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| 53 | <H3><A NAME="SECTION004231000000000000000"> |
| 54 | 2.2.3.1 Generic Attribute Management</A> |
| 55 | </H3> |
| 56 | |
| 57 | <P> |
| 58 | |
| 59 | <span class="versionnote">New in version 2.2.</span> |
| 60 | |
| 61 | <P> |
| 62 | Most extension types only use <em>simple</em> attributes. So, what |
| 63 | makes the attributes simple? There are only a couple of conditions |
| 64 | that must be met: |
| 65 | |
| 66 | <P> |
| 67 | |
| 68 | <OL> |
| 69 | <LI>The name of the attributes must be known when |
| 70 | <tt class="cfunction">PyType_Ready()</tt> is called. |
| 71 | |
| 72 | <P> |
| 73 | </LI> |
| 74 | <LI>No special processing is needed to record that an attribute |
| 75 | was looked up or set, nor do actions need to be taken based |
| 76 | on the value. |
| 77 | </LI> |
| 78 | </OL> |
| 79 | |
| 80 | <P> |
| 81 | Note that this list does not place any restrictions on the values of |
| 82 | the attributes, when the values are computed, or how relevant data is |
| 83 | stored. |
| 84 | |
| 85 | <P> |
| 86 | When <tt class="cfunction">PyType_Ready()</tt> is called, it uses three tables |
| 87 | referenced by the type object to create <em>descriptors</em> which are |
| 88 | placed in the dictionary of the type object. Each descriptor controls |
| 89 | access to one attribute of the instance object. Each of the tables is |
| 90 | optional; if all three are <tt class="constant">NULL</tt>, instances of the type will only have |
| 91 | attributes that are inherited from their base type, and should leave |
| 92 | the <tt class="member">tp_getattro</tt> and <tt class="member">tp_setattro</tt> fields <tt class="constant">NULL</tt> as |
| 93 | well, allowing the base type to handle attributes. |
| 94 | |
| 95 | <P> |
| 96 | The tables are declared as three fields of the type object: |
| 97 | |
| 98 | <P> |
| 99 | <div class="verbatim"><pre> |
| 100 | struct PyMethodDef *tp_methods; |
| 101 | struct PyMemberDef *tp_members; |
| 102 | struct PyGetSetDef *tp_getset; |
| 103 | </pre></div> |
| 104 | |
| 105 | <P> |
| 106 | If <tt class="member">tp_methods</tt> is not <tt class="constant">NULL</tt>, it must refer to an array of |
| 107 | <tt class="ctype">PyMethodDef</tt> structures. Each entry in the table is an |
| 108 | instance of this structure: |
| 109 | |
| 110 | <P> |
| 111 | <div class="verbatim"><pre> |
| 112 | typedef struct PyMethodDef { |
| 113 | char *ml_name; /* method name */ |
| 114 | PyCFunction ml_meth; /* implementation function */ |
| 115 | int ml_flags; /* flags */ |
| 116 | char *ml_doc; /* docstring */ |
| 117 | } PyMethodDef; |
| 118 | </pre></div> |
| 119 | |
| 120 | <P> |
| 121 | One entry should be defined for each method provided by the type; no |
| 122 | entries are needed for methods inherited from a base type. One |
| 123 | additional entry is needed at the end; it is a sentinel that marks the |
| 124 | end of the array. The <tt class="member">ml_name</tt> field of the sentinel must be |
| 125 | <tt class="constant">NULL</tt>. |
| 126 | |
| 127 | <P> |
| 128 | XXX Need to refer to some unified discussion of the structure fields, |
| 129 | shared with the next section. |
| 130 | |
| 131 | <P> |
| 132 | The second table is used to define attributes which map directly to |
| 133 | data stored in the instance. A variety of primitive C types are |
| 134 | supported, and access may be read-only or read-write. The structures |
| 135 | in the table are defined as: |
| 136 | |
| 137 | <P> |
| 138 | <div class="verbatim"><pre> |
| 139 | typedef struct PyMemberDef { |
| 140 | char *name; |
| 141 | int type; |
| 142 | int offset; |
| 143 | int flags; |
| 144 | char *doc; |
| 145 | } PyMemberDef; |
| 146 | </pre></div> |
| 147 | |
| 148 | <P> |
| 149 | For each entry in the table, a descriptor will be constructed and |
| 150 | added to the type which will be able to extract a value from the |
| 151 | instance structure. The <tt class="member">type</tt> field should contain one of the |
| 152 | type codes defined in the <span class="file">structmember.h</span> header; the value will |
| 153 | be used to determine how to convert Python values to and from C |
| 154 | values. The <tt class="member">flags</tt> field is used to store flags which control |
| 155 | how the attribute can be accessed. |
| 156 | |
| 157 | <P> |
| 158 | XXX Need to move some of this to a shared section! |
| 159 | |
| 160 | <P> |
| 161 | The following flag constants are defined in <span class="file">structmember.h</span>; |
| 162 | they may be combined using bitwise-OR. |
| 163 | |
| 164 | <P> |
| 165 | <div class="center"><table class="realtable"> |
| 166 | <thead> |
| 167 | <tr> |
| 168 | <th class="left" >Constant</th> |
| 169 | <th class="left" >Meaning</th> |
| 170 | </tr> |
| 171 | </thead> |
| 172 | <tbody> |
| 173 | <tr><td class="left" valign="baseline"><tt class="constant">READONLY <a id='l2h-10' xml:id='l2h-10'></a></tt></td> |
| 174 | <td class="left" >Never writable.</td></tr> |
| 175 | <tr><td class="left" valign="baseline"><tt class="constant">RO <a id='l2h-11' xml:id='l2h-11'></a></tt></td> |
| 176 | <td class="left" >Shorthand for <tt class="constant">READONLY</tt>.</td></tr> |
| 177 | <tr><td class="left" valign="baseline"><tt class="constant">READ_RESTRICTED <a id='l2h-12' xml:id='l2h-12'></a></tt></td> |
| 178 | <td class="left" >Not readable in restricted mode.</td></tr> |
| 179 | <tr><td class="left" valign="baseline"><tt class="constant">WRITE_RESTRICTED <a id='l2h-13' xml:id='l2h-13'></a></tt></td> |
| 180 | <td class="left" >Not writable in restricted mode.</td></tr> |
| 181 | <tr><td class="left" valign="baseline"><tt class="constant">RESTRICTED <a id='l2h-14' xml:id='l2h-14'></a></tt></td> |
| 182 | <td class="left" >Not readable or writable in restricted mode.</td></tr></tbody> |
| 183 | </table></div> |
| 184 | |
| 185 | <P> |
| 186 | An interesting advantage of using the <tt class="member">tp_members</tt> table to |
| 187 | build descriptors that are used at runtime is that any attribute |
| 188 | defined this way can have an associated doc string simply by providing |
| 189 | the text in the table. An application can use the introspection API |
| 190 | to retrieve the descriptor from the class object, and get the |
| 191 | doc string using its <tt class="member">__doc__</tt> attribute. |
| 192 | |
| 193 | <P> |
| 194 | As with the <tt class="member">tp_methods</tt> table, a sentinel entry with a |
| 195 | <tt class="member">name</tt> value of <tt class="constant">NULL</tt> is required. |
| 196 | |
| 197 | <P> |
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