我在Django上构建SPA,我有一个巨大的函数,有很多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.我现在阅读好书“Fluent python”,我提到了@singledispatch装饰器,它看起来很棒,但它只能用不同类型的参数如str,int等覆盖函数.
问题是,如果在python或Django方式中分离逻辑就像在我的庞大函数中使用像singledispatch那样的重叠函数吗？
解决方法:
有,但你必须写它.一种可能性是创建一个基于instance.state或任何选择的state_attr进行调度的descriptor：
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
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.
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,这是一种基于状态调度的非常恶劣的方法,因为对于未来的代码读者来说,整个机制将很难遵循.
另一种调度文本状态的方法是对方法名称中的状态进行编码,并在调度函数中根据该方法选择正确的方法.许多python类使用此模式(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()
来源：http://www.icode9.com/content-1-231851.html