Lazy class property decorator

问题

I have one django model which needs to do some processing referring the custom user model.

I can't work with the class of this model at class loading time because the loading order of the classes is unknown.

So I need to add some class attributes at runtime, at the moment I'm adding them in the __init__ or __new__ like:

def __new__(cls, *args, **kwargs):
    # hack to avoid INSTALLED_APPS initialization conflicts.
    # get_user_model() can't be called from this module at class loading time,
    # so some class attributes must be added later.
    # Metaclasses could me more appropiate but I don't want to override
    # dango's metaclasses.
    if not hasattr(cls, '_reverse_field_name_to_user'):
        cls._find_reverse_field_name_to_user()
    return Group.__new__(cls, *args, **kwargs)

It works but looks horrible so I've thought about using something like @lazyclassproperty for these attributes.

I've found several @classproperty and @lazyproperty decorators but not one for both and I have no idea how to write one myself.

Question: How could I code such decorator? or suggest another cleaner alternative to my current silly implementation.

回答1:

Pyramid framework has a very nice decorator called reify, but it only works at instance level, and you want class level, so let's modify it a bit

class class_reify(object):
    def __init__(self, wrapped):
        self.wrapped = wrapped
        try:
            self.__doc__ = wrapped.__doc__
        except: # pragma: no cover
            pass

    # original sets the attributes on the instance
    # def __get__(self, inst, objtype=None):
    #    if inst is None:
    #        return self
    #    val = self.wrapped(inst)
    #    setattr(inst, self.wrapped.__name__, val)
    #    return val

    # ignore the instance, and just set them on the class
    # if called on a class, inst is None and objtype is the class
    # if called on an instance, inst is the instance, and objtype 
    # the class
    def __get__(self, inst, objtype=None):
        # ask the value from the wrapped object, giving it
        # our class
        val = self.wrapped(objtype)

        # and set the attribute directly to the class, thereby
        # avoiding the descriptor to be called multiple times
        setattr(objtype, self.wrapped.__name__, val)

        # and return the calculated value
        return val

class Test(object):
    @class_reify
    def foo(cls):
        print("foo called for class", cls)
        return 42

print(Test.foo)
print(Test.foo)

Run the program and it prints

foo called for class <class '__main__.Test'>
42
42

来源：https://stackoverflow.com/questions/18289871/lazy-class-property-decorator

标签

python

django

decorator

lazy-evaluation

class-attributes