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<H1><A NAME="SECTION003300000000000000000"></A><A NAME="backToExample"></A>
<BR>
1.3 Back to the Example
</H1>
<P>
Going back to our example function, you should now be able to
understand this statement:
<P>
<div class="verbatim"><pre>
if (!PyArg_ParseTuple(args, "s", &command))
return NULL;
</pre></div>
<P>
It returns <tt class="constant">NULL</tt> (the error indicator for functions returning
object pointers) if an error is detected in the argument list, relying
on the exception set by <tt class="cfunction">PyArg_ParseTuple()</tt>. Otherwise the
string value of the argument has been copied to the local variable
<tt class="cdata">command</tt>. This is a pointer assignment and you are not supposed
to modify the string to which it points (so in Standard C, the variable
<tt class="cdata">command</tt> should properly be declared as "<tt class="samp">const char
*command</tt>").
<P>
The next statement is a call to the <span class="Unix">Unix</span> function
<tt class="cfunction">system()</tt>, passing it the string we just got from
<tt class="cfunction">PyArg_ParseTuple()</tt>:
<P>
<div class="verbatim"><pre>
sts = system(command);
</pre></div>
<P>
Our <tt class="function">spam.system()</tt> function must return the value of
<tt class="cdata">sts</tt> as a Python object. This is done using the function
<tt class="cfunction">Py_BuildValue()</tt>, which is something like the inverse of
<tt class="cfunction">PyArg_ParseTuple()</tt>: it takes a format string and an
arbitrary number of C values, and returns a new Python object.
More info on <tt class="cfunction">Py_BuildValue()</tt> is given later.
<P>
<div class="verbatim"><pre>
return Py_BuildValue("i", sts);
</pre></div>
<P>
In this case, it will return an integer object. (Yes, even integers
are objects on the heap in Python!)
<P>
If you have a C function that returns no useful argument (a function
returning <tt class="ctype">void</tt>), the corresponding Python function must return
<code>None</code>. You need this idiom to do so (which is implemented by the
Py_RETURN_NONE macro):
<P>
<div class="verbatim"><pre>
Py_INCREF(Py_None);
return Py_None;
</pre></div>
<P>
<tt class="cdata">Py_None</tt> is the C name for the special Python object
<code>None</code>. It is a genuine Python object rather than a <tt class="constant">NULL</tt>
pointer, which means ``error'' in most contexts, as we have seen.
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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