One use-case I have where __setattr__ would be useful is for a configuration attribute that at one point had permissible values True, False and 'auto'. 'auto' was deprecated, but the DeprecationWarning is only emitted when someone tries to use a function that depends on that value. With __setattr__, I would be able to emit a warning at the point someone tries to set a deprecated value. One can imagine a case where a downstream tool sets the value, but only a few users ever trigger the behavior, so it might not get caught through normal inspection of test logs.
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