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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' /> |
| 416 | <div class='online-navigation'> |
| 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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