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1.2.1.1 Reference Count Details

The reference count behavior of functions in the Python/C API is best explained in terms of ownership of references. Ownership pertains to references, never to objects (objects are not owned: they are always shared). "Owning a reference" means being responsible for calling Py_DECREF on it when the reference is no longer needed. Ownership can also be transferred, meaning that the code that receives ownership of the reference then becomes responsible for eventually decref'ing it by calling `Py_DECREF()` or `Py_XDECREF()` when it's no longer needed--or passing on this responsibility (usually to its caller). 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. Stealing a reference means that when you pass a reference to a function, that function assumes that it now owns that reference, and you are not responsible for it any longer.

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"));

Here, `PyInt_FromLong()` returns a new reference which is immediately stolen by `PyTuple_SetItem()`. When you want to keep using an object although the reference to it will be stolen, use `Py_INCREF()` to grab another reference before calling the reference-stealing function.

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 reference 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 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;
    }