問題描述

2017/2/6

比如, 考慮這樣一段代碼, 它的執行流程是怎樣的呢 ?

class Foo(object): @cached_property def foo(self):# calculate something important herereturn 42f = Foo()f.foof.foo 相關代碼

以class為基礎的緩存裝飾器

class cached_property(property): '''A decorator that converts a function into a lazy property. The function wrapped is called the first time to retrieve the result and then that calculated result is used the next time you access the value::class Foo(object): @cached_property def foo(self):# calculate something important herereturn 42 The class has to have a `__dict__` in order for this property to work. ''' # implementation detail: A subclass of python’s builtin property # decorator, we override __get__ to check for a cached value. If one # choses to invoke __get__ by hand the property will still work as # expected because the lookup logic is replicated in __get__ for # manual invocation. def __init__(self, func, name=None, doc=None):self.__name__ = name or func.__name__self.__module__ = func.__module__self.__doc__ = doc or func.__doc__self.func = func def __set__(self, obj, value):obj.__dict__[self.__name__] = value def __get__(self, obj, type=None):if obj is None: return selfvalue = obj.__dict__.get(self.__name__, _missing)if value is _missing: value = self.func(obj) obj.__dict__[self.__name__] = valuereturn value上下文環境

產品版本: Python2

操作系統: Linux

相似的問題: http://stackoverflow.com/ques...

問題解答

cached_property 是 property 的subclass, 復寫了 __get__, __set__ 方法.

cached_property 是一個描述器(資料描述器),獲取屬性的時候優先從描述器獲取,即(__get__).

所以執行流程就是:f.foo -> __get__ -> 從實例字典(f.__dict__)獲取 -> 如果沒有則保存到字典并調用實際方法返回

def cached_property(func): def _deco(*args, **kwargs):print(22222222222222)ret = func(*args, **kwargs) #這是調用foo方法print(44444444444444)return ret return _decoclass Foo(object): def __init__(self):print (111111111111111111) @cached_property def foo(self):print(3333333333333)return 42f = Foo()f.foo()