2017/2/6
比如, 考虑这样一段代码, 它的执行流程是怎样的呢 ?
class Foo(object): @cached_property def foo(self): # calculate something important here return 42 f = Foo() f.foo f.foo
以class为基础的缓存装饰器
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 here return 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 self value = obj.__dict__.get(self.__name__, _missing) if value is _missing: value = self.func(obj) obj.__dict__[self.__name__] = value return value
产品版本: Python2
操作系统: Linux
相似的问题: http://stackoverflow.com/ques...
def cached_property(func): def _deco(*args, **kwargs): print(22222222222222) ret = func(*args, **kwargs) #这是调用foo方法 print(44444444444444) return ret return _deco class Foo(object): def __init__(self): print (111111111111111111) @cached_property def foo(self): print(3333333333333) return 42 f = Foo() f.foo()
cached_property
是 property
的subclass, 复写了 __get__
, __set__
方法.
cached_property
是一个描述器(资料描述器),获取属性的时候优先从描述器获取,即(__get__
).
所以执行流程就是:f.foo
-> __get__
-> 从实例字典(f.__dict__
)获取 -> 如果没有则保存到字典并调用实际方法返回