Decorator @Cached_property
Intro
Defination: Decorator that converts a method with a single self argument into a property cached on the instance.
PYTHON
from functools import cached_propertyimport json5 as jsonclass ConfigLoader: @cached_property def config(self): with open('settings.json', 'r') as f: return json.load(f)config = ConfigLoader().configprint(config)Principle
源码逻辑比较简单，当首次访问被装饰的属性时，执行原始方法，将计算结果存储在instance的 __dict__ 字典中，后续访问时直接返回缓存值，不再重新计算
Two related Dunder methods
Source Code
PYTHON
# python3.13 functools.py################################################################################### cached_property() - property result cached as instance attribute################################################################################_NOT_FOUND = object()class cached_property: def __init__(self, func): self.func = func self.attrname = None self.__doc__ = func.__doc__ self.__module__ = func.__module__ def __set_name__(self, owner, name): # Automatically called at the time the owning class owner is created. if self.attrname is None: self.attrname = name elif name != self.attrname: raise TypeError( "Cannot assign the same cached_property to two different names " f"({self.attrname!r} and {name!r})." ) def __get__(self, instance, owner=None): # Automatically called when the property is accessed. if instance is None: # True when accessed through the class, e.g. MyClass.attr return self if self.attrname is None: raise TypeError( "Cannot use cached_property instance without calling __set_name__ on it.") try: cache = instance.__dict__ except AttributeError: # not all objects have __dict__ (e.g. class defines slots) msg = ( f"No '__dict__' attribute on {type(instance).__name__!r} " f"instance to cache {self.attrname!r} property." ) raise TypeError(msg) from None val = cache.get(self.attrname, _NOT_FOUND) if val is _NOT_FOUND: val = self.func(instance) try: cache[self.attrname] = val except TypeError: msg = ( f"The '__dict__' attribute on {type(instance).__name__!r} instance " f"does not support item assignment for caching {self.attrname!r} property." ) raise TypeError(msg) from None return val __class_getitem__ = classmethod(GenericAlias)View Mermaid diagram code
sequenceDiagram
participant User as 代码
participant Property as cached_property
participant Dict as instance.__dict__
Note over User,Dict: 首次访问属性
User->>Property: 访问属性 (obj.attr)
Property->>Dict: 检查是否有缓存值
Dict-->>Property: 返回 _NOT_FOUND (无缓存)
Property->>Property: 调用原始方法
Property->>Dict: 存储结果
Property-->>User: 返回结果
Note over User,Dict: 后续访问属性
User->>Dict: 访问属性 (obj.attr)
Dict-->>User: 直接返回缓存值
Note right of Dict: Python 查找属性时优先检查 __dict__Scenarios
访问频繁、计算耗时，通常为只读属性
Danger
Note: 如果对数据进行写入缓存并不会自动失效，需要手动del/set缓存
Examples
| Case | Description |
|---|---|
| Cache ORM related fields | 将数据库查询的结果中存到实例中 |
| Cache HTTP Request Object | 解析 JSON、表单数据等计算成本较高的属性 |
PYTHON
# 并非同一个module，但思路是一致的# Demo Ref: https://medium.com/@esatyilmaz/introduction-c1306df1a84cfrom django.utils.functional import cached_propertyclass Book(models.Model): title = models.CharField(max_length=50) author = models.ForeignKey(Author, on_delete=models.CASCADE) @cached_property def author_full_name(self): return self.author.full_nameDecorator @Cached_property