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55 | <H1><A NAME="SECTION005000000000000000000"></A><A NAME="countingRefs"></A> | |
56 | <BR> | |
57 | 3. Reference Counting | |
58 | </H1> | |
59 | ||
60 | <P> | |
61 | The macros in this section are used for managing reference counts | |
62 | of Python objects. | |
63 | ||
64 | <P> | |
65 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-84' xml:id='l2h-84' class="cfunction">Py_INCREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt> | |
66 | <dd> | |
67 | Increment the reference count for object <var>o</var>. The object must | |
68 | not be <tt class="constant">NULL</tt>; if you aren't sure that it isn't <tt class="constant">NULL</tt>, use | |
69 | <tt class="cfunction">Py_XINCREF()</tt>. | |
70 | </dd></dl> | |
71 | ||
72 | <P> | |
73 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-85' xml:id='l2h-85' class="cfunction">Py_XINCREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt> | |
74 | <dd> | |
75 | Increment the reference count for object <var>o</var>. The object may be | |
76 | <tt class="constant">NULL</tt>, in which case the macro has no effect. | |
77 | </dd></dl> | |
78 | ||
79 | <P> | |
80 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-86' xml:id='l2h-86' class="cfunction">Py_DECREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt> | |
81 | <dd> | |
82 | Decrement the reference count for object <var>o</var>. The object must | |
83 | not be <tt class="constant">NULL</tt>; if you aren't sure that it isn't <tt class="constant">NULL</tt>, use | |
84 | <tt class="cfunction">Py_XDECREF()</tt>. If the reference count reaches zero, the | |
85 | object's type's deallocation function (which must not be <tt class="constant">NULL</tt>) is | |
86 | invoked. | |
87 | ||
88 | <P> | |
89 | <span class="warning"><b class="label">Warning:</b> | |
90 | The deallocation function can cause arbitrary Python code | |
91 | to be invoked (e.g. when a class instance with a <tt class="method">__del__()</tt> | |
92 | method is deallocated). While exceptions in such code are not | |
93 | propagated, the executed code has free access to all Python global | |
94 | variables. This means that any object that is reachable from a | |
95 | global variable should be in a consistent state before | |
96 | <tt class="cfunction">Py_DECREF()</tt> is invoked. For example, code to delete an | |
97 | object from a list should copy a reference to the deleted object in | |
98 | a temporary variable, update the list data structure, and then call | |
99 | <tt class="cfunction">Py_DECREF()</tt> for the temporary variable.</span> | |
100 | </dd></dl> | |
101 | ||
102 | <P> | |
103 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-87' xml:id='l2h-87' class="cfunction">Py_XDECREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt> | |
104 | <dd> | |
105 | Decrement the reference count for object <var>o</var>. The object may be | |
106 | <tt class="constant">NULL</tt>, in which case the macro has no effect; otherwise the effect | |
107 | is the same as for <tt class="cfunction">Py_DECREF()</tt>, and the same warning | |
108 | applies. | |
109 | </dd></dl> | |
110 | ||
111 | <P> | |
112 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-88' xml:id='l2h-88' class="cfunction">Py_CLEAR</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt> | |
113 | <dd> | |
114 | Decrement the reference count for object <var>o</var>. The object may be | |
115 | <tt class="constant">NULL</tt>, in which case the macro has no effect; otherwise the effect | |
116 | is the same as for <tt class="cfunction">Py_DECREF()</tt>, except that the argument | |
117 | is also set to <tt class="constant">NULL</tt>. The warning for <tt class="cfunction">Py_DECREF()</tt> does | |
118 | not apply with respect to the object passed because the macro | |
119 | carefully uses a temporary variable and sets the argument to <tt class="constant">NULL</tt> | |
120 | before decrementing its reference count. | |
121 | ||
122 | <P> | |
123 | It is a good idea to use this macro whenever decrementing the value | |
124 | of a variable that might be traversed during garbage collection. | |
125 | ||
126 | <P> | |
127 | ||
128 | <span class="versionnote">New in version 2.4.</span> | |
129 | ||
130 | </dd></dl> | |
131 | ||
132 | <P> | |
133 | The following functions are for runtime dynamic embedding of Python: | |
134 | <tt class="cfunction">Py_IncRef(PyObject *o)</tt>, <tt class="cfunction">Py_DecRef(PyObject *o)</tt>. | |
135 | They are simply exported function versions of <tt class="cfunction">Py_XINCREF()</tt> and | |
136 | <tt class="cfunction">Py_XDECREF()</tt>, respectively. | |
137 | ||
138 | <P> | |
139 | The following functions or macros are only for use within the | |
140 | interpreter core: <tt class="cfunction">_Py_Dealloc()</tt>, | |
141 | <tt class="cfunction">_Py_ForgetReference()</tt>, <tt class="cfunction">_Py_NewReference()</tt>, as | |
142 | well as the global variable <tt class="cdata">_Py_RefTotal</tt>. | |
143 | ||
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