[Python-Dev] Q: why doesn't list.extend use the sequence interface?

[Fredrik Lundh]

jeremy wrote:

>   FL> has anyone benchmarked the abstract sequence API?  how much
>   FL> slower is it?  would it be a good idea to treat lists as a
>   FL> special case, to avoid slowing down existing code?
>=20
> The abstract interface is fairly expensive.  The current code uses
> PyList_GET_ITEM to access the elements of the list.  The abstract
> interface adds two function calls (PySequence_GetItem,
> PyList_GetItem).  The first function just does a couple of sanity
> checks (verify arg is sequence & that it has get_item).  The second
> function verifies that the arg is a list, then does a bounds check.
> None of the work that either of these functions does is necessary for
> the list case!

just for the record, I just changed some generic sequence code
to something like:

    if (PyList_Check(seq) || PyTuple_Check(seq))
        Py_INCREF(seq);
    else if (PySequence_Check(seq)) {
        seq =3D PySequence_Tuple(seq);
        if (!seq)
            return NULL;
    } else {
        PyErr_SetString(PyExc_TypeError, "argument must be a sequence");
        return NULL;
    }

    ... allocate PyObject_Length(seq) slots ...

    if (PyList_Check(seq))
        ... list code using PyList_GET_ITEM
    else
        ... tuple code using PyTuple_GET_ITEM

    Py_DECREF(seq);

for the standard case (lists, in this case), this resulted in a
total speedup of 4 times (this includes some other overhead).

so in other words, this construct is more than 4 times faster
than plain use of PySequence_GetItem.

guess it's time to fix a couple of things in PIL...

</F>