Example: MultiMapping objects

  "Copyright (C) 1996-1998, Digital Creations":COPYRIGHT.html.

  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.