New submission from Yonatan Goldschmidt <yon.goldschm...@gmail.com>:
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. ---------- components: Library (Lib) messages: 379439 nosy: Yonatan Goldschmidt priority: normal severity: normal status: open title: functools.cached_property possibly disables key-sharing instance dictionaries type: performance versions: Python 3.10 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue42127> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
