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54 | <H1><A NAME="SECTION004100000000000000000"></A><A NAME="dnt-basics"></A> | |
55 | <BR> | |
56 | 2.1 The Basics | |
57 | ||
58 | </H1> | |
59 | ||
60 | <P> | |
61 | The Python runtime sees all Python objects as variables of type | |
62 | <tt class="ctype">PyObject*</tt>. A <tt class="ctype">PyObject</tt> is not a very magnificent | |
63 | object - it just contains the refcount and a pointer to the object's | |
64 | ``type object''. This is where the action is; the type object | |
65 | determines which (C) functions get called when, for instance, an | |
66 | attribute gets looked up on an object or it is multiplied by another | |
67 | object. These C functions are called ``type methods'' to distinguish | |
68 | them from things like <code>[].append</code> (which we call ``object | |
69 | methods''). | |
70 | ||
71 | <P> | |
72 | So, if you want to define a new object type, you need to create a new | |
73 | type object. | |
74 | ||
75 | <P> | |
76 | This sort of thing can only be explained by example, so here's a | |
77 | minimal, but complete, module that defines a new type: | |
78 | ||
79 | <P> | |
80 | <div class="verbatim"> | |
81 | <pre>#include <Python.h> | |
82 | ||
83 | typedef struct { | |
84 | PyObject_HEAD | |
85 | /* Type-specific fields go here. */ | |
86 | } noddy_NoddyObject; | |
87 | ||
88 | static PyTypeObject noddy_NoddyType = { | |
89 | PyObject_HEAD_INIT(NULL) | |
90 | 0, /*ob_size*/ | |
91 | "noddy.Noddy", /*tp_name*/ | |
92 | sizeof(noddy_NoddyObject), /*tp_basicsize*/ | |
93 | 0, /*tp_itemsize*/ | |
94 | 0, /*tp_dealloc*/ | |
95 | 0, /*tp_print*/ | |
96 | 0, /*tp_getattr*/ | |
97 | 0, /*tp_setattr*/ | |
98 | 0, /*tp_compare*/ | |
99 | 0, /*tp_repr*/ | |
100 | 0, /*tp_as_number*/ | |
101 | 0, /*tp_as_sequence*/ | |
102 | 0, /*tp_as_mapping*/ | |
103 | 0, /*tp_hash */ | |
104 | 0, /*tp_call*/ | |
105 | 0, /*tp_str*/ | |
106 | 0, /*tp_getattro*/ | |
107 | 0, /*tp_setattro*/ | |
108 | 0, /*tp_as_buffer*/ | |
109 | Py_TPFLAGS_DEFAULT, /*tp_flags*/ | |
110 | "Noddy objects", /* tp_doc */ | |
111 | }; | |
112 | ||
113 | static PyMethodDef noddy_methods[] = { | |
114 | {NULL} /* Sentinel */ | |
115 | }; | |
116 | ||
117 | #ifndef PyMODINIT_FUNC /* declarations for DLL import/export */ | |
118 | #define PyMODINIT_FUNC void | |
119 | #endif | |
120 | PyMODINIT_FUNC | |
121 | initnoddy(void) | |
122 | { | |
123 | PyObject* m; | |
124 | ||
125 | noddy_NoddyType.tp_new = PyType_GenericNew; | |
126 | if (PyType_Ready(&noddy_NoddyType) < 0) | |
127 | return; | |
128 | ||
129 | m = Py_InitModule3("noddy", noddy_methods, | |
130 | "Example module that creates an extension type."); | |
131 | ||
132 | Py_INCREF(&noddy_NoddyType); | |
133 | PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType); | |
134 | } | |
135 | </pre> | |
136 | <div class="footer"> | |
137 | <a href="noddy.txt" type="text/plain">Download as text (original file name: <span class="file">noddy.c</span>).</a> | |
138 | </div></div> | |
139 | ||
140 | <P> | |
141 | Now that's quite a bit to take in at once, but hopefully bits will | |
142 | seem familiar from the last chapter. | |
143 | ||
144 | <P> | |
145 | The first bit that will be new is: | |
146 | ||
147 | <P> | |
148 | <div class="verbatim"><pre> | |
149 | typedef struct { | |
150 | PyObject_HEAD | |
151 | } noddy_NoddyObject; | |
152 | </pre></div> | |
153 | ||
154 | <P> | |
155 | This is what a Noddy object will contain--in this case, nothing more | |
156 | than every Python object contains, namely a refcount and a pointer to a type | |
