test_cached_property.py

# -*- coding: utf-8 -*-
import time


# copy out from django
# https://docs.djangoproject.com/zh-hans/2.0/_modules/django/utils/functional/#cached_property
class cached_property:
    """
    Decorator that converts a method with a single self argument into a
    property cached on the instance.

    Optional ``name`` argument allows you to make cached properties of other
    methods. (e.g.  url = cached_property(get_absolute_url, name='url') )
    """

    def __init__(self, func, name=None):
        self.func = func
        self.__doc__ = getattr(func, "__doc__")
        self.name = name or func.__name__

    def __get__(self, instance, cls=None):
        """
        Call the function and put the return value in instance.__dict__ so that
        subsequent attribute access on the instance returns the cached value
        instead of calling cached_property.__get__().
        """
        if instance is None:
            return self
        res = instance.__dict__[self.name] = self.func(instance)
        return res


class Foo:
    @cached_property
    def foo(self):
        return time.time()


print(dir(Foo))
print(type(Foo.foo))

f = Foo()
print(f.foo)
time.sleep(0.03)
print(f.foo)

# delete L27 and L28
➜ [takachiho] playground git:(master) ✗ python test_cached_property.py
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'foo']
Traceback (most recent call last):
  File "test_cached_property.py", line 40, in <module>
    print(type(Foo.foo))
  File "test_cached_property.py", line 29, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
AttributeError: 'NoneType' object has no attribute '__dict__'

# original code
➜ [takachiho] playground git:(master) ✗ python test_cached_property.py
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'foo']
<class '__main__.cached_property'>
1530107803.403837
1530107803.403837
➜ [takachiho] playground git:(master) ✗

Some related post：

1. Understanding Django's cached_property Decorator
2. django ticket: https://code.djangoproject.com/ticket/19872#no2
3. the pr: https://github.com/django/django/pull/812/files