I was wondering what would happen, so I tried this out for the heck of it with: Python 3.0a2 (py3k:59572M, Dec 19 2007, 15:54:07) [MSC v.1500 32 bit (Intel)] on win32
class a(int): def __new__(cls,number): return int.__new__(cls,number) for x in range(0,a(5)): print(x) Which resulted in a: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "a.py", line 5, in <module> for x in range(0,a(5)): SystemError: ..\Objects\longobject.c:400: bad argument to internal function [41030 refs] It looks like the rangeobject performs a FitsInLong test on each of the parameters to range, which uses the function _PyLong_FitsInLong(PyObject *vv) within longobject.c. In tern, this performs a typecheck: #define PyLong_CheckExact(op) (Py_TYPE(op) == &PyLong_Type) that fails. Interesting! -- http://mail.python.org/mailman/listinfo/python-list