python - 基于值而不是类型的单一调度

Question

我在 Django 上构建 SPA，我有一个巨大的函数，其中包含许多 if 语句，用于检查我的对象字段的状态名称。像这样:

if self.state == 'new':
    do some logic
if self.state == 'archive':
    do some logic

等等。我现在正在读一本好书“Fluent python”，我提到了 @singledispatch 装饰器，它看起来很棒，但它只能使用不同类型的参数覆盖函数，比如 str , int 等
问题是，如果在 python 或 Django 中有分离逻辑的方式，就像在我的巨大函数中一样，覆盖函数像 singledispatch 呢？

Answer 1

有，虽然你必须写。一种可能性是创建一个 descriptor根据 instance.state 或任何选定的 state_attr 进行调度:

class StateDispatcher(object):

    def __init__(self, state_attr='state'):
        self.registry = {}
        self._state_attr = state_attr

    def __get__(self, instance, owner):
        if instance is None:
            return self

        method = self.registry[getattr(instance, self._state_attr)]
        return method.__get__(instance, owner)

    def register(self, state):
        def decorator(method):
            self.registry[state] = method
            return method

        return decorator

https://docs.python.org/3/howto/descriptor.html#functions-and-methods :

To support method calls, functions include the __get__() method for binding methods during attribute access. This means that all functions are non-data descriptors which return bound or unbound methods depending whether they are invoked from an object or a class.

然后在您的有状态类中，您可以创建一个调度程序并注册方法:

class StateMachine(object):

    dispatcher = StateDispatcher()
    state = None

    @dispatcher.register('test')
    def test(self):
        print('Hello, World!', self.state)

    @dispatcher.register('working')
    def do_work(self):
        print('Working hard, or hardly working?', self.state)

让我们看看它的实际效果:

>>> sm = StateMachine()
>>> sm.state = 'test'
>>> sm.dispatcher()
Hello, World! test
>>> sm.state = 'working'
>>> sm.dispatcher()
Working hard, or hardly working? working
>>> sm.state = None
>>> sm.dispatcher()
Traceback (most recent call last):
  ...
  File "dispatcher.py", line 11, in __get__
    method = self.registry[getattr(instance, self._state_attr)]
KeyError: None

请注意，这是一种基于状态的非常邪恶的调度方法，因为对于您代码的 future 读者来说，整个机制将很难理解。

另一种调度文本状态的方法是在方法名称中对状态进行编码，并根据调度函数中的状态选择正确的方法。许多 Python 类使用这种模式(例如 ast.NodeVisitor):

class StateMachine(object):

    def dispatch(self, *args, **kwgs):
        getattr(self, 'do_{}'.format(self.state))(*args, **kwgs)

    def do_new(self):
        print('new')

    def do_archive(self):
        print('archive')


sm = StateMachine()
sm.state = 'new'
sm.dispatch()
sm.state = 'archive'
sm.dispatch()