Python, 这一个缓存装饰器, 其执行流程是怎样的?
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问题描述
问 题
2017/2/6
描述
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
相关代码
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
上下文环境
产品版本: Python2
操作系统: Linux
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解决方案
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
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