python-resource-management

Python 资源管理

使用上下文管理器确定性地管理资源。数据库连接、文件句柄和网络套接字等资源应可靠释放，即使发生异常也不例外。

何时使用本技能

• 管理数据库连接和连接池
• 处理文件句柄和 I/O
• 实现自定义上下文管理器
• 构建带状态的流式响应
• 处理嵌套资源清理
• 创建异步上下文管理器

核心概念

1. 上下文管理器

with 语句确保资源自动释放，即使发生异常也不例外。

2. 协议方法

__enter__/__exit__ 用于同步，__aenter__/__aexit__ 用于异步资源管理。

3. 无条件清理

无论是否发生异常，__exit__ 始终执行。

4. 异常处理

从 __exit__ 返回 True 可抑制异常，返回 False 则传播异常。

快速开始

from contextlib import contextmanager

@contextmanager
def managed_resource():
    resource = acquire_resource()
    try:
        yield resource
    finally:
        resource.cleanup()

with managed_resource() as r:
    r.do_work()

基本模式

模式 1：基于类的上下文管理器

为复杂资源实现上下文管理器协议。

class DatabaseConnection:
    """Database connection with automatic cleanup."""

    def __init__(self, dsn: str) -> None:
        self._dsn = dsn
        self._conn: Connection | None = None

    def connect(self) -> None:
        """Establish database connection."""
        self._conn = psycopg.connect(self._dsn)

    def close(self) -> None:
        """Close connection if open."""
        if self._conn is not None:
            self._conn.close()
            self._conn = None

    def __enter__(self) -> "DatabaseConnection":
        """Enter context: connect and return self."""
        self.connect()
        return self

    def __exit__(
        self,
        exc_type: type[BaseException] | None,
        exc_val: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> None:
        """Exit context: always close connection."""
        self.close()

# Usage with context manager (preferred)
with DatabaseConnection(dsn) as db:
    result = db.execute(query)

# Manual management when needed
db = DatabaseConnection(dsn)
db.connect()
try:
    result = db.execute(query)
finally:
    db.close()

模式 2：异步上下文管理器

对于异步资源，实现异步协议。

class AsyncDatabasePool:
    """Async database connection pool."""

    def __init__(self, dsn: str, min_size: int = 1, max_size: int = 10) -> None:
        self._dsn = dsn
        self._min_size = min_size
        self._max_size = max_size
        self._pool: asyncpg.Pool | None = None

    async def __aenter__(self) -> "AsyncDatabasePool":
        """Create connection pool."""
        self._pool = await asyncpg.create_pool(
            self._dsn,
            min_size=self._min_size,
            max_size=self._max_size,
        )
        return self

    async def __aexit__(
        self,
        exc_type: type[BaseException] | None,
        exc_val: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> None:
        """Close all connections in pool."""
        if self._pool is not None:
            await self._pool.close()

    async def execute(self, query: str, *args) -> list[dict]:
        """Execute query using pooled connection."""
        async with self._pool.acquire() as conn:
            return await conn.fetch(query, *args)

# Usage
async with AsyncDatabasePool(dsn) as pool:
    users = await pool.execute("SELECT * FROM users WHERE active = $1", True)

模式 3：使用 @contextmanager 装饰器

对于简单直接的情况，使用该装饰器简化上下文管理器。

from contextlib import contextmanager, asynccontextmanager
import time
import structlog

logger = structlog.get_logger()

@contextmanager
def timed_block(name: str):
    """Time a block of code."""
    start = time.perf_counter()
    try:
        yield
    finally:
        elapsed = time.perf_counter() - start
        logger.info(f"{name} completed", duration_seconds=round(elapsed, 3))

# Usage
with timed_block("data_processing"):
    process_large_dataset()

@asynccontextmanager
async def database_transaction(conn: AsyncConnection):
    """Manage database transaction."""
    await conn.execute("BEGIN")
    try:
        yield conn
        await conn.execute("COMMIT")
    except Exception:
        await conn.execute("ROLLBACK")
        raise

# Usage
async with database_transaction(conn) as tx:
    await tx.execute("INSERT INTO users ...")
    await tx.execute("INSERT INTO audit_log ...")

模式 4：无条件释放资源

无论是否发生异常，始终在 __exit__ 中清理资源。

class FileProcessor:
    """Process file with guaranteed cleanup."""

    def __init__(self, path: str) -> None:
        self._path = path
        self._file: IO | None = None
        self._temp_files: list[Path] = []

    def __enter__(self) -> "FileProcessor":
        self._file = open(self._path, "r")
        return self

    def __exit__(
        self,
        exc_type: type[BaseException] | None,
        exc_val: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> None:
        """Clean up all resources unconditionally."""
        # Close main file
        if self._file is not None:
            self._file.close()

        # Clean up any temporary files
        for temp_file in self._temp_files:
            try:
                temp_file.unlink()
            except OSError:
                pass  # Best effort cleanup

        # Return None/False to propagate any exception

详细示例与模式

详细章节（以 ## Advanced Patterns 开头）位于 references/details.md。当上述导航摘要不够用时，请阅读该文件。

最佳实践总结

1. 始终使用上下文管理器 - 适用于任何需要清理的资源
2. 无条件清理 - __exit__ 即使发生异常也会执行
3. 不要意外抑制异常 - 除非有意抑制，否则返回 False
4. 使用 @contextmanager - 适用于简单的资源模式
5. 实现两种协议 - 同时支持 with 和手动管理
6. 使用 ExitStack - 适用于资源数量动态变化的情况
7. 高效累积 - 使用列表 + join，而非字符串拼接
8. 跟踪指标 - 首字节时间对流式处理至关重要
9. 记录行为 - 尤其是异常抑制
10. 测试清理路径 - 验证出错时资源能够释放