Yesterday at the SciPy conference I suggested it would be a good idea to add a function to replace one of the inner loops in a ufunc so that an extension module could test an optimized inner loop more easily. It turns out that this is such a good idea that it's actually already in the code. The UFUNC_API call that does this is called: PyUFunc_ReplaceLoopBySignature You would use it something like this static void _new_add_loopfunc(char **args, intp *dimensions, intp *steps, void *func) { /* my new faster code */ } Then in the extension module (either in the init or in a method call) something like this: PyUFuncObject *ufunc; PyUFuncGenericFunction oldloopfunc, dummy; int signature[3] /* This should be the number of arguments to the ufunc you are replacing */ PyObject *module, *dict; module = PyImport_ImportModule("umath"); dict = PyModule_GetDict(module); ufunc = PyDict_GetItemString(dict, "add") /* get the ufunc where you want to replace one of the loops */ signature[0] = NPY_DOUBLE; signature[1] = NPY_DOUBLE; signature[2] = NPY_DOUBLE; if (PyUFunc_ReplaceLoopBySignature(ufunc, _new_add_loopfunc, signature, &old1dloop) < 0) { PyErr_SetString(PyExc_RuntimeError, "problem replacing loop"); /* error return i.e. return NULL or return or return -1*/ } Then to restore the original function: if (PyUFunc_ReplaceLoopBySignature(ufunc, old1dloop, signature, &dummy) < 0) { /* error */ } Remember to use import_umath() in the module you are using the UFUNC_API in. Now, lot's of folks should be able to test optimizations. -Travis