New submission from Tal Einat <taleinat+pyt...@gmail.com>:
I recently noticed that some functions use sentinel values to differentiate between not having been passed any value vs. None. One example of this is traceback.print_exception(), hence I'm adding Irit to the nosy list. However, using e.g. `sentinel = object()` or a single instance of a dedicated class/type can break when combined with pickling (see examples below). Since these sentinel are usually compared using the `is` operator, having more than a single instance will break things, sometimes in very confusing ways. I suggest ensuring that a single instance is always used, probably by using a class with a __new__() method making sure to always return a single instance. >>> sentinel = object() >>> sentinel2 = pickle.loads(pickle.dumps(sentinel)) >>> sentinel is sentinel2 False >>> sentinel <object object at 0x7fd00a986560> >>> sentinel2 <object object at 0x7fd00a986500> >>> class A: ... pass ... >>> a = A() >>> a2 = pickle.loads(pickle.dumps(a)) >>> a is a2 False >>> a <__main__.A object at 0x7fd00a9972f0> >>> a2 <__main__.A object at 0x7fd009599450> ---------- components: Library (Lib) messages: 393580 nosy: iritkatriel, taleinat priority: normal severity: normal status: open title: make function parameter sentinel value true singletons type: behavior versions: Python 3.10, Python 3.11 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue44123> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
