Eryk Sun <eryksun@gmail.com> added the comment: The error code for `1e+300 ** 2` is ERANGE, from calling libm pow(). Since pow() returns a double, there's no way to indicate an error in the return value. Instead, C errno is set to 0 beforehand and checked for a non-zero error value after the call. If the error code isn't ERANGE, then float.__pow__() raises ValueError instead of OverflowError. Here's the relevant snippet from float_pow() in Objects/floatobject.c: /* Now iv and iw are finite, iw is nonzero, and iv is * positive and not equal to 1.0. We finally allow * the platform pow to step in and do the rest. */ errno = 0; ix = pow(iv, iw); _Py_ADJUST_ERANGE1(ix); if (negate_result) ix = -ix; if (errno != 0) { /* We don't expect any errno value other than ERANGE, but * the range of libm bugs appears unbounded. */ PyErr_SetFromErrno(errno == ERANGE ? PyExc_OverflowError : PyExc_ValueError); return NULL; } return PyFloat_FromDouble(ix); Here's a direct example using ctypes: import ctypes import errno libm = ctypes.CDLL('libm.so.6', use_errno=True) libm.pow.argtypes = (ctypes.c_double, ctypes.c_double) libm.pow.restype = ctypes.c_double >>> errno.ERANGE 34 >>> ctypes.set_errno(0) 0 >>> libm.pow(1e300, 2) inf >>> ctypes.get_errno() == errno.ERANGE True ---------- nosy: +eryksun _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue47134> _______________________________________