Key-sharing dictionaries, defined by https://www.python.org/dev/peps/pep-0412/, require that any resizing of the shared dictionary keys will happen before a second instance of the class is created.

cached_property inserts its resolved result into the instance dict after it is called. This is likely to happen *after* a second instance has been created, and it is also likely to cause a resize of the dict, as demonstrated by this snippet:

    from functools import cached_property
    import sys

    def dict_size(o):
        return sys.getsizeof(o.__dict__)

    class X:
        def __init__(self):
            self.a = 1
            self.b = 2
            self.c = 3
            self.d = 4
            self.e = 5

        @cached_property
        def f(self):
            return id(self)

    x1 = X()
    x2 = X()

    print(dict_size(x1))
    print(dict_size(x2))

    x1.f

    print(dict_size(x1))
    print(dict_size(x2))

    x3 = X()
    print(dict_size(x3))

Essentially it means that types using cached_property are less likely to enjoy the benefits of shared keys. It may also incur a certain performance hit, because a resize + unshare will happen every time.

A simple way I've thought of to let cached_property play more nicely with shared keys, is to first create a single object of the class, and set the cached_property attribute to some value (so the key is added to the shared dict). In the snippet above, if you add "x0 = X(); x0.f = None" before creating x1 and x2, you'll see that the cached_property resolving does not unshare the dicts.

But I wonder if there's a way to do so without requiring user code changes.

> Essentially it means that types using cached_property are less
> likely to enjoy the benefits of shared keys.

I don't think anything can be done about it.  @cached_property and key-sharing dicts are intrinsically at odds with one another.  Likewise, @cached_property doesn't work with classes that define __slots__.

FWIW, there is an alternative that works with both key-sharing dicts and __slots__.  You can stack property() on top of functools.cache():

    class A:
        def __init__(self, x):
                self.x = x

        @property
        @cache
        def square(self):
                print('Called!')
                return self.x ** 2

            
    >>> a = A(10)
    >>> a.square
    Called!
    100
    >>> b = A(11)
    >>> b.square
    Called
    121
    >>> a.square
    100
    >>> b.square
    121

New changeset 48be6b1ef7a6201e13c87a317361cdb60bd5faa8 by Raymond Hettinger in branch 'master':
bpo-42127:  Document effect of cached_property on key-sharing dictionaries (GH-22930)
https://github.com/python/cpython/commit/48be6b1ef7a6201e13c87a317361cdb60bd5faa8

New changeset 427cb0aa78813b89a3f100073bf7d70a53510f57 by Miss Skeleton (bot) in branch '3.9':
bpo-42127:  Document effect of cached_property on key-sharing dictionaries (GH-22930) (GH-22955)
https://github.com/python/cpython/commit/427cb0aa78813b89a3f100073bf7d70a53510f57