Python進階——什麼是上下文管理器？

{"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 12 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，我們經常會使用到 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 語法塊，例如在讀寫文件時，保證文件描述符的正確關閉，避免資源泄露問題。","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":"你有沒有思考過， ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 背後是如何實現的？我們常常聽到的上下文管理器究竟是什麼？","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":"這篇文章我們就來學習一下 Python 上下文管理器，以及 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 的運行原理。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"with語法塊","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":"在講解 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 語法之前，我們先來看一下不使用 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後面的表達式是可以任意寫的嗎？","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":"答案是否定的。要想使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 語法塊，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 後面的的對象需要實現「上下文管理器協議」。","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":"什麼是「上下文管理器協議」？","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":"一個類在 Python 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contextmanager\n\n@contextmanager\ndef test():\n print('before')\n yield 'hello'\n print('after')\n\nwith test() as t:\n print(t)\n\n# Output:\n# before\n# hello\n# after","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，我們使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"contextmanager","attrs":{}}],"attrs":{}},{"type":"text","text":" 裝飾器和 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"yield","attrs":{}}],"attrs":{}},{"type":"text","text":"配合，實現了和前面上下文管理器相同的功能，它的執行流程如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":"1","normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"執行 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內發生了異常\n else:\n if value is None:\n value = type()\n try:\n # 拋出異常\n self.gen.throw(type, value, traceback)\n except StopIteration as exc:\n return exc is not value\n except RuntimeError as exc:\n if exc is value:\n return False\n if type is StopIteration and exc.__cause__ is value:\n return False\n raise\n except:\n if sys.exc_info()[1] is value:\n return False\n raise\n raise RuntimeError(\"generator didn't stop after throw()\")\n\ndef contextmanager(func):\n @wraps(func)\n def helper(*args, **kwds):\n return _GeneratorContextManager(func, args, kwds)\n return helper\n\nclass closing(AbstractContextManager):\n def __init__(self, thing):\n self.thing = thing\n def __enter__(self):\n return self.thing\n def __exit__(self, *exc_info):\n 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contextlib import contextmanager\n\n@contextmanager\ndef lock(redis, lock_key, expire):\n try:\n locked = redis.set(lock_key, 'locked', expire)\n yield locked\n finally:\n redis.delete(lock_key)\n\n# 業務調用 with 代碼塊執行結束後 自動釋放鎖資源\nwith lock(redis, 'locked', 3) as locked:\n if not locked:\n return\n # do something ...","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，我們實現了 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"lock","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法，用於在 Redis 上申請一個分佈式鎖，然後使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"contextmanager","attrs":{}}],"attrs":{}},{"type":"text","text":" 裝飾器裝飾了這個方法。","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":"之後我們業務在調用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"lock","attrs":{}}],"attrs":{}},{"type":"text","text":" 方法時，就可以使用 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 語法塊了。","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":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 語法塊的第一步，首先判斷是否申請到了分佈式鎖，如果申請失敗，則業務邏輯直接返回。如果申請成功，則執行具體的業務邏輯，當業務邏輯執行完成後，","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"with","attrs":{}}],"attrs":{}},{"type":"text","text":" 退出時會自動釋放分佈式鎖，就不需要我們每次都手動釋放鎖了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"Redis事物和管道","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"python"},"content":[{"type":"text","text":"from contextlib import contextmanager\n\n@contextmanager\ndef pipeline(redis):\n pipe = redis.pipeline()\n try:\n yield pipe\n pipe.execute()\n except Exception as exc:\n pipe.reset()\n \n# 業務調用 with 代碼塊執行結束後 自動執行 execute 方法\nwith pipeline(redis) as pipe:\n pipe.set('key1', 'a', 30)\n pipe.zadd('key2', 'a', 1)\n pipe.sadd('key3', 'a')","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 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