Python上下文管理器高级用法全解析

# 使用上下文管理器
with open('file.txt', 'r') as f:
    content = f.read()
print(f.closed)  # 输出: True

class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        import time
        self.end = time.time()
        print(f"Elapsed time: {self.end - self.start:.4f} seconds")
        // 返回 False 表示不抑制异常
        return False
# 使用自定义上下文管理器
with Timer():
    import time
    time.sleep(1)
    print("Operation completed")

from contextlib import contextmanager
@contextmanager
def timer():
    import time
    start = time.time()
    try:
        yield
    finally:
        end = time.time()
        print(f"Elapsed time: {end - start:.4f} seconds")
# 使用装饰器实现的上下文管理器
with timer():
    import time
    time.sleep(1)
    print("Operation completed")

class File:
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
    def __enter__(self):
        self.file = open(self.filename, self.mode)
        return self.file
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()
# 使用带参数的上下文管理器
with File('output.txt', 'w') as f:
    f.write('Hello, World!')

from contextlib import nested
with nested(open('file1.txt', 'r'), open('file2.txt', 'w')) as (f1, f2):
    content = f1.read()
    f2.write(content)
# Python 3.1+ 支持直接嵌套
with open('file1.txt', 'r') as f1, open('file2.txt', 'w') as f2:
    content = f1.read()
    f2.write(content)

def process_file(file):
    content = file.read()
    return len(content)
# 上下文管理器作为函数参数
with open('file.txt', 'r') as f:
    length = process_file(f)
print(f"File length: {length}")

import sqlite3
class DatabaseConnection:
    def __init__(self, db_path):
        self.db_path = db_path
    def __enter__(self):
        self.conn = sqlite3.connect(self.db_path)
        self.cursor = self.conn.cursor()
        return self.cursor
    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_type is None:
            self.conn.commit()
        else:
            self.conn.rollback()
        self.cursor.close()
        self.conn.close()
# 使用数据库连接上下文管理器
with DatabaseConnection('example.db') as cursor:
    cursor.execute('CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, name TEXT)')
    cursor.execute('INSERT INTO users (name) VALUES (?)', ('Alice',))
    cursor.execute('SELECT * FROM users')
    print(cursor.fetchall())

import socket
class SocketConnection:
    def __init__(self, host, port):
        self.host = host
        self.port = port
    def __enter__(self):
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.connect((self.host, self.port))
        return self.sock
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.sock.close()
# 使用网络连接上下文管理器
with SocketConnection('localhost', 8080) as sock:
    sock.sendall(b'Hello, Server!')
    data = sock.recv(1024)
    print(f"Received: {data.decode()}")

class ResourceManager:
    def __init__(self, resource):
        self.resource = resource
    def __enter__(self):
        print(f"Acquiring resource: {self.resource}")
        // 模拟资源获取
        return self.resource
    def __exit__(self, exc_type, exc_val, exc_tb):
        print(f"Releasing resource: {self.resource}")
        // 模拟资源释放
# 使用资源管理上下文管理器
with ResourceManager('database') as resource:
    print(f"Using resource: {resource}")

import os
from contextlib import contextmanager
@contextmanager
def temporary_env_var(key, value):
    original_value = os.environ.get(key)
    os.environ[key] = value
    try:
        yield
    finally:
        if original_value is None:
            del os.environ[key]
        else:
            os.environ[key] = original_value
# 使用临时环境变量上下文管理器
print(f"Before: {os.environ.get('TEST_VAR', 'Not set')}")
with temporary_env_var('TEST_VAR', 'test_value'):
    print(f"During: {os.environ.get('TEST_VAR')}")
print(f"After: {os.environ.get('TEST_VAR', 'Not set')}")