I always intended for int() and str() to case subclasses to the built-in base class, and I don't want to change that rule.
Consider a subclass of int() that overrides __repr__() and __str__() to print something fancy (maybe it defaults to hex; maybe it's an enum :-). I want to be able to say repr(int(x)) and get the standard decimal representation. Same with strings. If int() or str() were allowed to return a subclass instance, this wouldn't work, and I'd have to resort to draconian measures. (Betcha the first few solutions you come up with don't even work. :-) There are plenty of other use cases where a trivial subclass of one of the built-in types is used as some kind of "flag" -- e.g. maybe for values that ought to be serialized differently, or to enable some kind of type checking -- but it should always be possible to convert such values to the underlying base class to remove the special treatment. -- --Guido van Rossum (python.org/~guido
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