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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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