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from typing import Any
_frozen_classes: dict[type, type] = {}
# make a derived class freezable (ie, disallow modifications).
# we do this by changing the class of an instance at runtime when freeze()
# is called, providing a derived class that is exactly the same except
# for a __setattr__ that raises an error when called. this beats having
# a field for frozenness and an unconditional __setattr__ that checks this
# field because it does not insert anything into the class dict.
class Freezeable:
def freeze(self) -> None:
cls = type(self)
if not (frozen := _frozen_classes.get(cls)):
def __setattr__(instance: Any, n: str, v: Any) -> None:
raise TypeError(f'{cls.__name__} is frozen')
frozen = type(cls.__name__, (cls,), {
'__setattr__': __setattr__,
})
_frozen_classes[cls] = frozen
self.__class__ = frozen
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