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<H1><A NAME="SECTION004100000000000000000"></A><A NAME="dnt-basics"></A>
<BR>
2.1 The Basics

</H1>

<P>
The Python runtime sees all Python objects as variables of type
<tt class="ctype">PyObject*</tt>.  A <tt class="ctype">PyObject</tt> is not a very magnificent
object - it just contains the refcount and a pointer to the object's
``type object''.  This is where the action is; the type object
determines which (C) functions get called when, for instance, an
attribute gets looked up on an object or it is multiplied by another
object.  These C functions are called ``type methods'' to distinguish
them from things like <code>[].append</code> (which we call ``object
methods'').

<P>
So, if you want to define a new object type, you need to create a new
type object.

<P>
This sort of thing can only be explained by example, so here's a
minimal, but complete, module that defines a new type:

<P>
<div class="verbatim">
<pre>#include &lt;Python.h&gt;

typedef struct {
    PyObject_HEAD
    /* Type-specific fields go here. */
} noddy_NoddyObject;

static PyTypeObject noddy_NoddyType = {
    PyObject_HEAD_INIT(NULL)
    0,                         /*ob_size*/
    "noddy.Noddy",             /*tp_name*/
    sizeof(noddy_NoddyObject), /*tp_basicsize*/
    0,                         /*tp_itemsize*/
    0,                         /*tp_dealloc*/
    0,                         /*tp_print*/
    0,                         /*tp_getattr*/
    0,                         /*tp_setattr*/
    0,                         /*tp_compare*/
    0,                         /*tp_repr*/
    0,                         /*tp_as_number*/
    0,                         /*tp_as_sequence*/
    0,                         /*tp_as_mapping*/
    0,                         /*tp_hash */
    0,                         /*tp_call*/
    0,                         /*tp_str*/
    0,                         /*tp_getattro*/
    0,                         /*tp_setattro*/
    0,                         /*tp_as_buffer*/
    Py_TPFLAGS_DEFAULT,        /*tp_flags*/
    "Noddy objects",           /* tp_doc */
};

static PyMethodDef noddy_methods[] = {
    {NULL}  /* Sentinel */
};

#ifndef PyMODINIT_FUNC  /* declarations for DLL import/export */
#define PyMODINIT_FUNC void
#endif
PyMODINIT_FUNC
initnoddy(void)
{
    PyObject* m;

    noddy_NoddyType.tp_new = PyType_GenericNew;
    if (PyType_Ready(&amp;noddy_NoddyType) &lt; 0)
        return;

    m = Py_InitModule3("noddy", noddy_methods,
                       "Example module that creates an extension type.");

    Py_INCREF(&amp;noddy_NoddyType);
    PyModule_AddObject(m, "Noddy", (PyObject *)&amp;noddy_NoddyType);
}
</pre>
<div class="footer">
<a href="noddy.txt" type="text/plain">Download as text (original file name: <span class="file">noddy.c</span>).</a>
</div></div>

<P>
Now that's quite a bit to take in at once, but hopefully bits will
seem familiar from the last chapter.

<P>
The first bit that will be new is:

<P>
<div class="verbatim"><pre>
typedef struct {
    PyObject_HEAD
} noddy_NoddyObject;
</pre></div>

<P>
This is what a Noddy object will contain--in this case, nothing more
than every Python object contains, namely a refcount and a pointer to a type
object.  These are the fields the <code>PyObject_HEAD</code> macro brings
in.  The reason for the macro is to standardize the layout and to
enable special debugging fields in debug builds.  Note that there is
no semicolon after the <code>PyObject_HEAD</code> macro; one is included in
the macro definition.  Be wary of adding one by accident; it's easy to
do from habit, and your compiler might not complain, but someone
else's probably will!  (On Windows, MSVC is known to call this an
error and refuse to compile the code.)

