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54 | <H2><A NAME="SECTION0031010000000000000000"></A><A NAME="refcountsInPython"></A> | |
55 | <BR> | |
56 | 1.10.1 Reference Counting in Python | |
57 | ||
58 | </H2> | |
59 | ||
60 | <P> | |
61 | There are two macros, <code>Py_INCREF(x)</code> and <code>Py_DECREF(x)</code>, | |
62 | which handle the incrementing and decrementing of the reference count. | |
63 | <tt class="cfunction">Py_DECREF()</tt> also frees the object when the count reaches zero. | |
64 | For flexibility, it doesn't call <tt class="cfunction">free()</tt> directly -- rather, it | |
65 | makes a call through a function pointer in the object's <i class="dfn">type | |
66 | object</i>. For this purpose (and others), every object also contains a | |
67 | pointer to its type object. | |
68 | ||
69 | <P> | |
70 | The big question now remains: when to use <code>Py_INCREF(x)</code> and | |
71 | <code>Py_DECREF(x)</code>? Let's first introduce some terms. Nobody | |
72 | ``owns'' an object; however, you can <i class="dfn">own a reference</i> to an | |
73 | object. An object's reference count is now defined as the number of | |
74 | owned references to it. The owner of a reference is responsible for | |
75 | calling <tt class="cfunction">Py_DECREF()</tt> when the reference is no longer | |
76 | needed. Ownership of a reference can be transferred. There are three | |
77 | ways to dispose of an owned reference: pass it on, store it, or call | |
78 | <tt class="cfunction">Py_DECREF()</tt>. Forgetting to dispose of an owned reference | |
79 | creates a memory leak. | |
80 | ||
81 | <P> | |
82 | It is also possible to <i class="dfn">borrow</i><A NAME="tex2html2" | |
83 | HREF="#foot403"><SUP>1.2</SUP></A> a reference to an object. The borrower | |
84 | of a reference should not call <tt class="cfunction">Py_DECREF()</tt>. The borrower must | |
85 | not hold on to the object longer than the owner from which it was | |
86 | borrowed. Using a borrowed reference after the owner has disposed of | |
87 | it risks using freed memory and should be avoided | |
88 | completely.<A NAME="tex2html3" | |
89 | HREF="#foot535"><SUP>1.3</SUP></A> | |
90 | <P> | |
91 | The advantage of borrowing over owning a reference is that you don't | |
92 | need to take care of disposing of the reference on all possible paths | |
93 | through the code -- in other words, with a borrowed reference you | |
94 | don't run the risk of leaking when a premature exit is taken. The | |
95 | disadvantage of borrowing over leaking is that there are some subtle | |
96 | situations where in seemingly correct code a borrowed reference can be | |
97 | used after the owner from which it was borrowed has in fact disposed | |
98 | of it. | |
99 | ||
100 | <P> | |
101 | A borrowed reference can be changed into an owned reference by calling | |
102 | <tt class="cfunction">Py_INCREF()</tt>. This does not affect the status of the owner from | |
103 | which the reference was borrowed -- it creates a new owned reference, | |
104 | and gives full owner responsibilities (the new owner must | |
105 | dispose of the reference properly, as well as the previous owner). | |
106 | ||
107 | <P> | |
108 | <BR><HR><H4>Footnotes</H4> | |
109 | <DL> | |
110 | <DT><A NAME="foot403">...borrow</A><A | |
111 | href="refcountsInPython.html#tex2html2"><SUP>1.2</SUP></A></DT> | |
112 | <DD>The metaphor of | |
113 | ``borrowing'' a reference is not completely correct: the owner still | |
114 | has a copy of the reference. | |
115 | ||
116 | </DD> | |
117 | <DT><A NAME="foot535">... | |
118 | completely.</A><A | |
119 | href="refcountsInPython.html#tex2html3"><SUP>1.3</SUP></A></DT> | |
120 | <DD>Checking that the reference count is at least 1 | |
121 | <strong>does not work</strong> -- the reference count itself could be in | |
122 | freed memory and may thus be reused for another object! | |
123 | ||
124 | </DD> | |
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159 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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