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53 | ||
54 | <H1><A NAME="SECTION003300000000000000000"></A><A NAME="backToExample"></A> | |
55 | <BR> | |
56 | 1.3 Back to the Example | |
57 | ||
58 | </H1> | |
59 | ||
60 | <P> | |
61 | Going back to our example function, you should now be able to | |
62 | understand this statement: | |
63 | ||
64 | <P> | |
65 | <div class="verbatim"><pre> | |
66 | if (!PyArg_ParseTuple(args, "s", &command)) | |
67 | return NULL; | |
68 | </pre></div> | |
69 | ||
70 | <P> | |
71 | It returns <tt class="constant">NULL</tt> (the error indicator for functions returning | |
72 | object pointers) if an error is detected in the argument list, relying | |
73 | on the exception set by <tt class="cfunction">PyArg_ParseTuple()</tt>. Otherwise the | |
74 | string value of the argument has been copied to the local variable | |
75 | <tt class="cdata">command</tt>. This is a pointer assignment and you are not supposed | |
76 | to modify the string to which it points (so in Standard C, the variable | |
77 | <tt class="cdata">command</tt> should properly be declared as "<tt class="samp">const char | |
78 | *command</tt>"). | |
79 | ||
80 | <P> | |
81 | The next statement is a call to the <span class="Unix">Unix</span> function | |
82 | <tt class="cfunction">system()</tt>, passing it the string we just got from | |
83 | <tt class="cfunction">PyArg_ParseTuple()</tt>: | |
84 | ||
85 | <P> | |
86 | <div class="verbatim"><pre> | |
87 | sts = system(command); | |
88 | </pre></div> | |
89 | ||
90 | <P> | |
91 | Our <tt class="function">spam.system()</tt> function must return the value of | |
92 | <tt class="cdata">sts</tt> as a Python object. This is done using the function | |
93 | <tt class="cfunction">Py_BuildValue()</tt>, which is something like the inverse of | |
94 | <tt class="cfunction">PyArg_ParseTuple()</tt>: it takes a format string and an | |
95 | arbitrary number of C values, and returns a new Python object. | |
96 | More info on <tt class="cfunction">Py_BuildValue()</tt> is given later. | |
97 | ||
98 | <P> | |
99 | <div class="verbatim"><pre> | |
100 | return Py_BuildValue("i", sts); | |
101 | </pre></div> | |
102 | ||
103 | <P> | |
104 | In this case, it will return an integer object. (Yes, even integers | |
105 | are objects on the heap in Python!) | |
106 | ||
107 | <P> | |
108 | If you have a C function that returns no useful argument (a function | |
109 | returning <tt class="ctype">void</tt>), the corresponding Python function must return | |
110 | <code>None</code>. You need this idiom to do so (which is implemented by the | |
111 | Py_RETURN_NONE macro): | |
112 | ||
113 | <P> | |
114 | <div class="verbatim"><pre> | |
115 | Py_INCREF(Py_None); | |
116 | return Py_None; | |
117 | </pre></div> | |
118 | ||
119 | <P> | |
120 | <tt class="cdata">Py_None</tt> is the C name for the special Python object | |
121 | <code>None</code>. It is a genuine Python object rather than a <tt class="constant">NULL</tt> | |
122 | pointer, which means ``error'' in most contexts, as we have seen. | |
123 | ||
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