<P>
For contrast, let's take a look at the corresponding definition for
standard Python integers:

<P>
<div class="verbatim"><pre>
typedef struct {
    PyObject_HEAD
    long ob_ival;
} PyIntObject;
</pre></div>

<P>
Moving on, we come to the crunch -- the type object.

<P>
<div class="verbatim"><pre>
static PyTypeObject noddy_NoddyType = {
    PyObject_HEAD_INIT(NULL)
    0,                         /*ob_size*/
    "noddy.Noddy",             /*tp_name*/
    sizeof(noddy_NoddyObject), /*tp_basicsize*/
    0,                         /*tp_itemsize*/
    0,                         /*tp_dealloc*/
    0,                         /*tp_print*/
    0,                         /*tp_getattr*/
    0,                         /*tp_setattr*/
    0,                         /*tp_compare*/
    0,                         /*tp_repr*/
    0,                         /*tp_as_number*/
    0,                         /*tp_as_sequence*/
    0,                         /*tp_as_mapping*/
    0,                         /*tp_hash */
    0,                         /*tp_call*/
    0,                         /*tp_str*/
    0,                         /*tp_getattro*/
    0,                         /*tp_setattro*/
    0,                         /*tp_as_buffer*/
    Py_TPFLAGS_DEFAULT,        /*tp_flags*/
    "Noddy objects",           /* tp_doc */
};
</pre></div>

<P>
Now if you go and look up the definition of <tt class="ctype">PyTypeObject</tt> in
<span class="file">object.h</span> you'll see that it has many more fields that the
definition above.  The remaining fields will be filled with zeros by
the C compiler, and it's common practice to not specify them
explicitly unless you need them.

<P>
This is so important that we're going to pick the top of it apart still
further:

<P>
<div class="verbatim"><pre>
    PyObject_HEAD_INIT(NULL)
</pre></div>

<P>
This line is a bit of a wart; what we'd like to write is:

<P>
<div class="verbatim"><pre>
    PyObject_HEAD_INIT(&amp;PyType_Type)
</pre></div>

<P>
as the type of a type object is ``type'', but this isn't strictly
conforming C and some compilers complain.  Fortunately, this member
will be filled in for us by <tt class="cfunction">PyType_Ready()</tt>.

<P>
<div class="verbatim"><pre>
    0,                          /* ob_size */
</pre></div>

<P>
The <tt class="member">ob_size</tt> field of the header is not used; its presence in
the type structure is a historical artifact that is maintained for
binary compatibility with extension modules compiled for older
versions of Python.  Always set this field to zero.

<P>
<div class="verbatim"><pre>
    "noddy.Noddy",              /* tp_name */
</pre></div>

<P>
The name of our type.  This will appear in the default textual
representation of our objects and in some error messages, for example:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; "" + noddy.new_noddy()
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
TypeError: cannot add type "noddy.Noddy" to string
</pre></div>

<P>
Note that the name is a dotted name that includes both the module name
and the name of the type within the module. The module in this case is
<tt class="module">noddy</tt> and the type is <tt class="class">Noddy</tt>, so we set the type name
to <tt class="class">noddy.Noddy</tt>.

<P>
<div class="verbatim"><pre>
    sizeof(noddy_NoddyObject),  /* tp_basicsize */
</pre></div>

<P>
This is so that Python knows how much memory to allocate when you call
<tt class="cfunction">PyObject_New()</tt>.

<P>
<span class="note"><b class="label">Note:</b>
If you want your type to be subclassable from Python, and your
type has the same <tt class="member">tp_basicsize</tt> as its base type, you may
have problems with multiple inheritance.  A Python subclass of your
type will have to list your type first in its <tt class="member">__bases__</tt>, or
else it will not be able to call your type's <tt class="method">__new__</tt> method
without getting an error.  You can avoid this problem by ensuring
that your type has a larger value for <tt class="member">tp_basicsize</tt> than
its base type does.  Most of the time, this will be true anyway,
because either your base type will be <tt class="class">object</tt>, or else you will
be adding data members to your base type, and therefore increasing its
size.</span>

<P>
<div class="verbatim"><pre>
    0,                          /* tp_itemsize */
</pre></div>

<P>
This has to do with variable length objects like lists and strings.
Ignore this for now.

