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<title>1.4 The Module's Method Table and Initialization Function
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<H1><A NAME="SECTION003400000000000000000"></A><A NAME="methodTable"></A>
<BR>
1.4 The Module's Method Table and Initialization Function
</H1>
<P>
I promised to show how <tt class="cfunction">spam_system()</tt> is called from Python
programs. First, we need to list its name and address in a ``method
table'':
<P>
<div class="verbatim"><pre>
static PyMethodDef SpamMethods[] = {
...
{"system", spam_system, METH_VARARGS,
"Execute a shell command."},
...
{NULL, NULL, 0, NULL} /* Sentinel */
};
</pre></div>
<P>
Note the third entry ("<tt class="samp">METH_VARARGS</tt>"). This is a flag telling
the interpreter the calling convention to be used for the C
function. It should normally always be "<tt class="samp">METH_VARARGS</tt>" or
"<tt class="samp">METH_VARARGS | METH_KEYWORDS</tt>"; a value of <code>0</code> means that an
obsolete variant of <tt class="cfunction">PyArg_ParseTuple()</tt> is used.
<P>
When using only "<tt class="samp">METH_VARARGS</tt>", the function should expect
the Python-level parameters to be passed in as a tuple acceptable for
parsing via <tt class="cfunction">PyArg_ParseTuple()</tt>; more information on this
function is provided below.
<P>
The <tt class="constant">METH_KEYWORDS</tt> bit may be set in the third field if
keyword arguments should be passed to the function. In this case, the
C function should accept a third "<tt class="samp">PyObject *</tt>" parameter which
will be a dictionary of keywords. Use
<tt class="cfunction">PyArg_ParseTupleAndKeywords()</tt> to parse the arguments to
such a function.
<P>
The method table must be passed to the interpreter in the module's
initialization function. The initialization function must be named
<tt class="cfunction">init<var>name</var>()</tt>, where <var>name</var> is the name of the
module, and should be the only non-<tt class="keyword">static</tt> item defined in
the module file:
<P>
<div class="verbatim"><pre>
PyMODINIT_FUNC
initspam(void)
{
(void) Py_InitModule("spam", SpamMethods);
}
</pre></div>
<P>
Note that PyMODINIT_FUNC declares the function as <code>void</code> return type,
declares any special linkage declarations required by the platform, and for
C++ declares the function as <code>extern "C"</code>.
<P>
When the Python program imports module <tt class="module">spam</tt> for the first
time, <tt class="cfunction">initspam()</tt> is called. (See below for comments about
embedding Python.) It calls
<tt class="cfunction">Py_InitModule()</tt>, which creates a ``module object'' (which
is inserted in the dictionary <code>sys.modules</code> under the key
<code>"spam"</code>), and inserts built-in function objects into the newly
created module based upon the table (an array of <tt class="ctype">PyMethodDef</tt>
structures) that was passed as its second argument.
<tt class="cfunction">Py_InitModule()</tt> returns a pointer to the module object
that it creates (which is unused here). It aborts with a fatal error
if the module could not be initialized satisfactorily, so the caller
doesn't need to check for errors.
<P>
When embedding Python, the <tt class="cfunction">initspam()</tt> function is not
called automatically unless there's an entry in the
<tt class="cdata">_PyImport_Inittab</tt> table. The easiest way to handle this is to
statically initialize your statically-linked modules by directly
calling <tt class="cfunction">initspam()</tt> after the call to
<tt class="cfunction">Py_Initialize()</tt>:
<P>
<div class="verbatim"><pre>
int
main(int argc, char *argv[])
{
/* Pass argv[0] to the Python interpreter */
Py_SetProgramName(argv[0]);
/* Initialize the Python interpreter. Required. */
Py_Initialize();
/* Add a static module */
initspam();
</pre></div>
<P>
An example may be found in the file <span class="file">Demo/embed/demo.c</span> in the
Python source distribution.
<P>
<span class="note"><b class="label">Note:</b>
Removing entries from <code>sys.modules</code> or importing
compiled modules into multiple interpreters within a process (or
following a <tt class="cfunction">fork()</tt> without an intervening
<tt class="cfunction">exec()</tt>) can create problems for some extension modules.
Extension module authors should exercise caution when initializing
internal data structures.
Note also that the <tt class="function">reload()</tt> function can be used with
extension modules, and will call the module initialization function
(<tt class="cfunction">initspam()</tt> in the example), but will not load the module
again if it was loaded from a dynamically loadable object file
(<span class="file">.so</span> on <span class="Unix">Unix</span>, <span class="file">.dll</span> on Windows).</span>
<P>
A more substantial example module is included in the Python source
distribution as <span class="file">Modules/xxmodule.c</span>. This file may be used as a
template or simply read as an example. The <b class="program">modulator.py</b>
script included in the source distribution or Windows install provides
a simple graphical user interface for declaring the functions and
objects which a module should implement, and can generate a template
which can be filled in. The script lives in the
<span class="file">Tools/modulator/</span> directory; see the <span class="file">README</span> file there
for more information.
<P>
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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