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55 | <H1><A NAME="SECTION0012200000000000000000"></A><A NAME="common-structs"></A> | |
56 | <BR> | |
57 | 10.2 Common Object Structures | |
58 | </H1> | |
59 | ||
60 | <P> | |
61 | There are a large number of structures which are used in the | |
62 | definition of object types for Python. This section describes these | |
63 | structures and how they are used. | |
64 | ||
65 | <P> | |
66 | All Python objects ultimately share a small number of fields at the | |
67 | beginning of the object's representation in memory. These are | |
68 | represented by the <tt class="ctype">PyObject</tt> and <tt class="ctype">PyVarObject</tt> types, | |
69 | which are defined, in turn, by the expansions of some macros also | |
70 | used, whether directly or indirectly, in the definition of all other | |
71 | Python objects. | |
72 | ||
73 | <P> | |
74 | <dl><dt><b><tt class="ctype"><a id='l2h-920' xml:id='l2h-920'>PyObject</a></tt></b></dt> | |
75 | <dd> | |
76 | All object types are extensions of this type. This is a type which | |
77 | contains the information Python needs to treat a pointer to an | |
78 | object as an object. In a normal ``release'' build, it contains | |
79 | only the objects reference count and a pointer to the corresponding | |
80 | type object. It corresponds to the fields defined by the | |
81 | expansion of the <code>PyObject_HEAD</code> macro. | |
82 | </dl> | |
83 | ||
84 | <P> | |
85 | <dl><dt><b><tt class="ctype"><a id='l2h-921' xml:id='l2h-921'>PyVarObject</a></tt></b></dt> | |
86 | <dd> | |
87 | This is an extension of <tt class="ctype">PyObject</tt> that adds the | |
88 | <tt class="member">ob_size</tt> field. This is only used for objects that have | |
89 | some notion of <em>length</em>. This type does not often appear in | |
90 | the Python/C API. It corresponds to the fields defined by the | |
91 | expansion of the <code>PyObject_VAR_HEAD</code> macro. | |
92 | </dl> | |
93 | ||
94 | <P> | |
95 | These macros are used in the definition of <tt class="ctype">PyObject</tt> and | |
96 | <tt class="ctype">PyVarObject</tt>: | |
97 | ||
98 | <P> | |
99 | <dl><dt><b><tt id='l2h-922' xml:id='l2h-922' class="macro">PyObject_HEAD</tt></b></dt> | |
100 | <dd> | |
101 | This is a macro which expands to the declarations of the fields of | |
102 | the <tt class="ctype">PyObject</tt> type; it is used when declaring new types which | |
103 | represent objects without a varying length. The specific fields it | |
104 | expands to depend on the definition of | |
105 | Py_TRACE_REFS. By default, that macro is not | |
106 | defined, and PyObject_HEAD expands to: | |
107 | <div class="verbatim"><pre> | |
108 | int ob_refcnt; | |
109 | PyTypeObject *ob_type; | |
110 | </pre></div> | |
111 | When Py_TRACE_REFS is defined, it expands to: | |
112 | <div class="verbatim"><pre> | |
113 | PyObject *_ob_next, *_ob_prev; | |
114 | int ob_refcnt; | |
115 | PyTypeObject *ob_type; | |
116 | </pre></div> | |
117 | </dl> | |
118 | ||
119 | <P> | |
120 | <dl><dt><b><tt id='l2h-923' xml:id='l2h-923' class="macro">PyObject_VAR_HEAD</tt></b></dt> | |
121 | <dd> | |
122 | This is a macro which expands to the declarations of the fields of | |
123 | the <tt class="ctype">PyVarObject</tt> type; it is used when declaring new types which | |
124 | represent objects with a length that varies from instance to | |
125 | instance. This macro always expands to: | |
126 | <div class="verbatim"><pre> | |
127 | PyObject_HEAD | |
128 | int ob_size; | |
129 | </pre></div> | |
130 | Note that PyObject_HEAD is part of the expansion, and | |
131 | that it's own expansion varies depending on the definition of | |
132 | Py_TRACE_REFS. | |
133 | </dl> | |
134 | ||
135 | <P> | |
136 | PyObject_HEAD_INIT | |
137 | ||
138 | <P> | |
139 | <dl><dt><b><tt class="ctype"><a id='l2h-924' xml:id='l2h-924'>PyCFunction</a></tt></b></dt> | |
140 | <dd> | |
141 | Type of the functions used to implement most Python callables in C. | |
142 | Functions of this type take two <tt class="ctype">PyObject*</tt> parameters and | |
143 | return one such value. If the return value is <tt class="constant">NULL</tt>, an exception | |
144 | shall have been set. If not <tt class="constant">NULL</tt>, the return value is interpreted | |
145 | as the return value of the function as exposed in Python. The | |
146 | function must return a new reference. | |
147 | </dl> | |
148 | ||
149 | <P> | |
150 | <dl><dt><b><tt class="ctype"><a id='l2h-925' xml:id='l2h-925'>PyMethodDef</a></tt></b></dt> | |
151 | <dd> | |
