Python从装饰器返回额外的值

Question

I have some functions and I want to do some computation based on the argument values passed to it in a decorator. I am able to achieve this but the decorator populates an important object which I would like to reuse later on. eg..

def my_awesome_decorator(*d):
    how_do_i_return_this_object_which_got_created_here
    def wrapping_function(func):
        def wrapper(*args, **kwargs):
            ...
        return wrapper
    return wrapping_function


class A:
    @my_awesome_decorator(d)
    def fun(a, b, c):
        ...

I want the object to be easily accessible using object_A.how_do_i_return_this_object_which_got_created_here

Answer 1

This is not something a decorator can do. All that a decorator can do is return something different for the given function object. You don't have a choice what the return value is assigned to, in the namespace.

Better options are:

• using an attribute on the decorated function:

   def my_awesome_decorator(*d): def wrapping_function(func): def wrapper(*args, **kwargs): # ... wrapper.some_attribute = ... return wrapper return wrapping_function 

  after which ClassObject.methodname.some_attribute and instance.methodname.some_attribute are available.

• For Python 3.6 or newer, you can add a __init_subclass__ method to a baseclass for your classes. It could combine with an attribute set by your decorator to copy an attribute over to the class:

   class BaseClass: def __init_subclass__(cls, **kwargs): super().__init_subclass__() special = None for attr in vars(cls).values(): special = getattr(attr, '_special_attribute_name', None) if special is not None: break cls.target_attribute = special 

  after which subclasses will copy across the first method._special_attribute_name attribute that is found to the class object; a decorator can set that attribute on a method.

• For earlier Python versions, use a metaclass to do the same:

   class SomeMetaClass(type): def __new__(mcls, name, bases, namespace, **kwds): special = None for attr in namespace.values(): special = getattr(attr, '_special_attribute_name', None) if special is not None: break namespace['target_attribute'] = special return super().__new__(mcls, name, bases, namespace, **kwds)