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装饰器模式

问题​

Python 的装饰器和 GoF 装饰器模式有什么区别？

答案​

GoF 装饰器模式 vs Python 装饰器​

GoF 装饰器模式通过类包装给对象动态添加功能；Python 的 @decorator 语法是函数包装器，用途更广。

函数装饰器（Python 风格）​

import time
import functools
from typing import Callable, TypeVar, ParamSpec

P = ParamSpec("P")
R = TypeVar("R")

def timer(func: Callable[P, R]) -> Callable[P, R]:
    @functools.wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
        start = time.perf_counter()
        result = func(*args, **kwargs)
        elapsed = time.perf_counter() - start
        print(f"{func.__name__} took {elapsed:.3f}s")
        return result
    return wrapper

def retry(max_attempts: int = 3):
    """参数化装饰器"""
    def decorator(func: Callable[P, R]) -> Callable[P, R]:
        @functools.wraps(func)
        def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
            for attempt in range(max_attempts):
                try:
                    return func(*args, **kwargs)
                except Exception as e:
                    if attempt == max_attempts - 1:
                        raise
                    time.sleep(2 ** attempt)
        return wrapper
    return decorator

# 装饰器叠加
@timer
@retry(max_attempts=3)
def fetch_data(url: str) -> dict:
    ...

类装饰器模式（GoF 风格）​

from abc import ABC, abstractmethod

class DataSource(ABC):
    @abstractmethod
    def read(self) -> str: ...
    @abstractmethod
    def write(self, data: str) -> None: ...

class FileDataSource(DataSource):
    def __init__(self, filename: str):
        self.filename = filename
    def read(self) -> str:
        with open(self.filename) as f:
            return f.read()
    def write(self, data: str) -> None:
        with open(self.filename, "w") as f:
            f.write(data)

# 装饰器：添加加密功能
class EncryptionDecorator(DataSource):
    def __init__(self, source: DataSource):
        self._source = source
    def read(self) -> str:
        data = self._source.read()
        return self._decrypt(data)
    def write(self, data: str) -> None:
        self._source.write(self._encrypt(data))
    def _encrypt(self, data: str) -> str: ...
    def _decrypt(self, data: str) -> str: ...

# 装饰器：添加压缩功能
class CompressionDecorator(DataSource):
    def __init__(self, source: DataSource):
        self._source = source
    def read(self) -> str:
        return self._decompress(self._source.read())
    def write(self, data: str) -> None:
        self._source.write(self._compress(data))
    # ...

# 组合使用
source = CompressionDecorator(EncryptionDecorator(FileDataSource("data.txt")))
source.write("hello")  # 先加密，再压缩，最后写文件

常见面试问题​

Q1: 两种"装饰器"的本质区别？​

答案：

GoF 装饰器模式Python @decorator
作用对象对象实例函数/类
实现方式接口+包装类高阶函数
时机运行时动态组合定义时静态包装
典型场景I/O 流包装、组件增强日志、缓存、权限检查

Q2: functools.wraps 有什么用？​

答案：

装饰器会把原函数的 __name__、__doc__ 等元信息替换为 wrapper 的。@functools.wraps(func) 把这些信息拷贝回来，确保调试和文档正常。

Python 装饰器的详细用法参考 Python 基础 - 装饰器。

相关链接​