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<h3>Calling Python from C</h3>
<p>Sometimes it抯 useful to call Python functions from C programs. To do this, the following functions can be used:</p>
<PRE>
<B>PyObject *PyEval_CallObject(PyObject *</b><b><i>func</i></B><B>, PyObject *</B><B><i>args</i></b><b>)</B> </PRE>
<p>Call <i><tt class="monofont">func</tt></i>
with arguments <i><tt class="monofont">args</Tt></i>
. <I><tt cLass="monofont">func</tT></i>
is a Python callable object (function, method, class, and so on). <i><tT CLAss="monofont">args</tt></I>
is a tuple of arguments.</P>
<PRe>
<b>PyObject *PyEval_CallObjectWithKeywords(PyObject *</b><b><I>func</I></B><B>, PyObject *</b><b><i>args</i></B><B>, PyObject *</B><B><i>kwargs</i></b><b>)</b> </pre>
<p>Call <i><tt class="monofont">func</tt></i>
with positional arguments <I><tt ClasS="monofont">args</tt></i>
and keyword arguments <i><Tt clASS="monofont">kwargs</Tt></i>
. <i><tT CLAss="monofont">func</tt></I>
is a callable object, <I><TT clasS="monofont">args</TT></I>
is a tuple, and <i><tt class="monofont">kwargs</tt></i>
is a dictionary.</p>
<p>The following example illustrates the use of these functions:</p>
<pre>
/* Call a python function */
PyObject *func; /* Callable object. */
PyObject *args;
PyObject *result;
int arg1, arg2;
func = get_python_function() /* See below */
args = Py_BuildValue("(ii)", arg1, arg2); /* Build argument list */
result = PyEval_CallObject(func,args); /* Call function */ </pre>
<P>The only remaining problem is that C code, at compile time, cannot know the address of a Python object that has not yet been created, since Python is dynamic. One approach is to let Python create the function object and then register the address with a callback function. To deal with this, extension code such as the following can be used to set the callback function:</p>
<pRe>
static PyObject *func = 0; /* Callback function */
static PyObject *
set_callback(PyObject *self, PyObject *args) {
PyObject *f;
if (PyArg_ParseTuple(args,"O",&f)) {
if (!PyCallable_Check(f)) {
PyErr_SetString(PyExc_TypeError, "expected a callable");
return NULL;
}
Py_XINCREF(f); /* Save reference to callback */
Py_XDECREF(func); /* Release any previous callback */
func = f;
Py_INCREF(Py_None);
return Py_None;
}
return NULL;
} </prE>
<p>This function would then be invoked from the interpreter as follows:</p>
<prE>
# Some function
def foo(x,y):
return x+y
...
set_callback(foo) </pre>
<P>Alternatively, it might be possible to obtain Python callable objects using functions in the embedding API, described later in this appendix.</P>
</FOnt>
<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, © 2002 New Riders Publishing</font></TD></TR></TABLE></P>
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