File: MultiMapping.html

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<html><head><title>Example: MultiMapping objects</title>
	    </head><body>
	    <h1>Example: MultiMapping objects</h1>
<p>  <a href="COPYRIGHT.html">Copyright &copy; 1996-1998, Digital Creations</a>.</p>

<p>  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.</p>

<p>  Consider an implementation of a MultiMapping extension type,
  without use of the extension class mechanism:</p>
<PRE>
    #include &quot;Python.h&quot;

    #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, &quot;O&quot;, &amp;src)) return NULL;
        UNLESS(-1 != PyList_Append(self-&gt;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, &quot;&quot;)) return NULL;
        UNLESS(-1 != (l=PyList_Size(self-&gt;data))) return NULL;
        l--;
        UNLESS(r=PySequence_GetItem(self-&gt;data,l)) return NULL;
        UNLESS(-1 != PyList_SetSlice(self-&gt;data,l,l+1,emptyList)) goto err;
        return r;
    err:
        Py_DECREF&reg;;
        return NULL;
    }

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

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

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

    static void
    MM_dealloc(MMobject *self){
        Py_XDECREF(self-&gt;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-&gt;data))) return -1;
        while(--i &gt;= 0)
          {
            e=PyList_GetItem(self-&gt;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-&gt;data))) return NULL;
        while(--i &gt;= 0)
          {
            e=PyList_GetItem(self-&gt;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__[] = 
    &quot;MultiMapping -- Combine multiple mapping objects for lookup&quot;
    ;

    static PyTypeObject MMtype = {
              PyObject_HEAD_INIT(&amp;PyType_Type)
              0,                            /*ob_size*/
              &quot;MultMapping&quot;,                        /*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*/
              &amp;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[] = {
        {&quot;MultiMapping&quot;, (PyCFunction)newMMobject, 1,
         &quot;MultiMapping() -- Create a new empty multi-mapping&quot;},
        {NULL,              NULL}           /* sentinel */
    };

    void
    initMultiMapping(){
        PyObject *m;

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

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

</PRE>

<p>  This module defines an extension type, <code>MultiMapping</code>, and exports a
  module function, <code>MultiMapping</code>, that creates <code>MultiMapping</code>
  Instances. The type provides two methods, <code>push</code>, and <code>pop</code>, 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.</p>

<p>  Now consider an extension class implementation of MultiMapping
  objects:</p>
<PRE>
    #include &quot;Python.h&quot;
    #include &quot;ExtensionClass.h&quot;

    #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, &quot;O&quot;, &amp;src)) return NULL;
        UNLESS(-1 != PyList_Append(self-&gt;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, &quot;&quot;)) return NULL;
        UNLESS(-1 != (l=PyList_Size(self-&gt;data))) return NULL;
        l--;
        UNLESS(r=PySequence_GetItem(self-&gt;data,l)) return NULL;
        UNLESS(-1 != PyList_SetSlice(self-&gt;data,l,l+1,emptyList)) goto err;
        return r;
    err:
        Py_DECREF&reg;;
        return NULL;
    }

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

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

    static void
    MM_dealloc(self)
           MMobject *self;
    {
        Py_XDECREF(self-&gt;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-&gt;data))) return -1;
        while(--i &gt;= 0)
          {
            e=PyList_GetItem(self-&gt;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-&gt;data))) return NULL;
        while(--i &gt;= 0)
          {
            e=PyList_GetItem(self-&gt;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__[] = 
    &quot;MultiMapping -- Combine multiple mapping objects for lookup&quot;
    ;

    static PyExtensionClass MMtype = {
              PyObject_HEAD_INIT(&amp;PyType_Type)
              0,                            /*ob_size*/
              &quot;MultMapping&quot;,                        /*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*/
              &amp;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(
            &quot;MultiMapping&quot;, MultiMapping_methods,
            &quot;MultiMapping -- Wrap multiple mapping objects for lookup&quot;,
            (PyObject*)NULL,PYTHON_API_VERSION);
        d = PyModule_GetDict(m);
        PyExtensionClass_Export(d,&quot;MultiMapping&quot;,MMtype);

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

</PRE>

<p>  This version includes <code>ExtensionClass.h</code>.  The two declarations of
  <code>MMtype</code> have been changed from <code>PyTypeObject</code> to <code>PyExtensionClass</code>.
  The <code>METHOD_CHAIN</code> macro has been used to add methods to the end of
  the definition for <code>MMtype</code>.  The module function, newMMobject has
  been replaced by the <code>MMtype</code> method, <code>MM__init__</code>.  Note that this
  method does not create or return a new object.  Finally, the lines:</p>
<PRE>
    d = PyModule_GetDict(m);
    PyExtensionClass_Export(d,&quot;MultiMapping&quot;,MMtype);

</PRE>

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

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

</PRE>

<p>  Creating the <code>MultiMapping</code> object took three steps, one to create
  an empty <code>MultiMapping</code>, 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:</p>
<PRE>
    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

</PRE>

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



	    </body></html>