| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> |
| 2 | <html> |
| 3 | <head> |
| 4 | <link rel="STYLESHEET" href="ext.css" type='text/css' /> |
| 5 | <link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" /> |
| 6 | <link rel='start' href='../index.html' title='Python Documentation Index' /> |
| 7 | <link rel="first" href="ext.html" title='Extending and Embedding the Python Interpreter' /> |
| 8 | <link rel='contents' href='contents.html' title="Contents" /> |
| 9 | <link rel='last' href='about.html' title='About this document...' /> |
| 10 | <link rel='help' href='about.html' title='About this document...' /> |
| 11 | <link rel="next" href="node28.html" /> |
| 12 | <link rel="prev" href="node26.html" /> |
| 13 | <link rel="parent" href="dnt-type-methods.html" /> |
| 14 | <link rel="next" href="node28.html" /> |
| 15 | <meta name='aesop' content='information' /> |
| 16 | <title>2.2.2 Object Presentation</title> |
| 17 | </head> |
| 18 | <body> |
| 19 | <DIV CLASS="navigation"> |
| 20 | <div id='top-navigation-panel' xml:id='top-navigation-panel'> |
| 21 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> |
| 22 | <tr> |
| 23 | <td class='online-navigation'><a rel="prev" title="2.2.1 Finalization and De-allocation" |
| 24 | href="node26.html"><img src='../icons/previous.png' |
| 25 | border='0' height='32' alt='Previous Page' width='32' /></A></td> |
| 26 | <td class='online-navigation'><a rel="parent" title="2.2 Type Methods" |
| 27 | href="dnt-type-methods.html"><img src='../icons/up.png' |
| 28 | border='0' height='32' alt='Up One Level' width='32' /></A></td> |
| 29 | <td class='online-navigation'><a rel="next" title="2.2.3 Attribute Management" |
| 30 | href="node28.html"><img src='../icons/next.png' |
| 31 | border='0' height='32' alt='Next Page' width='32' /></A></td> |
| 32 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> |
| 33 | <td class='online-navigation'><a rel="contents" title="Table of Contents" |
| 34 | href="contents.html"><img src='../icons/contents.png' |
| 35 | border='0' height='32' alt='Contents' width='32' /></A></td> |
| 36 | <td class='online-navigation'><img src='../icons/blank.png' |
| 37 | border='0' height='32' alt='' width='32' /></td> |
| 38 | <td class='online-navigation'><img src='../icons/blank.png' |
| 39 | border='0' height='32' alt='' width='32' /></td> |
| 40 | </tr></table> |
| 41 | <div class='online-navigation'> |
| 42 | <b class="navlabel">Previous:</b> |
| 43 | <a class="sectref" rel="prev" href="node26.html">2.2.1 Finalization and De-allocation</A> |
| 44 | <b class="navlabel">Up:</b> |
| 45 | <a class="sectref" rel="parent" href="dnt-type-methods.html">2.2 Type Methods</A> |
| 46 | <b class="navlabel">Next:</b> |
| 47 | <a class="sectref" rel="next" href="node28.html">2.2.3 Attribute Management</A> |
| 48 | </div> |
| 49 | <hr /></div> |
| 50 | </DIV> |
| 51 | <!--End of Navigation Panel--> |
| 52 | |
| 53 | <H2><A NAME="SECTION004220000000000000000"> |
| 54 | 2.2.2 Object Presentation</A> |
| 55 | </H2> |
| 56 | |
| 57 | <P> |
| 58 | In Python, there are three ways to generate a textual representation |
| 59 | of an object: the <tt class="function">repr()</tt><a id='l2h-8' xml:id='l2h-8'></a> function (or |
| 60 | equivalent back-tick syntax), the <tt class="function">str()</tt><a id='l2h-9' xml:id='l2h-9'></a>function, and the <tt class="keyword">print</tt> statement. For most objects, the |
| 61 | <tt class="keyword">print</tt> statement is equivalent to the <tt class="function">str()</tt> |
| 62 | function, but it is possible to special-case printing to a |
| 63 | <tt class="ctype">FILE*</tt> if necessary; this should only be done if efficiency is |
| 64 | identified as a problem and profiling suggests that creating a |
| 65 | temporary string object to be written to a file is too expensive. |
| 66 | |
| 67 | <P> |
| 68 | These handlers are all optional, and most types at most need to |
| 69 | implement the <tt class="member">tp_str</tt> and <tt class="member">tp_repr</tt> handlers. |
| 70 | |
| 71 | <P> |
| 72 | <div class="verbatim"><pre> |
| 73 | reprfunc tp_repr; |
| 74 | reprfunc tp_str; |
| 75 | printfunc tp_print; |
| 76 | </pre></div> |
| 77 | |
| 78 | <P> |
| 79 | The <tt class="member">tp_repr</tt> handler should return a string object containing |
| 80 | a representation of the instance for which it is called. Here is a |
| 81 | simple example: |
| 82 | |
| 83 | <P> |
| 84 | <div class="verbatim"><pre> |
| 85 | static PyObject * |
| 86 | newdatatype_repr(newdatatypeobject * obj) |
| 87 | { |
| 88 | return PyString_FromFormat("Repr-ified_newdatatype{{size:\%d}}", |
| 89 | obj->obj_UnderlyingDatatypePtr->size); |
| 90 | } |
| 91 | </pre></div> |
| 92 | |
| 93 | <P> |
| 94 | If no <tt class="member">tp_repr</tt> handler is specified, the interpreter will |
