Because after tuple packing returns (a C return) you still need to perform a Python return. The C tuple packing function cannot return directly from the Python

Because after tuple packing returns (a C return) you still need to perform a Python return. The C tuple packing function cannot return directly from the Python function. Looking at https://github.com/python/cpython/blob/master/Python/ceval.c https://github.com/python/cpython/blob/master/Python/ceval.c, here is the code that needs to be executed after the tuple is constructed: case TARGET(RETURN_VALUE): { retval = POP(); assert(f->f_iblock == 0); assert(EMPTY()); f->f_state = FRAME_RETURNED; f->f_stackdepth = 0; goto exiting; } // ... exiting: if (tstate->use_tracing) { if (tstate->c_tracefunc) { if (call_trace_protected(tstate->c_tracefunc, tstate->c_traceobj, tstate, f, PyTrace_RETURN, retval)) { Py_CLEAR(retval); } } if (tstate->c_profilefunc) { if (call_trace_protected(tstate->c_profilefunc, tstate->c_profileobj, tstate, f, PyTrace_RETURN, retval)) { Py_CLEAR(retval); } } } /* pop frame */ exit_eval_frame: if (PyDTrace_FUNCTION_RETURN_ENABLED()) dtrace_function_return(f); _Py_LeaveRecursiveCall(tstate); tstate->frame = f->f_back; return _Py_CheckFunctionResult(tstate, NULL, retval, __func__); I guess you could duplicate all this code in a special "pack tuple and return from Python function" C function which you could then really tail call.