Patch #1671450: add a section about subclassing builtin types to the
"extending and embedding" tutorial. (backport from rev. 54150)
This commit is contained in:
Georg Brandl
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@ -489,7 +489,6 @@ this?
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garbage collection, there are calls that can be made to ``untrack''
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the object from garbage collection, however, these calls are
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advanced and not covered here.}
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\item
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\end{itemize}
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@ -930,6 +929,102 @@ That's pretty much it. If we had written custom \member{tp_alloc} or
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collection. Most extensions will use the versions automatically
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provided.
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\subsection{Subclassing other types}
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It is possible to create new extension types that are derived from existing
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types. It is easiest to inherit from the built in types, since an extension
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can easily use the \class{PyTypeObject} it needs. It can be difficult to
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share these \class{PyTypeObject} structures between extension modules.
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In this example we will create a \class{Shoddy} type that inherits from
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the builtin \class{list} type. The new type will be completely compatible
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with regular lists, but will have an additional \method{increment()} method
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that increases an internal counter.
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\begin{verbatim}
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>>> import shoddy
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>>> s = shoddy.Shoddy(range(3))
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>>> s.extend(s)
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>>> print len(s)
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6
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>>> print s.increment()
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1
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>>> print s.increment()
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2
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\end{verbatim}
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\verbatiminput{shoddy.c}
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As you can see, the source code closely resembles the \class{Noddy} examples in previous
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sections. We will break down the main differences between them.
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\begin{verbatim}
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typedef struct {
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PyListObject list;
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int state;
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} Shoddy;
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\end{verbatim}
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The primary difference for derived type objects is that the base type's
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object structure must be the first value. The base type will already
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include the \cfunction{PyObject_HEAD} at the beginning of its structure.
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When a Python object is a \class{Shoddy} instance, its \var{PyObject*} pointer
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can be safely cast to both \var{PyListObject*} and \var{Shoddy*}.
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\begin{verbatim}
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static int
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Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
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{
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if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
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return -1;
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self->state = 0;
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return 0;
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}
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\end{verbatim}
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In the \member{__init__} method for our type, we can see how to call through
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to the \member{__init__} method of the base type.
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This pattern is important when writing a type with custom \member{new} and
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\member{dealloc} methods. The \member{new} method should not actually create the
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memory for the object with \member{tp_alloc}, that will be handled by
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the base class when calling its \member{tp_new}.
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When filling out the \cfunction{PyTypeObject} for the \class{Shoddy} type,
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you see a slot for \cfunction{tp_base}. Due to cross platform compiler
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issues, you can't fill that field directly with the \cfunction{PyList_Type};
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it can be done later in the module's \cfunction{init} function.
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\begin{verbatim}
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PyMODINIT_FUNC
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initshoddy(void)
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{
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PyObject *m;
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ShoddyType.tp_base = &PyList_Type;
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if (PyType_Ready(&ShoddyType) < 0)
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return;
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m = Py_InitModule3("shoddy", NULL, "Shoddy module");
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if (m == NULL)
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return;
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Py_INCREF(&ShoddyType);
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PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
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}
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\end{verbatim}
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Before calling \cfunction{PyType_Ready}, the type structure must have the
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\member{tp_base} slot filled in. When we are deriving a new type, it is
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not necessary to fill out the \member{tp_alloc} slot with
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\cfunction{PyType_GenericNew} -- the allocate function from the base type
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will be inherited.
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After that, calling \cfunction{PyType_Ready} and adding the type object
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to the module is the same as with the basic \class{Noddy} examples.
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\section{Type Methods
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\label{dnt-type-methods}}
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@ -0,0 +1,91 @@
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#include <Python.h>
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typedef struct {
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PyListObject list;
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int state;
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} Shoddy;
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static PyObject *
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Shoddy_increment(Shoddy *self, PyObject *unused)
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{
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self->state++;
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return PyInt_FromLong(self->state);
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}
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static PyMethodDef Shoddy_methods[] = {
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{"increment", (PyCFunction)Shoddy_increment, METH_NOARGS,
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PyDoc_STR("increment state counter")},
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{NULL, NULL},
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};
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static int
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Shoddy_init(Shoddy *self, PyObject *args, PyObject *kwds)
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{
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if (PyList_Type.tp_init((PyObject *)self, args, kwds) < 0)
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return -1;
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self->state = 0;
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return 0;
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}
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static PyTypeObject ShoddyType = {
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PyObject_HEAD_INIT(NULL)
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0, /* ob_size */
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"shoddy.Shoddy", /* tp_name */
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sizeof(Shoddy), /* tp_basicsize */
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0, /* tp_itemsize */
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0, /* tp_dealloc */
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0, /* tp_print */
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0, /* tp_getattr */
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0, /* tp_setattr */
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0, /* tp_compare */
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0, /* tp_repr */
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0, /* tp_as_number */
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0, /* tp_as_sequence */
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0, /* tp_as_mapping */
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0, /* tp_hash */
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0, /* tp_call */
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0, /* tp_str */
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0, /* tp_getattro */
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0, /* tp_setattro */
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0, /* tp_as_buffer */
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Py_TPFLAGS_DEFAULT |
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Py_TPFLAGS_BASETYPE, /* tp_flags */
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0, /* tp_doc */
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0, /* tp_traverse */
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0, /* tp_clear */
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0, /* tp_richcompare */
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0, /* tp_weaklistoffset */
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0, /* tp_iter */
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0, /* tp_iternext */
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Shoddy_methods, /* tp_methods */
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0, /* tp_members */
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0, /* tp_getset */
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0, /* tp_base */
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0, /* tp_dict */
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0, /* tp_descr_get */
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0, /* tp_descr_set */
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0, /* tp_dictoffset */
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(initproc)Shoddy_init, /* tp_init */
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0, /* tp_alloc */
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0, /* tp_new */
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};
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PyMODINIT_FUNC
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initshoddy(void)
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{
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PyObject *m;
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ShoddyType.tp_base = &PyList_Type;
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if (PyType_Ready(&ShoddyType) < 0)
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return;
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m = Py_InitModule3("shoddy", NULL, "Shoddy module");
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if (m == NULL)
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return;
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Py_INCREF(&ShoddyType);
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PyModule_AddObject(m, "Shoddy", (PyObject *) &ShoddyType);
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}
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