Extending and Embedding the Python Interpreter

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2.2.3 Attribute Management Functions

    getattrfunc tp_getattr;
    setattrfunc tp_setattr;

The tp_getattr handle is called when the object requires an attribute look-up. It is called in the same situations where the __getattr__() method of a class would be called.

A likely way to handle this is (1) to implement a set of functions (such as newdatatype_getSize() and newdatatype_setSize() in the example below), (2) provide a method table listing these functions, and (3) provide a getattr function that returns the result of a lookup in that table.

Here is an example:

static PyMethodDef newdatatype_methods[] = {
    {"getSize", (PyCFunction)newdatatype_getSize, METH_VARARGS,
     "Return the current size."},
    {"setSize", (PyCFunction)newdatatype_setSize, METH_VARARGS,
     "Set the size."},
    {NULL, NULL, 0, NULL}           /* sentinel */
};

static PyObject *
newdatatype_getattr(newdatatypeobject *obj, char *name)
{
    return Py_FindMethod(newdatatype_methods, (PyObject *)obj, name);
}

The tp_setattr handler is called when the __setattr__() or __delattr__() method of a class instance would be called. When an attribute should be deleted, the third parameter will be NULL. Here is an example that simply raises an exception; if this were really all you wanted, the tp_setattr handler should be set to NULL.

static int
newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v)
{
    (void)PyErr_Format(PyExc_RuntimeError, "Read-only attribute: \%s", name);
    return -1;
}

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Extending and Embedding the Python Interpreter

Previous: 2.2.2 Object Representation Up: 2.2 Type Methods Next: 2.2.4 Object Comparison

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Release 2.2.3, documentation updated on 30 May 2003.