157 | object. These are the fields the <code>PyObject_HEAD</code> macro brings | |
158 | in. The reason for the macro is to standardize the layout and to | |
159 | enable special debugging fields in debug builds. Note that there is | |
160 | no semicolon after the <code>PyObject_HEAD</code> macro; one is included in | |
161 | the macro definition. Be wary of adding one by accident; it's easy to | |
162 | do from habit, and your compiler might not complain, but someone | |
163 | else's probably will! (On Windows, MSVC is known to call this an | |
164 | error and refuse to compile the code.) | |
165 | ||
166 | <P> | |
167 | For contrast, let's take a look at the corresponding definition for | |
168 | standard Python integers: | |
169 | ||
170 | <P> | |
171 | <div class="verbatim"><pre> | |
172 | typedef struct { | |
173 | PyObject_HEAD | |
174 | long ob_ival; | |
175 | } PyIntObject; | |
176 | </pre></div> | |
177 | ||
178 | <P> | |
179 | Moving on, we come to the crunch -- the type object. | |
180 | ||
181 | <P> | |
182 | <div class="verbatim"><pre> | |
183 | static PyTypeObject noddy_NoddyType = { | |
184 | PyObject_HEAD_INIT(NULL) | |
185 | 0, /*ob_size*/ | |
186 | "noddy.Noddy", /*tp_name*/ | |
187 | sizeof(noddy_NoddyObject), /*tp_basicsize*/ | |
188 | 0, /*tp_itemsize*/ | |
189 | 0, /*tp_dealloc*/ | |
190 | 0, /*tp_print*/ | |
191 | 0, /*tp_getattr*/ | |
192 | 0, /*tp_setattr*/ | |
193 | 0, /*tp_compare*/ | |
194 | 0, /*tp_repr*/ | |
195 | 0, /*tp_as_number*/ | |
196 | 0, /*tp_as_sequence*/ | |
197 | 0, /*tp_as_mapping*/ | |
198 | 0, /*tp_hash */ | |
199 | 0, /*tp_call*/ | |
200 | 0, /*tp_str*/ | |
201 | 0, /*tp_getattro*/ | |
202 | 0, /*tp_setattro*/ | |
203 | 0, /*tp_as_buffer*/ | |
204 | Py_TPFLAGS_DEFAULT, /*tp_flags*/ | |
205 | "Noddy objects", /* tp_doc */ | |
206 | }; | |
207 | </pre></div> | |
208 | ||
209 | <P> | |
210 | Now if you go and look up the definition of <tt class="ctype">PyTypeObject</tt> in | |
211 | <span class="file">object.h</span> you'll see that it has many more fields that the | |
212 | definition above. The remaining fields will be filled with zeros by | |
213 | the C compiler, and it's common practice to not specify them | |
214 | explicitly unless you need them. | |
215 | ||
216 | <P> | |
217 | This is so important that we're going to pick the top of it apart still | |
218 | further: | |
219 | ||
220 | <P> | |
221 | <div class="verbatim"><pre> | |
222 | PyObject_HEAD_INIT(NULL) | |
223 | </pre></div> | |
224 | ||
225 | <P> | |
226 | This line is a bit of a wart; what we'd like to write is: | |
227 | ||
228 | <P> | |
229 | <div class="verbatim"><pre> | |
230 | PyObject_HEAD_INIT(&PyType_Type) | |
231 | </pre></div> | |
232 | ||
233 | <P> | |
234 | as the type of a type object is ``type'', but this isn't strictly | |
235 | conforming C and some compilers complain. Fortunately, this member | |
236 | will be filled in for us by <tt class="cfunction">PyType_Ready()</tt>. | |
237 | ||
238 | <P> | |
239 | <div class="verbatim"><pre> | |
240 | 0, /* ob_size */ | |
241 | </pre></div> | |
242 | ||
243 | <P> | |
244 | The <tt class="member">ob_size</tt> field of the header is not used; its presence in | |
245 | the type structure is a historical artifact that is maintained for | |
246 | binary compatibility with extension modules compiled for older | |
247 | versions of Python. Always set this field to zero. | |
248 | ||
249 | <P> | |
250 | <div class="verbatim"><pre> | |
251 | "noddy.Noddy", /* tp_name */ | |
252 | </pre></div> | |
253 | ||
254 | <P> | |
255 | The name of our type. This will appear in the default textual | |
