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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 | |
| 124 | <P> |
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| 159 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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