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I don't disagree that OverflowError describes what's happening, but the need to convert to an int in the first place is a detail of the machine - you'd have to know that this is a limitation of whatever internal "protocol" CPython implements - not a description of the cause of the error, which is what the old-style message describes quite clearly.
On the subject of whether __len__ should be able to return long integers, GvR seems to like the idea (from the related bug mentioned earlier):
http://bugs.python.org/issue2690#msg70525
I'll take a closer look at the mechanisms for error reporting around this situation later, but my assertion is that the new-style message isn't as helpful as the old-style one.