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<title>1.10.2 Ownership Rules
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<H2><A NAME=
"SECTION0031020000000000000000"></A><A NAME=
"ownershipRules"></A>
Whenever an object reference is passed into or out of a function, it
is part of the function's interface specification whether ownership is
transferred with the reference or not.
Most functions that return a reference to an object pass on ownership
with the reference. In particular, all functions whose function it is
to create a new object, such as
<tt class=
"cfunction">PyInt_FromLong()
</tt> and
<tt class=
"cfunction">Py_BuildValue()
</tt>, pass ownership to the receiver. Even if
the object is not actually new, you still receive ownership of a new
reference to that object. For instance,
<tt class=
"cfunction">PyInt_FromLong()
</tt>
maintains a cache of popular values and can return a reference to a
Many functions that extract objects from other objects also transfer
ownership with the reference, for instance
<tt class=
"cfunction">PyObject_GetAttrString()
</tt>. The picture is less clear, here,
however, since a few common routines are exceptions:
<tt class=
"cfunction">PyTuple_GetItem()
</tt>,
<tt class=
"cfunction">PyList_GetItem()
</tt>,
<tt class=
"cfunction">PyDict_GetItem()
</tt>, and
<tt class=
"cfunction">PyDict_GetItemString()
</tt>
all return references that you borrow from the tuple, list or
The function
<tt class=
"cfunction">PyImport_AddModule()
</tt> also returns a borrowed
reference, even though it may actually create the object it returns:
this is possible because an owned reference to the object is stored in
<code>sys.modules
</code>.
When you pass an object reference into another function, in general,
the function borrows the reference from you -- if it needs to store
it, it will use
<tt class=
"cfunction">Py_INCREF()
</tt> to become an independent
owner. There are exactly two important exceptions to this rule:
<tt class=
"cfunction">PyTuple_SetItem()
</tt> and
<tt class=
"cfunction">PyList_SetItem()
</tt>. These
functions take over ownership of the item passed to them -- even if
they fail! (Note that
<tt class=
"cfunction">PyDict_SetItem()
</tt> and friends don't
take over ownership -- they are ``normal.'')
When a C function is called from Python, it borrows references to its
arguments from the caller. The caller owns a reference to the object,
so the borrowed reference's lifetime is guaranteed until the function
returns. Only when such a borrowed reference must be stored or passed
on, it must be turned into an owned reference by calling
<tt class=
"cfunction">Py_INCREF()
</tt>.
The object reference returned from a C function that is called from
Python must be an owned reference -- ownership is transferred from
the function to its caller.
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<span class=
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2.4.2, documentation updated on
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2005.
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