# Example: MultiMapping objects

Copyright (C) 1996-1998, Digital Creations.

As an example, consider an extension class that implements a "MultiMapping". A multi-mapping is an object that encapsulates 0 or more mapping objects. When an attempt is made to lookup an object, the encapsulated mapping objects are searched until an object is found.

Consider an implementation of a MultiMapping extension type, without use of the extension class mechanism:

        #include "Python.h"

        #define UNLESS(E) if(!(E))

        typedef struct {
            PyObject_HEAD
            PyObject *data;
        } MMobject;

        staticforward PyTypeObject MMtype;

        static PyObject *
        MM_push(MMobject *self, PyObject *args){
            PyObject *src;
            UNLESS(PyArg_ParseTuple(args, "O", &src)) return NULL;
            UNLESS(-1 != PyList_Append(self->data,src)) return NULL;
            Py_INCREF(Py_None);
            return Py_None;
        }

        static PyObject *
        MM_pop(MMobject *self, PyObject *args){
            long l;
            PyObject *r;
            static PyObject *emptyList=0;

            UNLESS(emptyList) UNLESS(emptyList=PyList_New(0)) return NULL;
            UNLESS(PyArg_ParseTuple(args, "")) return NULL;
            UNLESS(-1 != (l=PyList_Size(self->data))) return NULL;
            l--;
            UNLESS(r=PySequence_GetItem(self->data,l)) return NULL;
            UNLESS(-1 != PyList_SetSlice(self->data,l,l+1,emptyList)) goto err;
            return r;
        err:
            Py_DECREF(r);
            return NULL;
        }

        static struct PyMethodDef MM_methods[] = {
            {"push", (PyCFunction) MM_push, 1,
             "push(mapping_object) -- Add a data source"},
            {"pop",  (PyCFunction) MM_pop,  1,
             "pop() -- Remove and return the last data source added"},
            {NULL,              NULL}           /* sentinel */
        };

        static PyObject *
        newMMobject(PyObject *ignored, PyObject *args){
            MMobject *self;

            UNLESS(PyArg_ParseTuple(args, "")) return NULL;
            UNLESS(self = PyObject_NEW(MMobject, &MMtype)) return NULL;
            UNLESS(self->data=PyList_New(0)) goto err;
            return (PyObject *)self;
        err:
            Py_DECREF(self);
            return NULL;
        }

        static void
        MM_dealloc(MMobject *self){
            Py_XDECREF(self->data);
            PyMem_DEL(self);
        }

        static PyObject *
        MM_getattr(MMobject *self, char *name){
            return Py_FindMethod(MM_methods, (PyObject *)self, name);
        }

        static int
        MM_length(MMobject *self){
            long l=0, el, i;
            PyObject *e=0;

            UNLESS(-1 != (i=PyList_Size(self->data))) return -1;
            while(--i >= 0)
              {
                e=PyList_GetItem(self->data,i);
                UNLESS(-1 != (el=PyObject_Length(e))) return -1;
                l+=el;
              }
            return l;
        }

        static PyObject *
        MM_subscript(MMobject *self, PyObject *key){
            long i;
            PyObject *e;

            UNLESS(-1 != (i=PyList_Size(self->data))) return NULL;
            while(--i >= 0)
              {
                e=PyList_GetItem(self->data,i);
                if(e=PyObject_GetItem(e,key)) return e;
                PyErr_Clear();
              }
            PyErr_SetObject(PyExc_KeyError,key);
            return NULL;
        }

        static PyMappingMethods MM_as_mapping = {
                  (inquiry)MM_length,           /*mp_length*/
                  (binaryfunc)MM_subscript,             /*mp_subscript*/
                  (objobjargproc)NULL,          /*mp_ass_subscript*/
        };

        /* -------------------------------------------------------- */

        static char MMtype__doc__[] =
        "MultiMapping -- Combine multiple mapping objects for lookup"
        ;

        static PyTypeObject MMtype = {
                  PyObject_HEAD_INIT(&PyType_Type)
                  0,                            /*ob_size*/
                  "MultMapping",                        /*tp_name*/
                  sizeof(MMobject),             /*tp_basicsize*/
                  0,                            /*tp_itemsize*/
                  /* methods */
                  (destructor)MM_dealloc,               /*tp_dealloc*/
                  (printfunc)0,                 /*tp_print*/
                  (getattrfunc)MM_getattr,      /*tp_getattr*/
                  (setattrfunc)0,                       /*tp_setattr*/
                  (cmpfunc)0,                   /*tp_compare*/
                  (reprfunc)0,                  /*tp_repr*/
                  0,                            /*tp_as_number*/
                  0,                            /*tp_as_sequence*/
                  &MM_as_mapping,                       /*tp_as_mapping*/
                  (hashfunc)0,                  /*tp_hash*/
                  (ternaryfunc)0,                       /*tp_call*/
                  (reprfunc)0,                  /*tp_str*/