| 95 | supply a representation that uses the type's <tt class="member">tp_name</tt> and a |
| 96 | uniquely-identifying value for the object. |
| 97 | |
| 98 | <P> |
| 99 | The <tt class="member">tp_str</tt> handler is to <tt class="function">str()</tt> what the |
| 100 | <tt class="member">tp_repr</tt> handler described above is to <tt class="function">repr()</tt>; that |
| 101 | is, it is called when Python code calls <tt class="function">str()</tt> on an |
| 102 | instance of your object. Its implementation is very similar to the |
| 103 | <tt class="member">tp_repr</tt> function, but the resulting string is intended for |
| 104 | human consumption. If <tt class="member">tp_str</tt> is not specified, the |
| 105 | <tt class="member">tp_repr</tt> handler is used instead. |
| 106 | |
| 107 | <P> |
| 108 | Here is a simple example: |
| 109 | |
| 110 | <P> |
| 111 | <div class="verbatim"><pre> |
| 112 | static PyObject * |
| 113 | newdatatype_str(newdatatypeobject * obj) |
| 114 | { |
| 115 | return PyString_FromFormat("Stringified_newdatatype{{size:\%d}}", |
| 116 | obj->obj_UnderlyingDatatypePtr->size); |
| 117 | } |
| 118 | </pre></div> |
| 119 | |
| 120 | <P> |
| 121 | The print function will be called whenever Python needs to "print" an |
| 122 | instance of the type. For example, if 'node' is an instance of type |
| 123 | TreeNode, then the print function is called when Python code calls: |
| 124 | |
| 125 | <P> |
| 126 | <div class="verbatim"><pre> |
| 127 | print node |
| 128 | </pre></div> |
| 129 | |
| 130 | <P> |
| 131 | There is a flags argument and one flag, <tt class="constant">Py_PRINT_RAW</tt>, and |
| 132 | it suggests that you print without string quotes and possibly without |
| 133 | interpreting escape sequences. |
| 134 | |
| 135 | <P> |
| 136 | The print function receives a file object as an argument. You will |
| 137 | likely want to write to that file object. |
| 138 | |
| 139 | <P> |
| 140 | Here is a sample print function: |
| 141 | |
| 142 | <P> |
| 143 | <div class="verbatim"><pre> |
| 144 | static int |
| 145 | newdatatype_print(newdatatypeobject *obj, FILE *fp, int flags) |
| 146 | { |
| 147 | if (flags & Py_PRINT_RAW) { |
| 148 | fprintf(fp, "<{newdatatype object--size: %d}>", |
| 149 | obj->obj_UnderlyingDatatypePtr->size); |
| 150 | } |
| 151 | else { |
| 152 | fprintf(fp, "\"<{newdatatype object--size: %d}>\"", |
| 153 | obj->obj_UnderlyingDatatypePtr->size); |
| 154 | } |
| 155 | return 0; |
| 156 | } |
| 157 | </pre></div> |
| 158 | |
| 159 | <P> |
| 160 | |
| 161 | <DIV CLASS="navigation"> |
| 162 | <div class='online-navigation'> |
| 163 | <p></p><hr /> |
| 164 | <table align="center" width="100%" cellpadding="0" cellspacing="2"> |
| 165 | <tr> |
| 166 | <td class='online-navigation'><a rel="prev" title="2.2.1 Finalization and De-allocation" |
| 167 | href="node26.html"><img src='../icons/previous.png' |
| 168 | border='0' height='32' alt='Previous Page' width='32' /></A></td> |
| 169 | <td class='online-navigation'><a rel="parent" title="2.2 Type Methods" |
| 170 | href="dnt-type-methods.html"><img src='../icons/up.png' |
| 171 | border='0' height='32' alt='Up One Level' width='32' /></A></td> |
| 172 | <td class='online-navigation'><a rel="next" title="2.2.3 Attribute Management" |
| 173 | href="node28.html"><img src='../icons/next.png' |
| 174 | border='0' height='32' alt='Next Page' width='32' /></A></td> |
| 175 | <td align="center" width="100%">Extending and Embedding the Python Interpreter</td> |
| 176 | <td class='online-navigation'><a rel="contents" title="Table of Contents" |
| 177 | href="contents.html"><img src='../icons/contents.png' |
| 178 | border='0' height='32' alt='Contents' width='32' /></A></td> |
| 179 | <td class='online-navigation'><img src='../icons/blank.png' |
| 180 | border='0' height='32' alt='' width='32' /></td> |
| 181 | <td class='online-navigation'><img src='../icons/blank.png' |
| 182 | border='0' height='32' alt='' width='32' /></td> |
| 183 | </tr></table> |
| 184 | <div class='online-navigation'> |
| 185 | <b class="navlabel">Previous:</b> |
| 186 | <a class="sectref" rel="prev" href="node26.html">2.2.1 Finalization and De-allocation</A> |
| 187 | <b class="navlabel">Up:</b> |
| 188 | <a class="sectref" rel="parent" href="dnt-type-methods.html">2.2 Type Methods</A> |
| 189 | <b class="navlabel">Next:</b> |
| 190 | <a class="sectref" rel="next" href="node28.html">2.2.3 Attribute Management</A> |
| 191 | </div> |
| 192 | </div> |
| 193 | <hr /> |
| 194 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
| 195 | </DIV> |
| 196 | <!--End of Navigation Panel--> |
| 197 | <ADDRESS> |
| 198 | See <i><a href="about.html">About this document...</a></i> for information on suggesting changes. |
| 199 | </ADDRESS> |
| 200 | </BODY> |
| 201 | </HTML> |