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Initial commit of OpenSPARC T2 architecture model.
[OpenSPARC-T2-SAM] / sam-t2 / devtools / v9 / html / python / ext / node30.html
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52<H3><A NAME="SECTION004232000000000000000">
532.2.3.2 Type-specific Attribute Management</A>
54</H3>
55
56<P>
57For simplicity, only the <tt class="ctype">char*</tt> version will be demonstrated
58here; the type of the name parameter is the only difference between
59the <tt class="ctype">char*</tt> and <tt class="ctype">PyObject*</tt> flavors of the interface.
60This example effectively does the same thing as the generic example
61above, but does not use the generic support added in Python 2.2. The
62value in showing this is two-fold: it demonstrates how basic attribute
63management can be done in a way that is portable to older versions of
64Python, and explains how the handler functions are called, so that if
65you do need to extend their functionality, you'll understand what
66needs to be done.
67
68<P>
69The <tt class="member">tp_getattr</tt> handler is called when the object requires an
70attribute look-up. It is called in the same situations where the
71<tt class="method">__getattr__()</tt> method of a class would be called.
72
73<P>
74A likely way to handle this is (1) to implement a set of functions
75(such as <tt class="cfunction">newdatatype_getSize()</tt> and
76<tt class="cfunction">newdatatype_setSize()</tt> in the example below), (2) provide a
77method table listing these functions, and (3) provide a getattr
78function that returns the result of a lookup in that table. The
79method table uses the same structure as the <tt class="member">tp_methods</tt> field
80of the type object.
81
82<P>
83Here is an example:
84
85<P>
86<div class="verbatim"><pre>
87static PyMethodDef newdatatype_methods[] = {
88 {"getSize", (PyCFunction)newdatatype_getSize, METH_VARARGS,
89 "Return the current size."},
90 {"setSize", (PyCFunction)newdatatype_setSize, METH_VARARGS,
91 "Set the size."},
92 {NULL, NULL, 0, NULL} /* sentinel */
93};
94
95static PyObject *
96newdatatype_getattr(newdatatypeobject *obj, char *name)
97{
98 return Py_FindMethod(newdatatype_methods, (PyObject *)obj, name);
99}
100</pre></div>
101
102<P>
103The <tt class="member">tp_setattr</tt> handler is called when the
104<tt class="method">__setattr__()</tt> or <tt class="method">__delattr__()</tt> method of a class
105instance would be called. When an attribute should be deleted, the
106third parameter will be <tt class="constant">NULL</tt>. Here is an example that simply raises
107an exception; if this were really all you wanted, the
108<tt class="member">tp_setattr</tt> handler should be set to <tt class="constant">NULL</tt>.
109
110<P>
111<div class="verbatim"><pre>
112static int
113newdatatype_setattr(newdatatypeobject *obj, char *name, PyObject *v)
114{
115 (void)PyErr_Format(PyExc_RuntimeError, "Read-only attribute: \%s", name);
116 return -1;
117}
118</pre></div>
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155<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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Full OpenSPARC architecture tarball including simulator, hypervisor, and OBP.