                  /* Space for future expansion */
                  0L,0L,0L,0L,
                  MMtype__doc__ /* Documentation string */
        };

        static struct PyMethodDef MultiMapping_methods[] = {
            {"MultiMapping", (PyCFunction)newMMobject, 1,
             "MultiMapping() -- Create a new empty multi-mapping"},
            {NULL,              NULL}           /* sentinel */
        };

        void
        initMultiMapping(){
            PyObject *m;

            m = Py_InitModule4(
                "MultiMapping", MultiMapping_methods,
                  "MultiMapping -- Wrap multiple mapping objects for lookup",
                  (PyObject*)NULL,PYTHON_API_VERSION);

            if (PyErr_Occurred())
               Py_FatalError("can't initialize module MultiMapping");
        }

This module defines an extension type, `MultiMapping`, and exports a module function, `MultiMapping`, that creates `MultiMapping` Instances. The type provides two methods, `push`, and `pop`, for adding and removing mapping objects to the multi-mapping. The type provides mapping behavior, implementing mapping length and subscript operators but not mapping a subscript assignment operator.

Now consider an extension class implementation of MultiMapping objects:

        #include "Python.h"
        #include "ExtensionClass.h"

        #define UNLESS(E) if(!(E))

        typedef struct {
            PyObject_HEAD
            PyObject *data;
        } MMobject;

        staticforward PyExtensionClass MMtype;

        static PyObject *
        MM_push(self, args)
                  MMobject *self;
                  PyObject *args;
        {
            PyObject *src;
            UNLESS(PyArg_ParseTuple(args, "O", &src)) return NULL;
            UNLESS(-1 != PyList_Append(self->data,src)) return NULL;
            Py_INCREF(Py_None);
            return Py_None;
        }

        static PyObject *
        MM_pop(self, args)
                  MMobject *self;
                  PyObject *args;
        {
            long l;
            PyObject *r;
            static PyObject *emptyList=0;

            UNLESS(emptyList) UNLESS(emptyList=PyList_New(0)) return NULL;
            UNLESS(PyArg_ParseTuple(args, "")) return NULL;
            UNLESS(-1 != (l=PyList_Size(self->data))) return NULL;
            l--;
            UNLESS(r=PySequence_GetItem(self->data,l)) return NULL;
            UNLESS(-1 != PyList_SetSlice(self->data,l,l+1,emptyList)) goto err;
            return r;
        err:
            Py_DECREF(r);
            return NULL;
        }

        static PyObject *
        MM__init__(self, args)
               MMobject *self;
               PyObject *args;
        {
            UNLESS(PyArg_ParseTuple(args, "")) return NULL;
            UNLESS(self->data=PyList_New(0)) goto err;
            Py_INCREF(Py_None);
            return Py_None;
        err:
            Py_DECREF(self);
            return NULL;
        }

        static struct PyMethodDef MM_methods[] = {
            {"__init__", (PyCFunction)MM__init__, 1,
             "__init__() -- Create a new empty multi-mapping"},
            {"push", (PyCFunction) MM_push, 1,
             "push(mapping_object) -- Add a data source"},
            {"pop",  (PyCFunction) MM_pop,  1,
             "pop() -- Remove and return the last data source added"},
            {NULL,              NULL}           /* sentinel */
        };

        static void
        MM_dealloc(self)
               MMobject *self;
        {
            Py_XDECREF(self->data);
            PyMem_DEL(self);
        }

        static PyObject *
        MM_getattr(self, name)
                  MMobject *self;
                  char *name;
        {
            return Py_FindMethod(MM_methods, (PyObject *)self, name);
        }

        static int
        MM_length(self)
                  MMobject *self;
        {
            long l=0, el, i;
            PyObject *e=0;

            UNLESS(-1 != (i=PyList_Size(self->data))) return -1;
            while(--i >= 0)
              {
                e=PyList_GetItem(self->data,i);
                UNLESS(-1 != (el=PyObject_Length(e))) return -1;
                l+=el;
              }
            return l;
        }

        static PyObject *
        MM_subscript(self, key)
                  MMobject *self;
                  PyObject *key;
        {
            long i;
            PyObject *e;