<P>
Skipping a number of type methods that we don't provide, we set the
class flags to <tt class="constant">Py_TPFLAGS_DEFAULT</tt>.

<P>
<div class="verbatim"><pre>
    Py_TPFLAGS_DEFAULT,        /*tp_flags*/
</pre></div>

<P>
All types should include this constant in their flags.  It enables all
of the members defined by the current version of Python.

<P>
We provide a doc string for the type in <tt class="member">tp_doc</tt>.

<P>
<div class="verbatim"><pre>
    "Noddy objects",           /* tp_doc */
</pre></div>

<P>
Now we get into the type methods, the things that make your objects
different from the others.  We aren't going to implement any of these
in this version of the module.  We'll expand this example later to
have more interesting behavior.

<P>
For now, all we want to be able to do is to create new <tt class="class">Noddy</tt>
objects. To enable object creation, we have to provide a
<tt class="member">tp_new</tt> implementation. In this case, we can just use the
default implementation provided by the API function
<tt class="cfunction">PyType_GenericNew()</tt>.  We'd like to just assign this to the
<tt class="member">tp_new</tt> slot, but we can't, for portability sake, On some
platforms or compilers, we can't statically initialize a structure
member with a function defined in another C module, so, instead, we'll
assign the <tt class="member">tp_new</tt> slot in the module initialization function
just before calling <tt class="cfunction">PyType_Ready()</tt>:

<P>
<div class="verbatim"><pre>
    noddy_NoddyType.tp_new = PyType_GenericNew;
    if (PyType_Ready(&amp;noddy_NoddyType) &lt; 0)
        return;
</pre></div>

<P>
All the other type methods are <tt class="constant">NULL</tt>, so we'll go over them later
-- that's for a later section!

<P>
Everything else in the file should be familiar, except for some code
in <tt class="cfunction">initnoddy()</tt>:

<P>
<div class="verbatim"><pre>
    if (PyType_Ready(&amp;noddy_NoddyType) &lt; 0)
        return;
</pre></div>

<P>
This initializes the <tt class="class">Noddy</tt> type, filing in a number of
members, including <tt class="member">ob_type</tt> that we initially set to <tt class="constant">NULL</tt>.

<P>
<div class="verbatim"><pre>
    PyModule_AddObject(m, "Noddy", (PyObject *)&amp;noddy_NoddyType);
</pre></div>

<P>
This adds the type to the module dictionary.  This allows us to create
<tt class="class">Noddy</tt> instances by calling the <tt class="class">Noddy</tt> class:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; import noddy
&gt;&gt;&gt; mynoddy = noddy.Noddy()
</pre></div>

<P>
That's it!  All that remains is to build it; put the above code in a
file called <span class="file">noddy.c</span> and

<P>
<div class="verbatim"><pre>
from distutils.core import setup, Extension
setup(name="noddy", version="1.0",
      ext_modules=[Extension("noddy", ["noddy.c"])])
</pre></div>

<P>
in a file called <span class="file">setup.py</span>; then typing

<P>
<div class="verbatim"><pre>
$ python setup.py build
</pre></div>
<P>
at a shell should produce a file <span class="file">noddy.so</span> in a subdirectory;
move to that directory and fire up Python -- you should be able to
<code>import noddy</code> and play around with Noddy objects.

<P>
That wasn't so hard, was it?

<P>
Of course, the current Noddy type is pretty uninteresting. It has no
data and doesn't do anything. It can't even be subclassed.

<P>

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<LI><A href="node22.html">2.1.1 Adding data and methods to the Basic example</a>
<LI><A href="node23.html">2.1.2 Providing finer control over data attributes</a>
<LI><A href="node24.html">2.1.3 Supporting cyclic garbage collection</a>
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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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