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)
===