            UNLESS(-1 != (i=PyList_Size(self->data))) return NULL;
            while(--i >= 0)
              {
                e=PyList_GetItem(self->data,i);
                if(e=PyObject_GetItem(e,key)) return e;
                PyErr_Clear();
              }
            PyErr_SetObject(PyExc_KeyError,key);
            return NULL;
        }

        static PyMappingMethods MM_as_mapping = {
                  (inquiry)MM_length,           /*mp_length*/
                  (binaryfunc)MM_subscript,             /*mp_subscript*/
                  (objobjargproc)NULL,          /*mp_ass_subscript*/
        };

        /* -------------------------------------------------------- */

        static char MMtype__doc__[] =
        "MultiMapping -- Combine multiple mapping objects for lookup"
        ;

        static PyExtensionClass MMtype = {
                  PyObject_HEAD_INIT(&PyType_Type)
                  0,                            /*ob_size*/
                  "MultMapping",                        /*tp_name*/
                  sizeof(MMobject),             /*tp_basicsize*/
                  0,                            /*tp_itemsize*/
                  /* methods */
                  (destructor)MM_dealloc,               /*tp_dealloc*/
                  (printfunc)0,                 /*tp_print*/
                  (getattrfunc)MM_getattr,      /*tp_getattr*/
                  (setattrfunc)0,                       /*tp_setattr*/
                  (cmpfunc)0,                   /*tp_compare*/
                  (reprfunc)0,                  /*tp_repr*/
                  0,                            /*tp_as_number*/
                  0,                            /*tp_as_sequence*/
                  &MM_as_mapping,                       /*tp_as_mapping*/
                  (hashfunc)0,                  /*tp_hash*/
                  (ternaryfunc)0,                       /*tp_call*/
                  (reprfunc)0,                  /*tp_str*/

                  /* Space for future expansion */
                  0L,0L,0L,0L,
                  MMtype__doc__, /* Documentation string */
                  METHOD_CHAIN(MM_methods)
        };

        static struct PyMethodDef MultiMapping_methods[] = {
            {NULL,              NULL}           /* sentinel */
        };

        void
        initMultiMapping()
        {
            PyObject *m, *d;

            m = Py_InitModule4(
                "MultiMapping", MultiMapping_methods,
                "MultiMapping -- Wrap multiple mapping objects for lookup",
                (PyObject*)NULL,PYTHON_API_VERSION);
            d = PyModule_GetDict(m);
            PyExtensionClass_Export(d,"MultiMapping",MMtype);

            if (PyErr_Occurred())
               Py_FatalError("can't initialize module MultiMapping");
        }

This version includes `ExtensionClass.h`. The two declarations of `MMtype` have been changed from `PyTypeObject` to `PyExtensionClass`. The `METHOD_CHAIN` macro has been used to add methods to the end of the definition for `MMtype`. The module function, newMMobject has been replaced by the `MMtype` method, `MM__init__`. Note that this method does not create or return a new object. Finally, the lines:

        d = PyModule_GetDict(m);
        PyExtensionClass_Export(d,"MultiMapping",MMtype);

Have been added to both initialize the extension class and to export it in the module dictionary.

To use this module, compile, link, and import it as with any other extension module. The following python code illustrates the module's use:

        from MultiMapping import MultiMapping
        m=MultiMapping()
        m.push({'spam':1, 'eggs':2})
        m.push({'spam':3, 'ham':4})

        m['spam'] # returns 3
        m['ham']  # returns 4
        m['foo']  # raises a key error

Creating the `MultiMapping` object took three steps, one to create an empty `MultiMapping`, and two to add mapping objects to it. We might wish to simplify the process of creating MultiMapping objects by providing a constructor that takes source mapping objects as parameters. We can do this by sub-classing MultiMapping in Python:

        from MultiMapping import MultiMapping
        class ExtendedMultiMapping(MultiMapping):
            def __init__(self,*data):
              MultiMapping.__init__(self)
              for d in data: self.push(d)

        m=ExtendedMultiMapping({'spam':1, 'eggs':2}, {'spam':3, 'ham':4})

        m['spam'] # returns 3
        m['ham']  # returns 4
        m['foo']  # raises a key error

Note that the source file included in the ExtensionClass distribution has numerous enhancements beyond the version shown in this document.