Possibly something like this? (Code for illustration purposes only, of course -- please no one use this.) class AntiAntiPattern: 2 """A quick and

Possibly something like this? (Code for illustration purposes only, of course -- please no one use this.) class AntiAntiPattern: 2 """A quick and dirty attempt to mock a Python class instance that 'undoes' the antipattern 3 described here: 4 https://docs.quantifiedcode.com/python-anti-patterns/correctness/mutable_default_value_as_argument.html)""" 5 __mutable_args__ = {} # TODO: a real implementation would probably use a WeakMap of some sort 6 def __init__(self, a, b=None, c={}, d=set()): 7 self.a = a 8 self.b = "foo" if b is None else b 9 self.c = c 10 self.d = "bar" if d is None else d 11 def __getattr__(self, k): 12 return self.__mutable_args__[k] if k in self.__mutable_args__ else super().getattr(k) 13 def __setattr__(self, k, v): 14 if v in self.__init__.__defaults__: 15 self.__mutable_args__[k] = v 16 else: 17 self.__dict__[k] = v 18 def __repr__(self): 19 return json.dumps(dict(self.__dict__, **self.__mutable_args__), default=str) >>> t = AntiAntiPattern(4, d = 'hello!') >>> t.__mutable_args__ {'b': None, 'c': {}} >>> t.__dict__ {'a': 4, 'd': 'hello!'} >>> t {"a": 4, "d": "hello!", "b": null, "c": {}} Edit: Oops, fixed a tiny logic bug in __init__. Trying to ween myself off of the 'a = a or "my_a_default"' syntax as instructed by previous commenters in this thread. :) Edit2: It dawned on me that this doesn't actually resolve the 'gotcha' unless `v` is replaced with `deepcopy(v)` on line 15; otherwise the same exact problem rears its head: >>> t = AntiAntiPattern(3, {}) >>> t.d set() >>> t.d.update([3,4]) >>> t {"a": 3, "b": {}, "c": {}, "d": "{3, 4}"} >>> t2 = AntiAntiPattern(4) >>> t2 {"a": 4, "b": {}, "c": {}, "d": "{3, 4}"} # aw, heck In which case I gather `__mutable_args__` is superfluous, and the solution involves simply making sure that class instances avoid pointing to the same memory objects?