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__del__ methods are in general tricky because they are in the general case run asynchronously. Therefore any proposal to "attach" the message to another message is a non-starter.
If a __del__ method depends on attributes set in the __init__, then the programmer needs to decide if they want to handle the possibility of __init__ failing, and therefore __del__ running without __init__ having completed. For the stdlib, I think I'd lean toward handling such cases, in which case IMO the Pythonic thing to do is indeed to have a class attribute to provide the pre-__init__ default.