成器時，這個 5 個元素其實還並沒有產生，什麼時候產生呢？只有在執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 循環遇到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 時，纔會依次生成每個元素。","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樣實現迭代數據之外，還包含了其他方法：","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":"：執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 時調用此方法，每次執行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 就會停止，然後返回 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 後面的值，如果沒有數據可迭代，拋出 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"StopIterator","attrs":{}}],"attrs":{}},{"type":"text","text":" 異常，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 循環結束","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"generator.send(value)","attrs":{}}],"attrs":{}},{"type":"text","text":"：外部傳入一個值到生成器內部，改變 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 前面的值","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"generator.throw(type[, value[, 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","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"__next__","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法，我們看下面這個例子。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"# coding: utf8\n\ndef gen(n):\n for i in range(n):\n print('yield before')\n yield i\n print('yield after')\n\ng = gen(3) # 創建一個生成器\nprint(g.__next__()) # 0\nprint('----')\nprint(g.__next__()) # 1\nprint('----')\nprint(g.__next__()) # 2\nprint('----')\nprint(g.__next__()) # StopIteration\n\n# Output:\n# yield before\n# 0\n# ----\n# yield after\n# yield before\n# 1\n# ----\n# yield after\n# yield before\n# 2\n# ----\n# yield after\n# Traceback (most recent call last):\n# File \"gen.py\", line 16, in \n# print(g.__next__()) # StopIteration\n# StopIteration","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":"在這個例子中，我們定義了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"gen","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法，這個方法包含了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 關鍵字。然後我們執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g = gen(3)","attrs":{}}],"attrs":{}},{"type":"text","text":" 創建一個生成器，但是這次沒有執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"for","attrs":{}}],"attrs":{}},{"type":"text","text":" 去迭代它，而是多次調用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"next","attrs":{}},{"type":"text","text":"()","attrs":{}}],"attrs":{}},{"type":"text","text":" 去輸出生成器中的元素。","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":"我們看到，當執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.__next__()","attrs":{}}],"attrs":{}},{"type":"text","text":"時，代碼就會執行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 處，然後返回 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 後面的值，如果繼續調用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.__next__()","attrs":{}}],"attrs":{}},{"type":"text","text":"，注意，你會發現，這次執行的開始位置，是上次 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 結束的地方，並且它還保留了上一次執行的上下文，繼續向後迭代。","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":"這就是使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 的作用，在迭代生成器時，每一次執行都可以保留上一次的狀態，而不是像普通方法那樣，遇到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"return","attrs":{}}],"attrs":{}},{"type":"text","text":" 就返回結果，下一次執行只能再次重複上一次的流程。","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":"生成器除了能保存狀態之外，我們還可以通過其他方式，改變其內部的狀態，這就是下面要講的 ","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，我們只展示了在 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":" 後有值的情況，其實還可以使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"j = yield i","attrs":{}}],"attrs":{}},{"type":"text","text":" 這種語法，我們看下面的代碼：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"# coding: utf8\n\ndef gen():\n i = 1\n while True:\n j = yield i\n i *= 2\n if j == -1:\n break","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":"此時如果我們執行下面的代碼：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"for i in gen():\n print(i)\n time.sleep(1)","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":"輸出結果會是 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"1 2 4 8 16 32 64 ...","attrs":{}}],"attrs":{}},{"type":"text","text":" 一直循環下去， 直到我們殺死這個進程才能停止。","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":"這段代碼一直循環的原因在於，它無法執行到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"j == -1","attrs":{}}],"attrs":{}},{"type":"text","text":" 這個分支裏 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"break","attrs":{}}],"attrs":{}},{"type":"text","text":" 出來，如果我們想讓代碼執行到這個地方，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":"這裏就要用到生成器的 ","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傳入生成器內部，從而改變生成器的狀態。","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":"代碼可以像下面這樣寫：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"g = gen() # 創建一個生成器\nprint(g.__next__()) # 1\nprint(g.__next__()) # 2\nprint(g.__next__()) # 4\n# send 把 -1 傳入生成器內部 走到了 j = -1 這個分支\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":"當我們執行 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.send(-1)","attrs":{}}],"attrs":{}},{"type":"text","text":" 時，相當於把 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-1","attrs":{}}],"attrs":{}},{"type":"text","text":" 傳入到了生成器內部，然後賦值給了 ","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":"，此時 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"j = -1","attrs":{}}],"attrs":{}},{"type":"text","text":"，然後這個方法就會 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"break","attrs":{}}],"attrs":{}},{"type":"text","text":" 出來，不會繼續迭代下去。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"throw","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":"外部除了可以向生成器內部傳入一個值外，還可以傳入一個異常，也就是調用 ","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":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"# coding: utf8\n\ndef gen():\n try:\n yield 1\n except ValueError:\n yield 'ValueError'\n finally:\n print('finally')\n\ng = gen() # 創建一個生成器\nprint(g.__next__()) # 1\n# 向生成器內部傳入異常 返回ValueError\nprint(g.throw(ValueError))\n\n# Output：\n# 1\n# ValueError\n# finally","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":"這個例子創建好生成器後，使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"g.throw(ValueError)","attrs":{}}],"attrs":{}},{"type":"text","text":" 的方式，向生成器內部傳入了一個異常，走到了生成器異常處理的分支邏輯。","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":" 方法也比較簡單，就是手動關閉這個生成器，關閉後的生成器無法再進行操作。","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() # 關閉生成器\n>>> g.__next__() # 無法迭代數據\nTraceback (most recent call last):\n File \"\", line 1, in 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和生成器一般用在哪些業務場景中呢？","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":"下面我介紹幾個例子，分別是大集合的生成、簡化代碼結構、協程與併發，你可以參考這些使用場景來使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"大集合的生成","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":"如果你想生成一個非常大的集合，如果使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"list","attrs":{}}],"attrs":{}},{"type":"text","text":" 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