152 | Structure used to describe a method of an extension type. This | |
153 | structure has four fields: | |
154 | ||
155 | <P> | |
156 | <div class="center"><table class="realtable"> | |
157 | <thead> | |
158 | <tr> | |
159 | <th class="left" >Field</th> | |
160 | <th class="left" >C Type</th> | |
161 | <th class="left" >Meaning</th> | |
162 | </tr> | |
163 | </thead> | |
164 | <tbody> | |
165 | <tr><td class="left" valign="baseline"><tt class="member">ml_name</tt></td> | |
166 | <td class="left" >char *</td> | |
167 | <td class="left" >name of the method</td></tr> | |
168 | <tr><td class="left" valign="baseline"><tt class="member">ml_meth</tt></td> | |
169 | <td class="left" >PyCFunction</td> | |
170 | <td class="left" >pointer to the C implementation</td></tr> | |
171 | <tr><td class="left" valign="baseline"><tt class="member">ml_flags</tt></td> | |
172 | <td class="left" >int</td> | |
173 | <td class="left" >flag bits indicating how the call should be | |
174 | constructed</td></tr> | |
175 | <tr><td class="left" valign="baseline"><tt class="member">ml_doc</tt></td> | |
176 | <td class="left" >char *</td> | |
177 | <td class="left" >points to the contents of the docstring</td></tr></tbody> | |
178 | </table></div> | |
179 | </dl> | |
180 | ||
181 | <P> | |
182 | The <tt class="member">ml_meth</tt> is a C function pointer. The functions may be of | |
183 | different types, but they always return <tt class="ctype">PyObject*</tt>. If the | |
184 | function is not of the <tt class="ctype">PyCFunction</tt>, the compiler will require | |
185 | a cast in the method table. Even though <tt class="ctype">PyCFunction</tt> defines | |
186 | the first parameter as <tt class="ctype">PyObject*</tt>, it is common that the method | |
187 | implementation uses a the specific C type of the <var>self</var> object. | |
188 | ||
189 | <P> | |
190 | The <tt class="member">ml_flags</tt> field is a bitfield which can include the | |
191 | following flags. The individual flags indicate either a calling | |
192 | convention or a binding convention. Of the calling convention flags, | |
193 | only <tt class="constant">METH_VARARGS</tt> and <tt class="constant">METH_KEYWORDS</tt> can be | |
194 | combined (but note that <tt class="constant">METH_KEYWORDS</tt> alone is equivalent | |
195 | to <code><tt class="constant">METH_VARARGS</tt> | <tt class="constant">METH_KEYWORDS</tt></code>). | |
196 | Any of the calling convention flags can be combined with a | |
197 | binding flag. | |
198 | ||
199 | <P> | |
200 | <dl><dt><b><tt id='l2h-926' xml:id='l2h-926'>METH_VARARGS</tt></b></dt> | |
201 | <dd> | |
202 | This is the typical calling convention, where the methods have the | |
203 | type <tt class="ctype">PyCFunction</tt>. The function expects two | |
204 | <tt class="ctype">PyObject*</tt> values. The first one is the <var>self</var> object for | |
205 | methods; for module functions, it has the value given to | |
206 | <tt class="cfunction">Py_InitModule4()</tt> (or <tt class="constant">NULL</tt> if | |
207 | <tt class="cfunction">Py_InitModule()</tt> was used). The second parameter | |
208 | (often called <var>args</var>) is a tuple object representing all | |
209 | arguments. This parameter is typically processed using | |
210 | <tt class="cfunction">PyArg_ParseTuple()</tt> or <tt class="cfunction">PyArg_UnpackTuple</tt>. | |
211 | </dd></dl> | |
212 | ||
213 | <P> | |
214 | <dl><dt><b><tt id='l2h-927' xml:id='l2h-927'>METH_KEYWORDS</tt></b></dt> | |
215 | <dd> | |
216 | Methods with these flags must be of type | |
217 | <tt class="ctype">PyCFunctionWithKeywords</tt>. The function expects three | |
218 | parameters: <var>self</var>, <var>args</var>, and a dictionary of all the | |
219 | keyword arguments. The flag is typically combined with | |
220 | <tt class="constant">METH_VARARGS</tt>, and the parameters are typically processed | |
221 | using <tt class="cfunction">PyArg_ParseTupleAndKeywords()</tt>. | |
222 | </dd></dl> | |
223 | ||
224 | <P> | |
225 | <dl><dt><b><tt id='l2h-928' xml:id='l2h-928'>METH_NOARGS</tt></b></dt> | |
226 | <dd> | |
227 | Methods without parameters don't need to check whether arguments are | |
228 | given if they are listed with the <tt class="constant">METH_NOARGS</tt> flag. They | |
229 | need to be of type <tt class="ctype">PyCFunction</tt>. When used with object | |
230 | methods, the first parameter is typically named <code>self</code> and will | |
231 | hold a reference to the object instance. In all cases the second | |
232 | parameter will be <tt class="constant">NULL</tt>. | |
233 | </dd></dl> | |
234 | ||
