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16 | <title>1.10.2 Ownership Rules | |
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54 | <H2><A NAME="SECTION0031020000000000000000"></A><A NAME="ownershipRules"></A> | |
55 | <BR> | |
56 | 1.10.2 Ownership Rules | |
57 | ||
58 | </H2> | |
59 | ||
60 | <P> | |
61 | Whenever an object reference is passed into or out of a function, it | |
62 | is part of the function's interface specification whether ownership is | |
63 | transferred with the reference or not. | |
64 | ||
65 | <P> | |
66 | Most functions that return a reference to an object pass on ownership | |
67 | with the reference. In particular, all functions whose function it is | |
68 | to create a new object, such as <tt class="cfunction">PyInt_FromLong()</tt> and | |
69 | <tt class="cfunction">Py_BuildValue()</tt>, pass ownership to the receiver. Even if | |
70 | the object is not actually new, you still receive ownership of a new | |
71 | reference to that object. For instance, <tt class="cfunction">PyInt_FromLong()</tt> | |
72 | maintains a cache of popular values and can return a reference to a | |
73 | cached item. | |
74 | ||
75 | <P> | |
76 | Many functions that extract objects from other objects also transfer | |
77 | ownership with the reference, for instance | |
78 | <tt class="cfunction">PyObject_GetAttrString()</tt>. The picture is less clear, here, | |
79 | however, since a few common routines are exceptions: | |
80 | <tt class="cfunction">PyTuple_GetItem()</tt>, <tt class="cfunction">PyList_GetItem()</tt>, | |
81 | <tt class="cfunction">PyDict_GetItem()</tt>, and <tt class="cfunction">PyDict_GetItemString()</tt> | |
82 | all return references that you borrow from the tuple, list or | |
83 | dictionary. | |
84 | ||
85 | <P> | |
86 | The function <tt class="cfunction">PyImport_AddModule()</tt> also returns a borrowed | |
87 | reference, even though it may actually create the object it returns: | |
88 | this is possible because an owned reference to the object is stored in | |
89 | <code>sys.modules</code>. | |
90 | ||
91 | <P> | |
92 | When you pass an object reference into another function, in general, | |
93 | the function borrows the reference from you -- if it needs to store | |
94 | it, it will use <tt class="cfunction">Py_INCREF()</tt> to become an independent | |
95 | owner. There are exactly two important exceptions to this rule: | |
96 | <tt class="cfunction">PyTuple_SetItem()</tt> and <tt class="cfunction">PyList_SetItem()</tt>. These | |
97 | functions take over ownership of the item passed to them -- even if | |
98 | they fail! (Note that <tt class="cfunction">PyDict_SetItem()</tt> and friends don't | |
99 | take over ownership -- they are ``normal.'') | |
100 | ||
101 | <P> | |
102 | When a C function is called from Python, it borrows references to its | |
103 | arguments from the caller. The caller owns a reference to the object, | |
104 | so the borrowed reference's lifetime is guaranteed until the function | |
105 | returns. Only when such a borrowed reference must be stored or passed | |
106 | on, it must be turned into an owned reference by calling | |
107 | <tt class="cfunction">Py_INCREF()</tt>. | |
108 | ||
109 | <P> | |
110 | The object reference returned from a C function that is called from | |
111 | Python must be an owned reference -- ownership is transferred from | |
112 | the function to its caller. | |
113 | ||
114 | <P> | |
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