[Python-Dev] tuple[int] optimization (original) (raw)

Andrew Svetlov andrew.svetlov at gmail.com
Sun Jul 24 00:20:08 CEST 2011

tuple[int] is 30% slower than list[int]. Please let me explain the problem.

(1, 2, 3)[1] has compiled as BINARY_SUBSCR operation. ceval.c has optimization for list:

    case BINARY_SUBSCR:
        w = POP();
        v = TOP();
        if (PyList_CheckExact(v) && PyInt_CheckExact(w)) {
            /* INLINE: list[int] */
            Py_ssize_t i = PyInt_AsSsize_t(w);
            if (i < 0)
                i += PyList_GET_SIZE(v);
            if (i >= 0 && i < PyList_GET_SIZE(v)) {
                x = PyList_GET_ITEM(v, i);
                Py_INCREF(x);
            }
            else
                goto slow_get;
        }
        else
          slow_get:
            x = PyObject_GetItem(v, w);
        Py_DECREF(v);
        Py_DECREF(w);
        SET_TOP(x);
        if (x != NULL) continue;
        break;

For tuple code follows slow way via tp_as_mapping slot and calls tuplesubscript from Objects/tupleobject.c

Looks like tuple is goot applicant to be optimized as list does. tuples is used in python programs very often and getting element from tuple by index as common operation as list[int].

Is there reasons to prevent special case for tuples in BINARY_SUBSCR?

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