前言
最近在查一个服务的问题时，看到有一段代码if .. elif ... 写了近百行，类似
if command == "xxx": obj = CommandX() obj.run() # ... elif command == "yyy": obj = CommandY() obj.run() # ... elif command == "zzz": obj = CommandZ() obj.run() # ... # ... 翻了下git记录，最开始其实只有两三个条件判断，后来command越加越多，就这么延续下来了。
代码逻辑其实没什么问题，也很简单明了，就是看起来有点丑，而且我还开了比较高的桌面缩放，导致一屏幕几乎都是这段if ... elif
看来看去越发觉得丑，先写个demo看看能不能跑通代码。
方式1, 字典映射
如果需要判断的条件比较少，用字典做映射还是挺方便的，但如果条件多，看起来还是挺丑的。
from abc import ABC, abstractmethod class AbstractCommand(ABC): @abstractmethod def run(self): pass class CommandA(AbstractCommand): def run(self): return "this is command A" class CommandB(AbstractCommand): def run(self): return "this is command B" class CommandC(AbstractCommand): def run(self): return "this is command C" class CommandFactory: @staticmethod def create_command(command_type: str) -> AbstractCommand: command_mapping = { "cmda": CommandA, "cmdb": CommandB, "cmdc": CommandC } cls = command_mapping.get(command_type.lower()) if not cls: raise ValueError(f"Unknown command type: {command_type}") return cls() if __name__ == "__main__": cmd = CommandFactory.create_command("cmda") assert cmd.run() == "this is command A" cmd = CommandFactory.create_command("cmdb") assert cmd.run() == "this is command B" cmd = CommandFactory.create_command("cmdc") assert cmd.run() == "this is command CD" # should be exception cmd = CommandFactory.create_command("cmdd") # should be exception assert cmd.run() == "this is command D" 方式2, __init_subclass__
《流畅的Python(第2版)》的最后一章提到了这个__init__subclass__，根据python官方文档：
当所在类派生子类时此方法就会被调用。cls 将指向新的子类。如果定义为一个普通实例方法，此方法将被隐式地转换为类方法。传给一个新类的关键字参数会被传给上级类的
__init_subclass__。 为了与其他使用__init_subclass__的类兼容，应当去掉需要的关键字参数再将其他参数传给基类。
借助这个机制，可以在实现抽象基类时自动注册子类，避免手动维护注册表。
from abc import ABCMeta, abstractmethod from threading import Lock from collections import UserDict class ThreadSafeDict(UserDict): """线程安全的字典""" def __init__(self): super().__init__() self._lock = Lock() def __setitem__(self, key, item): with self._lock: super().__setitem__(key, item) class Command(metaclass=ABCMeta): registry = ThreadSafeDict() def __init__(self): pass @abstractmethod def run(self): pass def __init_subclass__(cls, **kwargs): super().__init_subclass__(**kwargs) cls.registry[cls.__name__.lower()] = cls # 自动注册子类 # 子类定义即自动注册 class CommandA(Command): def run(self): return "this is command a!" class CommandB(Command): def run(self): return "this is command b!" class CommandC(Command): def run(self): return "this is command b!" def create_command(command_type: str) -> Command: """工厂函数""" cls = Command.registry.get(command_type.lower()) if not cls: raise ValueError(f"Unknown command type: {command_type}") return cls() if __name__ == "__main__": cmd = create_command("CommandA") assert cmd.run() == "this is command a!" cmd = create_command("CommandB") assert cmd.run() == "this is command b!" cmd = create_command("CommandC") assert cmd.run() == "this is command cc!" # should be exception cmd = create_command("CommandD") assert cmd.run() == "this is command b!" # should be exception 乍一看还是挺不错的，但是也有个缺点，那就是如果各个类分散在不同模块中，那么工厂函数所在的模块就要写一堆from xxx import ...
如果module和类命名比较规范，也可以这么动态加载类
import importlib def create_class(module_name, class_name): module = importlib.import_module(module_name) cls = getattr(module, class_name) return cls() 补充
自动注册类看起来炫，但是对代码阅读来说不是很直观。易读还是美观？这是一个问题。
