I'm glad you caught that! First things first: the converted code should behave identically to the existing code, including raising the same exceptions. If you examine the exception hierarchy: http://docs.python.org/3.4/library/exceptions.html#exception-hierarchy you'll see that "OverflowError" is a subclass of "ArithmeticError". In other words, it represents when you perform an arithmetic operation that overflows the result type. Using it to also represent "you specified a value that is out of range for this conversion" seems wrong. So I like #3 as well. Could _PyLong_UnsignedInt_Converter catch the OverflowError raised by PyLong_AsUnsignedLong and reraise it as ValueError?