2025-09-10:仓库迁移

8.类与对象/3.让对象支持上下文管理协议.py

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+# 有时候我们想让类支持with触发的上下文管理协议实现自动启动和关闭
+# 这时候就要实现类的__enter__和__exit__方法
+
+from socket import socket, AF_INET, SOCK_STREAM
+# 比如下面这个网络连接类
+class LazyConnection:
+    def __init__(self, address, family=AF_INET, type=SOCK_STREAM):
+        self.address = address
+        self.family = family
+        self.type = type
+        self.sock = None
+
+    def __enter__(self):
+        if self.sock is not None:
+            raise RuntimeError
+        self.sock = socket(self.family, self.type)
+        self.sock.connect(self.address)
+        print("链接被拉起")
+        return self.sock
+
+    def __exit__(self, exc_ty, exc_va, tb):
+        self.sock.close()
+        self.sock = None
+        print("链接关闭")
+
+from functools import partial
+conn = LazyConnection(('www.python.org', 80))
+with conn as s:
+    # 链接被拉起
+    s.send(b'GET /index.html HTTP/1.0\r\n')
+    s.send(b'Host: www.python.org\r\n')
+    s.send(b'\r\n')
+    resp = b''.join(iter(partial(s.recv, 1024), b''))
+    # 链接被关闭
+
+# 但是这个类只能使用单层的with，如果有多个with嵌套，它就会失效
+# 可以对原来的类进行一次修改，做一个工厂类，每次从一个栈中存取一个链接
+class LazyConnectionV2():
+    def __init__(self, address, family=AF_INET, type=SOCK_STREAM):
+        self.address = address
+        self.family = family
+        self.type = type
+        # 单个的sock被替换成空列表
+        self.connections = []
+
+    def __enter__(self):
+        # 建立一个socket链接
+        sock = socket(self.family, self.type)
+        sock.connect(self.address)
+        # 将链接压入栈中
+        self.connections.append(sock)
+        print("链接被拉起")
+        return sock
+
+    def __exit__(self, exc_ty, exc_va, tb):
+        # 将最近被压入栈的链接弹出并关闭
+        self.connections.pop().close()
+        print("链接关闭")