How to create "immutable" constants in Python

immutable_consts.py

class ContainerOfConstants(type):
    def __new__(metacls, name, bases, namespace):
        namespace["_const_values"] = frozenset(
            value
            for key, value in namespace.items()
            if not key.startswith("_")
        )
        cls = super().__new__(metacls, name, bases, namespace)
        cls._initializing = True
        return cls

    def __init__(cls, name, bases, namespace):
        super().__init__(name, bases, namespace)
        cls._initializing = False
        cls._frozen = True

    def __setattr__(cls, key, value):
        if getattr(cls, "_frozen", False) and not getattr(cls, "_initializing", False):
            if hasattr(cls, key):
                raise AttributeError(f"{key} is immutable and cannot be changed")
        super().__setattr__(key, value)

    def __contains__(cls, item):
        return item in cls._const_values

class Stuff(metaclass=ContainerOfConstants):
    FIRST_CONST = 42
    SECOND_CONST = 17
    THIRD_CONST = 105
    ANOTHER_CONST = -11
    
    
####### test it out #######
try:
    Stuff.FIRST_CONST = 100
except AttributeError as e:
    print(e)

print(Stuff.THIRD_CONST)
print(Stuff._const_values)
print(42 in Stuff)
print(999 in Stuff)
Stuff._const_values.add(999)

####### stdout #######
FIRST_CONST is immutable and cannot be changed
105
frozenset({17, 42, -11, 105})
True
False

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[15], line 10
      8 print(42 in Stuff)
      9 print(999 in Stuff)
---> 10 Stuff._const_values.add(999)

AttributeError: 'frozenset' object has no attribute 'add'