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(python2.1-api.info)Reference Count Details

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Reference Count Details
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The reference count behavior of functions in the Python/C API is best
explained in terms of _ownership of references_.  Note that we talk of
owning references, never of owning objects; objects are always shared!
When a function owns a reference, it has to dispose of it properly --
either by passing ownership on (usually to its caller) or by calling
`Py_DECREF()' or `Py_XDECREF()'.  When a function passes ownership of a
reference on to its caller, the caller is said to receive a _new_
reference.  When no ownership is transferred, the caller is said to
_borrow_ the reference.  Nothing needs to be done for a borrowed
reference.

Conversely, when a calling function passes it a reference to an object,
there are two possibilities: the function _steals_ a reference to the
object, or it does not.  Few functions steal references; the two
notable exceptions are `PyList_SetItem()'  and `PyTuple_SetItem()' ,
which steal a reference to the item (but not to the tuple or list into
which the item is put!).  These functions were designed to steal a
reference because of a common idiom for populating a tuple or list with
newly created objects; for example, the code to create the tuple `(1,
2, "three")' could look like this (forgetting about error handling for
the moment; a better way to code this is shown below):

     PyObject *t;
     
     t = PyTuple_New(3);
     PyTuple_SetItem(t, 0, PyInt_FromLong(1L));
     PyTuple_SetItem(t, 1, PyInt_FromLong(2L));
     PyTuple_SetItem(t, 2, PyString_FromString("three"));

Incidentally, `PyTuple_SetItem()' is the _only_ way to set tuple items;
`PySequence_SetItem()' and `PyObject_SetItem()' refuse to do this since
tuples are an immutable data type.  You should only use
`PyTuple_SetItem()' for tuples that you are creating yourself.

Equivalent code for populating a list can be written using
`PyList_New()' and `PyList_SetItem()'.  Such code can also use
`PySequence_SetItem()'; this illustrates the difference between the two
(the extra `Py_DECREF()' calls):

     PyObject *l, *x;
     
     l = PyList_New(3);
     x = PyInt_FromLong(1L);
     PySequence_SetItem(l, 0, x); Py_DECREF(x);
     x = PyInt_FromLong(2L);
     PySequence_SetItem(l, 1, x); Py_DECREF(x);
     x = PyString_FromString("three");
     PySequence_SetItem(l, 2, x); Py_DECREF(x);

You might find it strange that the "recommended" approach takes more
code.  However, in practice, you will rarely use these ways of creating
and populating a tuple or list.  There's a generic function,
`Py_BuildValue()', that can create most common objects from C values,
directed by a "format string".  For example, the above two blocks of
code could be replaced by the following (which also takes care of the
error checking):

     PyObject *t, *l;
     
     t = Py_BuildValue("(iis)", 1, 2, "three");
     l = Py_BuildValue("[iis]", 1, 2, "three");

It is much more common to use `PyObject_SetItem()' and friends with
items whose references you are only borrowing, like arguments that were
passed in to the function you are writing.  In that case, their
behaviour regarding reference counts is much saner, since you don't
have to increment a reference count so you can give a reference away
("have it be stolen").  For example, this function sets all items of a
list (actually, any mutable sequence) to a given item:

     int set_all(PyObject *target, PyObject *item)
     {
         int i, n;
     
         n = PyObject_Length(target);
         if (n < 0)
             return -1;
         for (i = 0; i < n; i++) {
             if (PyObject_SetItem(target, i, item) < 0)
                 return -1;
         }
         return 0;
     }

The situation is slightly different for function return values.  While
passing a reference to most functions does not change your ownership
responsibilities for that reference, many functions that return a
referece to an object give you ownership of the reference.  The reason
is simple: in many cases, the returned object is created on the fly,
and the reference you get is the only reference to the object.
Therefore, the generic functions that return object references, like
`PyObject_GetItem()' and `PySequence_GetItem()', always return a new
reference (the caller becomes the owner of the reference).

It is important to realize that whether you own a reference returned by
a function depends on which function you call only -- _the plumage_
(the type of the type of the object passed as an argument to the
function) _doesn't enter into it!_  Thus, if you extract an item from a
list using `PyList_GetItem()', you don't own the reference -- but if
you obtain the same item from the same list using
`PySequence_GetItem()' (which happens to take exactly the same
arguments), you do own a reference to the returned object.

Here is an example of how you could write a function that computes the
sum of the items in a list of integers; once using `PyList_GetItem()' ,
and once using `PySequence_GetItem()' .

     long sum_list(PyObject *list)
     {
         int i, n;
         long total = 0;
         PyObject *item;
     
         n = PyList_Size(list);
         if (n < 0)
             return -1; /* Not a list */
         for (i = 0; i < n; i++) {
             item = PyList_GetItem(list, i); /* Can't fail */
             if (!PyInt_Check(item)) continue; /* Skip non-integers */
             total += PyInt_AsLong(item);
         }
         return total;
     }

     long sum_sequence(PyObject *sequence)
     {
         int i, n;
         long total = 0;
         PyObject *item;
         n = PySequence_Length(sequence);
         if (n < 0)
             return -1; /* Has no length */
         for (i = 0; i < n; i++) {
             item = PySequence_GetItem(sequence, i);
             if (item == NULL)
                 return -1; /* Not a sequence, or other failure */
             if (PyInt_Check(item))
                 total += PyInt_AsLong(item);
             Py_DECREF(item); /* Discard reference ownership */
         }
         return total;
     }