235 | <P> | |
236 | <dl><dt><b><tt id='l2h-929' xml:id='l2h-929'>METH_O</tt></b></dt> | |
237 | <dd> | |
238 | Methods with a single object argument can be listed with the | |
239 | <tt class="constant">METH_O</tt> flag, instead of invoking | |
240 | <tt class="cfunction">PyArg_ParseTuple()</tt> with a <code>"O"</code> argument. They have | |
241 | the type <tt class="ctype">PyCFunction</tt>, with the <var>self</var> parameter, and a | |
242 | <tt class="ctype">PyObject*</tt> parameter representing the single argument. | |
243 | </dd></dl> | |
244 | ||
245 | <P> | |
246 | <dl><dt><b><tt id='l2h-930' xml:id='l2h-930'>METH_OLDARGS</tt></b></dt> | |
247 | <dd> | |
248 | This calling convention is deprecated. The method must be of type | |
249 | <tt class="ctype">PyCFunction</tt>. The second argument is <tt class="constant">NULL</tt> if no arguments | |
250 | are given, a single object if exactly one argument is given, and a | |
251 | tuple of objects if more than one argument is given. There is no | |
252 | way for a function using this convention to distinguish between a | |
253 | call with multiple arguments and a call with a tuple as the only | |
254 | argument. | |
255 | </dd></dl> | |
256 | ||
257 | <P> | |
258 | These two constants are not used to indicate the calling convention | |
259 | but the binding when use with methods of classes. These may not be | |
260 | used for functions defined for modules. At most one of these flags | |
261 | may be set for any given method. | |
262 | ||
263 | <P> | |
264 | <dl><dt><b><tt id='l2h-931' xml:id='l2h-931'>METH_CLASS</tt></b></dt> | |
265 | <dd> | |
266 | The method will be passed the type object as the first parameter | |
267 | rather than an instance of the type. This is used to create | |
268 | <em>class methods</em>, similar to what is created when using the | |
269 | <tt class="function">classmethod()</tt><a id='l2h-932' xml:id='l2h-932'></a> built-in | |
270 | function. | |
271 | ||
272 | <span class="versionnote">New in version 2.3.</span> | |
273 | ||
274 | </dd></dl> | |
275 | ||
276 | <P> | |
277 | <dl><dt><b><tt id='l2h-933' xml:id='l2h-933'>METH_STATIC</tt></b></dt> | |
278 | <dd> | |
279 | The method will be passed <tt class="constant">NULL</tt> as the first parameter rather than | |
280 | an instance of the type. This is used to create <em>static | |
281 | methods</em>, similar to what is created when using the | |
282 | <tt class="function">staticmethod()</tt><a id='l2h-934' xml:id='l2h-934'></a> built-in | |
283 | function. | |
284 | ||
285 | <span class="versionnote">New in version 2.3.</span> | |
286 | ||
287 | </dd></dl> | |
288 | ||
289 | <P> | |
290 | One other constant controls whether a method is loaded in place of | |
291 | another definition with the same method name. | |
292 | ||
293 | <P> | |
294 | <dl><dt><b><tt id='l2h-935' xml:id='l2h-935'>METH_COEXIST</tt></b></dt> | |
295 | <dd> | |
296 | The method will be loaded in place of existing definitions. Without | |
297 | <var>METH_COEXIST</var>, the default is to skip repeated definitions. Since | |
298 | slot wrappers are loaded before the method table, the existence of a | |
299 | <var>sq_contains</var> slot, for example, would generate a wrapped method | |
300 | named <tt class="method">__contains__()</tt> and preclude the loading of a | |
301 | corresponding PyCFunction with the same name. With the flag defined, | |
302 | the PyCFunction will be loaded in place of the wrapper object and will | |
303 | co-exist with the slot. This is helpful because calls to PyCFunctions | |
304 | are optimized more than wrapper object calls. | |
305 | ||
306 | <span class="versionnote">New in version 2.4.</span> | |
307 | ||
308 | </dd></dl> | |
309 | ||
310 | <P> | |
311 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject* <b><tt id='l2h-936' xml:id='l2h-936' class="cfunction">Py_FindMethod</tt></b>(</nobr></td><td>PyMethodDef table[], | |
312 | PyObject *<var>ob</var>, char *<var>name</var>)</td></tr></table></dt> | |
313 | <dd> | |
314 | <div class="refcount-info"> | |
315 | <span class="label">Return value:</span> | |
316 | <span class="value">New reference.</span> | |
317 | </div> | |
318 | Return a bound method object for an extension type implemented in | |
319 | C. This can be useful in the implementation of a | |
320 | <tt class="member">tp_getattro</tt> or <tt class="member">tp_getattr</tt> handler that does not | |
321 | use the <tt class="cfunction">PyObject_GenericGetAttr()</tt> function. | |
322 | </dd></dl> | |
323 | ||
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