Python：对装饰器非常困惑

Question

我原以为我懂装饰器，但现在不太确定了。装饰器是只在函数创建的时候才起作用吗？

我想创建一系列函数，这些函数都有一个必需的参数，叫做'ticket_params'，它是一个字典。然后我想用类似@param_checker(['req_param_1', 'req_param_2'])的装饰器来装饰这些函数。如果字典里没有'req_param_1'和'req_param_2'，就抛出一个自定义的异常子类。我这样想是不是错了？

在调用代码中大概是这样的：

@param_checker(['req_param_1', 'req_param_2'])
def my_decorated_function(params):
    # do stuff

params = {'req_param_1': 'Some Value'}
my_decorated_function(params)

# exception would be raised here from decorator.

Answer 1

这里有一个完整的例子，基于@AndiDog的示例。记住，任何可以调用的东西都可以用作装饰器，它不一定要是一个类。

class MyCustomException(Exception):
    pass

# The decorator - instances of this class are callable as it implements __call__
class param_checker:
    # In this example l is the parameter you pass to the decorator. 
    # For example, l could be ['req_param_1', 'req_param_2'].
    def __init__(self, l):
        self.l = l

    # This makes the instance callable
    def __call__(self, functionToBeDecorated):
        def wrapper(*args, **kwargs):
            # For the successful call below args = () and
            # kwargs = {'ticket_params': {'req_param_1': 'param_1', 'req_param_2': 'param_2'}}
            if "ticket_params" not in kwargs or any(necessary not in kwargs["ticket_params"] for necessary in self.l):
                # if the ticket params parameter has not been specified, or if
                # any of the required parameters are not present raise an exception
                raise MyCustomException
            return functionToBeDecorated(*args, **kwargs)
        return wrapper

@param_checker(['req_param_1', 'req_param_2'])
def myfunction(ticket_params=None): 
    # if the two required params are present this will print
    print "params ", ticket_params

if __name__ == "__main__":
    try:
        myfunction()
    except MyCustomException:
        print "all required params not supplied"
    try:
        myfunction(ticket_params={'req_param_1': 'param_1'})
    except MyCustomException:
        print "all required params not supplied"
    myfunction(ticket_params={'req_param_1': 'param_1', 'req_param_2': 'param_2'})

Answer 2

装饰器只会在一个函数上被调用一次，也就是说，当我们定义这个函数的时候，它会被解析成这样：

@mydecorator
def myfunction(): ...

我猜你是想说类似这样的意思：

class param_checker:
  def __init__(self, l):
    self.l = l

  def __call__(self, functionToBeDecorated):
    def wrapper(*args, **kwargs):
      if any(necessary not in kwargs["ticket_params"] for necessary in self.l):
        raise MyCustomException
      return functionToBeDecorated(*args, **kwargs)

    return wrapper

如果你不明白，请告诉我哦；)

Answer 3

装饰器是在def语句后面立即应用的；它们的关系是：

@param_checker(['req_param_1', 'req_param_2'])
def my_decorated_function(params):
    # do stuff

这完全是一样的意思：

def my_decorated_function(params):
    # do stuff
my_decorated_function = param_checker(['req_param_1', 'req_param_2'])(my_decorated_function)

所以param_checker的工作是返回一个函数，这个函数接收要被装饰的函数作为参数，并返回另一个函数，这个新函数会执行你需要的操作。到这里明白了吗？

编辑：那么，这里有一个实现的例子...：

import functools

def param_checker(reqs):
  reqs = set(reqs)
  def middling(f):
    @functools.wraps(f)
    def wrapper(params):
      missing = reqs.difference(params)
      if missing:
        raise TypeError('Missing parms: %s' % ', '.join(sorted(missing)))
      return f(params)
    return wrapper
  return middling