GammaLib

Here the code that allows slicing of the GObservations container and that also allows to return elements from the back of the container:

    GObservation* __getitem__(const int& index) {
        if (index >= 0 && index < self->size()) {        // counting from start
            return (*self)[index];
        }
        else if (index < 0 && self->size()+index >= 0) { // counting from end
            return (*self)[self->size()+index];
        }
        else {
            throw GException::out_of_range("__getitem__(int)", index, self->size());
        }
    }
    GObservations* __getitem__(PyObject *param) {
        if (PySlice_Check(param)) {
            Py_ssize_t start = 0;
            Py_ssize_t stop  = 0;
            Py_ssize_t step  = 0;
            Py_ssize_t len   = self->size();
            if (PySlice_GetIndices((PySliceObject*)param, len, &start, &stop, &step) == 0) {
                GObservations* obs = new GObservations;
                if (step > 0) {
                    for (int i = (int)start; i < (int)stop; i += (int)step) {
                        obs->append(*(*self)[i]);
                    }
                }
                else {
                    for (int i = (int)start; i > (int)stop; i += (int)step) {
                        obs->append(*(*self)[i]);
                    }
                }
                return obs;
            }
            else {
                throw GException::invalid_argument("__getitem__(PyObject)","Invalid slice indices");
            }
        }
        else {
            throw GException::invalid_argument("__getitem__(PyObject)","");
        }
    }