To add to the suggestions already given in this thread I dug into code I wrote some time ago. Offered as an inspiration. === missing.py === from typing import Any def MISSING(klass: Any) -> Any: """ create a sentinel to indicate a missing instance of a class :param klass: the class of which an instance is missing :return: missing class object """ g = globals() missing_klass_name = f"_MISSING_{klass.__module__}_{klass.__name__}_MISSING_" if missing_klass_name not in g: g[missing_klass_name] = type( missing_klass_name, (klass,), { "__repr__": lambda x: f"MISSING({klass.__name__})", } ) return g[missing_klass_name]() === and as a demo: === demo_missing.py === import pickle from missing import MISSING x = MISSING(str) y = "bar" print(f"{x!r} == {y!r}: {x == y}") print(f"{x!r} is {y!r}: {x is y}") # MISSING(str) == 'bar': False # MISSING(str) is 'bar': False with open("object.pickled", "wb") as f: pickle.dump(x, f) with open("object.pickled", "rb") as f: y = pickle.load(f) print(f"{x!r} == {y!r}: {x == y}") print(f"{x!r} is {y!r}: {x is y}") # MISSING(str) == MISSING(str): True # MISSING(str) is MISSING(str): False def foo(a: int = MISSING(int), b: int = MISSING(int)): print(f"{a=} {isinstance(a, int)}") print(f"{b=} {isinstance(b, int)}") print(f"{a!r} == {b!r}: {a == b}") print(f"{a!r} is {b!r}: {a is b}") foo() # a=MISSING(int) True # b=MISSING(int) True # MISSING(int) == MISSING(int): True # MISSING(int) is MISSING(int): False foo(1) # a=1 True # b=MISSING(int) True # 1 == MISSING(int): False # 1 is MISSING(int): False class Test: ... t = MISSING(Test) print(f"{t=}") # t=MISSING(Test) ===