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<H2><A NAME="SECTION0031010000000000000000"></A><A NAME="refcountsInPython"></A>
<BR>
1.10.1 Reference Counting in Python

</H2>

<P>
There are two macros, <code>Py_INCREF(x)</code> and <code>Py_DECREF(x)</code>,
which handle the incrementing and decrementing of the reference count.
<tt class="cfunction">Py_DECREF()</tt> also frees the object when the count reaches zero.
For flexibility, it doesn't call <tt class="cfunction">free()</tt> directly -- rather, it
makes a call through a function pointer in the object's <i class="dfn">type
object</i>.  For this purpose (and others), every object also contains a
pointer to its type object.

<P>
The big question now remains: when to use <code>Py_INCREF(x)</code> and
<code>Py_DECREF(x)</code>?  Let's first introduce some terms.  Nobody
``owns'' an object; however, you can <i class="dfn">own a reference</i> to an
object.  An object's reference count is now defined as the number of
owned references to it.  The owner of a reference is responsible for
calling <tt class="cfunction">Py_DECREF()</tt> when the reference is no longer
needed.  Ownership of a reference can be transferred.  There are three
ways to dispose of an owned reference: pass it on, store it, or call
<tt class="cfunction">Py_DECREF()</tt>.  Forgetting to dispose of an owned reference
creates a memory leak.

<P>
It is also possible to <i class="dfn">borrow</i><A NAME="tex2html2"
  HREF="#foot403"><SUP>1.2</SUP></A> a reference to an object.  The borrower
of a reference should not call <tt class="cfunction">Py_DECREF()</tt>.  The borrower must
not hold on to the object longer than the owner from which it was
borrowed.  Using a borrowed reference after the owner has disposed of
it risks using freed memory and should be avoided
completely.<A NAME="tex2html3"
  HREF="#foot535"><SUP>1.3</SUP></A>
<P>
The advantage of borrowing over owning a reference is that you don't
need to take care of disposing of the reference on all possible paths
through the code -- in other words, with a borrowed reference you
don't run the risk of leaking when a premature exit is taken.  The
disadvantage of borrowing over leaking is that there are some subtle
situations where in seemingly correct code a borrowed reference can be
used after the owner from which it was borrowed has in fact disposed
of it.

<P>
A borrowed reference can be changed into an owned reference by calling
<tt class="cfunction">Py_INCREF()</tt>.  This does not affect the status of the owner from
which the reference was borrowed -- it creates a new owned reference,
and gives full owner responsibilities (the new owner must
dispose of the reference properly, as well as the previous owner).

<P>
<BR><HR><H4>Footnotes</H4>
<DL>
<DT><A NAME="foot403">...borrow</A><A
 href="refcountsInPython.html#tex2html2"><SUP>1.2</SUP></A></DT>
<DD>The metaphor of
``borrowing'' a reference is not completely correct: the owner still
has a copy of the reference.

</DD>
<DT><A NAME="foot535">...
completely.</A><A
 href="refcountsInPython.html#tex2html3"><SUP>1.3</SUP></A></DT>
<DD>Checking that the reference count is at least 1
<strong>does not work</strong> -- the reference count itself could be in
freed memory and may thus be reused for another object!

</DD>
</DL>
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