Question

如何实现对象属性的延迟加载，即如果属性被访问但尚不存在，则调用一些应该加载这些属性的对象方法？

我的第一次尝试是

def lazyload(cls):
    def _getattr(obj, attr):
        if "_loaded" not in obj.__dict__:
            obj._loaded=True
            try:
                obj.load()
            except Exception as e:
                raise Exception("Load method failed when trying to access attribute '{}' of object\n{}".format(attr, e))
            if attr not in obj.__dict__:
                AttributeError("No attribute '{}' in '{}' (after loading)".format(attr, type(obj))) # TODO: infinite recursion if obj fails
            return getattr(obj, attr)
        else:
            raise AttributeError("No attribute '{}' in '{}' (already loaded)".format(attr, type(obj)))

    cls.__getattr__=_getattr
    return cls

@lazyload
class Test:
    def load(self):
         self.x=1

t=Test()     # not loaded yet
print(t.x)   # will load as x isnt known yet

我将仅对某些属性名称进行lazyload。由于我还没有做过很多元分类，我不确定这是不是正确的方法。你会建议什么？

Answer 1

看起来像一个简单的property会更好地解决这个问题：

@property
def my_attribute():
    if not hasattr(self, '_my_attribute'):
        do_expensive_operation_to_get_attribute()
    return self._my_attribute

Answer 2

从lazy看django/utils/functionals.py

https://docs.djangoproject.com/en/2.1/_modules/django/utils/functional

延迟加载属性

2 个答案: