Passing list with flexible length to C extension

Fredrik Lundh fredrik at pythonware.com
Fri Nov 21 14:23:43 CET 2003


Bostjan Jerko wrote:

> Is there a way to pass list with flexible length to C extension?
> I want to pass flexible length list of floats to method and just can't get info
> if it is possible and how to do it.

use a PyObject* variable to hold the list object (use "O" for the type
code), and use the sequence API to loop over the list contents:

    http://www.python.org/doc/current/api/sequence.html

unless your sequences are really large, the PySequence_Fast API is the
most efficient way to extract the items (it applies tuple() to the source,
unless it's already a tuple or a list, and uses direct access to the object
structure to pick out individual items):

...

    PyObject* obj;
    PyObject* seq;
    int i, len;

    if (!PyArg_ParseTuple(args, "O", &obj))
        return NULL;

    seq = PySequence_Fast(obj, type_error_msg);
    len = PySequence_Size(obj);
    for (i = 0; i < len; i++) {
        item = PySequence_Fast_GET_ITEM(seq, i);
        ...
        /* DON'T DECREF item here */
    }
    Py_DECREF(seq);

...

you'll find plenty of examples in the source code for the standard library
grep for PySequence and look for GetItem, GET_ITEM, and Fast_GET_ITEM
calls...)

you can also use the PyList API, but that only works for lists, and is only
marginally faster than the PySequence_Fast API.

    http://www.python.org/doc/current/api/listObjects.html

(to squeeze out the last cycles from PySequence_Fast, expand the loop
body yourself:

    seq = PySequence_Fast(obj, type_error_msg);
    len = PySequence_Size(obj);
    if (PyList_Check(seq))
        for (i = 0; i < len; i++) {
            item = PyList_GET_ITEM(seq, i);
            ...
        }
    else
        for (i = 0; i < len; i++) {
            item = PyTuple_GET_ITEM(seq, i);
            ...
        }
    Py_DECREF(seq);

this design is as fast as a list-only implementation, but still supports
tuples (fast) and other sequence objects, including arbitrary iterable
objects (slower)).

</F>








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