256 | representation of our objects and in some error messages, for example: | |
257 | ||
258 | <P> | |
259 | <div class="verbatim"><pre> | |
260 | >>> "" + noddy.new_noddy() | |
261 | Traceback (most recent call last): | |
262 | File "<stdin>", line 1, in ? | |
263 | TypeError: cannot add type "noddy.Noddy" to string | |
264 | </pre></div> | |
265 | ||
266 | <P> | |
267 | Note that the name is a dotted name that includes both the module name | |
268 | and the name of the type within the module. The module in this case is | |
269 | <tt class="module">noddy</tt> and the type is <tt class="class">Noddy</tt>, so we set the type name | |
270 | to <tt class="class">noddy.Noddy</tt>. | |
271 | ||
272 | <P> | |
273 | <div class="verbatim"><pre> | |
274 | sizeof(noddy_NoddyObject), /* tp_basicsize */ | |
275 | </pre></div> | |
276 | ||
277 | <P> | |
278 | This is so that Python knows how much memory to allocate when you call | |
279 | <tt class="cfunction">PyObject_New()</tt>. | |
280 | ||
281 | <P> | |
282 | <span class="note"><b class="label">Note:</b> | |
283 | If you want your type to be subclassable from Python, and your | |
284 | type has the same <tt class="member">tp_basicsize</tt> as its base type, you may | |
285 | have problems with multiple inheritance. A Python subclass of your | |
286 | type will have to list your type first in its <tt class="member">__bases__</tt>, or | |
287 | else it will not be able to call your type's <tt class="method">__new__</tt> method | |
288 | without getting an error. You can avoid this problem by ensuring | |
289 | that your type has a larger value for <tt class="member">tp_basicsize</tt> than | |
290 | its base type does. Most of the time, this will be true anyway, | |
291 | because either your base type will be <tt class="class">object</tt>, or else you will | |
292 | be adding data members to your base type, and therefore increasing its | |
293 | size.</span> | |
294 | ||
295 | <P> | |
296 | <div class="verbatim"><pre> | |
297 | 0, /* tp_itemsize */ | |
298 | </pre></div> | |
299 | ||
300 | <P> | |
301 | This has to do with variable length objects like lists and strings. | |
302 | Ignore this for now. | |
303 | ||
304 | <P> | |
305 | Skipping a number of type methods that we don't provide, we set the | |
306 | class flags to <tt class="constant">Py_TPFLAGS_DEFAULT</tt>. | |
307 | ||
308 | <P> | |
309 | <div class="verbatim"><pre> | |
310 | Py_TPFLAGS_DEFAULT, /*tp_flags*/ | |
311 | </pre></div> | |
312 | ||
313 | <P> | |
314 | All types should include this constant in their flags. It enables all | |
315 | of the members defined by the current version of Python. | |
316 | ||
317 | <P> | |
318 | We provide a doc string for the type in <tt class="member">tp_doc</tt>. | |
319 | ||
320 | <P> | |
321 | <div class="verbatim"><pre> | |
322 | "Noddy objects", /* tp_doc */ | |
323 | </pre></div> | |
324 | ||
325 | <P> | |
326 | Now we get into the type methods, the things that make your objects | |
327 | different from the others. We aren't going to implement any of these | |
328 | in this version of the module. We'll expand this example later to | |
329 | have more interesting behavior. | |
330 | ||
331 | <P> | |
332 | For now, all we want to be able to do is to create new <tt class="class">Noddy</tt> | |
333 | objects. To enable object creation, we have to provide a | |
334 | <tt class="member">tp_new</tt> implementation. In this case, we can just use the | |
335 | default implementation provided by the API function | |
336 | <tt class="cfunction">PyType_GenericNew()</tt>. We'd like to just assign this to the | |
337 | <tt class="member">tp_new</tt> slot, but we can't, for portability sake, On some | |
338 | platforms or compilers, we can't statically initialize a structure | |
339 | member with a function defined in another C module, so, instead, we'll | |
340 | assign the <tt class="member">tp_new</tt> slot in the module initialization function | |
341 | just before calling <tt class="cfunction">PyType_Ready()</tt>: | |
342 | ||
343 | <P> | |
344 | <div class="verbatim"><pre> | |
345 | noddy_NoddyType.tp_new = PyType_GenericNew; | |
346 | if (PyType_Ready(&noddy_NoddyType) < 0) | |
347 | return; | |
348 | </pre></div> | |
349 | ||
350 | <P> | |
351 | All the other type methods are <tt class="constant">NULL</tt>, so we'll go over them later | |
352 | -- that's for a later section! | |
353 | ||
354 | <P> | |
355 | Everything else in the file should be familiar, except for some code | |
356 | in <tt class="cfunction">initnoddy()</tt>: | |
357 | ||
358 | <P> | |
359 | <div class="verbatim"><pre> | |
360 | if (PyType_Ready(&noddy_NoddyType) < 0) | |
361 | return; | |
362 | </pre></div> | |
363 | ||
364 | <P> | |
365 | This initializes the <tt class="class">Noddy</tt> type, filing in a number of | |
366 | members, including <tt class="member">ob_type</tt> that we initially set to <tt class="constant">NULL</tt>. | |
367 | ||
368 | <P> | |
369 | <div class="verbatim"><pre> | |
370 | PyModule_AddObject(m, "Noddy", (PyObject *)&noddy_NoddyType); | |
371 | </pre></div> | |
372 | ||
373 | <P> | |
374 | This adds the type to the module dictionary. This allows us to create | |
375 | <tt class="class">Noddy</tt> instances by calling the <tt class="class">Noddy</tt> class: | |
376 | ||
377 | <P> | |
378 | <div class="verbatim"><pre> | |
379 | >>> import noddy | |
380 | >>> mynoddy = noddy.Noddy() | |
381 | </pre></div> | |
382 | ||
383 | <P> | |
384 | That's it! All that remains is to build it; put the above code in a | |
385 | file called <span class="file">noddy.c</span> and | |
386 | ||
387 | <P> | |
388 | <div class="verbatim"><pre> | |
389 | from distutils.core import setup, Extension | |
390 | setup(name="noddy", version="1.0", | |
391 | ext_modules=[Extension("noddy", ["noddy.c"])]) | |
392 | </pre></div> | |
393 | ||
394 | <P> | |
395 | in a file called <span class="file">setup.py</span>; then typing | |
396 | ||
397 | <P> | |
398 | <div class="verbatim"><pre> | |
399 | $ python setup.py build | |
400 | </pre></div> | |
401 | <P> | |
402 | at a shell should produce a file <span class="file">noddy.so</span> in a subdirectory; | |
403 | move to that directory and fire up Python -- you should be able to | |
404 | <code>import noddy</code> and play around with Noddy objects. | |
405 | ||
406 | <P> | |
407 | That wasn't so hard, was it? | |
408 | ||
409 | <P> | |
410 | Of course, the current Noddy type is pretty uninteresting. It has no | |
411 | data and doesn't do anything. It can't even be subclassed. | |
412 | ||
413 | <P> | |
414 | ||
415 | <p><br /></p><hr class='online-navigation' /> | |
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417 | <!--Table of Child-Links--> | |
418 | <A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a> | |
419 | ||
420 | <UL CLASS="ChildLinks"> | |
421 | <LI><A href="node22.html">2.1.1 Adding data and methods to the Basic example</a> | |
422 | <LI><A href="node23.html">2.1.2 Providing finer control over data attributes</a> | |
423 | <LI><A href="node24.html">2.1.3 Supporting cyclic garbage collection</a> | |
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461 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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