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Python C API and references

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lallous at lgwm

Nov 12, 2009, 1:23 AM

Post #1 of 4 (75 views)
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Python C API and references

Hello,

Everytime I use PyObject_SetAttrString(obj, attr_name, py_val) and I don't
need the reference to py_val I should decrement the reference after this
call?

So for example:

PyObject *py_val = PyInt_FromLong(5)
PyObject_SetAttrString(py_obj, "val", py_val);
Py_DECREF(py_val)

Right?

If so, take sysmodule.c:

if (PyObject_SetAttrString(builtins, "_", Py_None) != 0)
return NULL;

Shouldn't they also call Py_DECREF(Py_None) ?

Same logic applies to PyDict_SetItemString() and the reference should be
decrement after setting the item (ofcourse if the value is not needed).

--
Elias

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lallous at lgwm

Nov 12, 2009, 2:56 AM

Post #2 of 4 (73 views)
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Re: Python C API and references [In reply to]

Hello Daniel,

Thanks for the reply.

>>
>> Everytime I use PyObject_SetAttrString(obj, attr_name, py_val) and I
>> don't
>> need the reference to py_val I should decrement the reference after this
>> call?
>
> It really depends on /how/ the object is created. If the
> method used to create *py_val* increases the reference count
> on the object and another function any other function is
> used to increase the reference count, you should use Py_DECREF
> or Py_XDECREF.
>
>>
>> So for example:
>>
>> PyObject *py_val = PyInt_FromLong(5)
>> PyObject_SetAttrString(py_obj, "val", py_val);
>> Py_DECREF(py_val)
>>
>> Right?
>
> In this case is right. *PyInt_FromLong()* returns a new
> reference: 'Return value: New reference.', which is increasing
> the reference count and PyObject_SetAttrString does it twice,
> then you have a reference count of two and you need to decrement
> the initial reference counting of the object, or you will have
> a memory leak.
>

[quote]
int PyObject_SetAttr(PyObject *o, PyObject *attr_name, PyObject *v)
Set the value of the attribute named attr_name, for object o, to the value
v. Returns -1 on failure. This is the equivalent of the Python statement
o.attr_name = v.
int PyObject_SetAttrString(PyObject *o, const char *attr_name, PyObject *v)
Set the value of the attribute named attr_name, for object o, to the value
v. Returns -1 on failure. This is the equivalent of the Python statement
o.attr_name = v.
[/quote]

Looking at the documentation, should I have understood that the passed value
reference will be incremented and that I should decrement it if I don't need
it?

Or I should have understood just because of the fact that whenever we have x
= b (be it from the C api in a form of SetAttr()) then we should know that
b's reference will be incremented. ?

Because, before this discussion I did not know I should decrease the
reference after SetAttr()

>>
>> If so, take sysmodule.c:
>>
>> if (PyObject_SetAttrString(builtins, "_", Py_None) != 0)
>> return NULL;
>>
>> Shouldn't they also call Py_DECREF(Py_None) ?
>
> No, I think that Py_None do not needs to decrease the
> reference count...
>

None is an object like other objects. I think its reference must be taken
into consideration too, for instance why there is the convenience macro:
Py_RETURN_NONE ?

--
Elias

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dickinsm at gmail

Nov 12, 2009, 3:02 AM

Post #3 of 4 (73 views)
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Re: Python C API and references [In reply to]

On Nov 12, 9:23 am, "lallous" <lall...@lgwm.org> wrote:
> Hello,
>
> Everytime I use PyObject_SetAttrString(obj, attr_name, py_val) and I don't
> need the reference to py_val I should decrement the reference after this
> call?

Not necessarily: it depends where py_val came from. I find the
'ownership' model described in the docs quite helpful:

http://docs.python.org/c-api/intro.html#reference-count-details

It's probably better to read the docs, but here's a short version:

If you create a reference to some Python object in a function in your
code, you then 'own' that reference, and you're responsible for
getting rid of it by the time the function exits. There are various
ways this can happen: you can return the reference, thereby
transferring ownership to the calling function; you can explicitly
discard the reference by calling Py_DECREF; or you can transfer
ownership by calling a function that 'steals' the reference (most
functions in the C-API don't steal references, but rather borrow them
for the duration of the function call).

>
> So for example:
>
> PyObject *py_val = PyInt_FromLong(5)
> PyObject_SetAttrString(py_obj, "val", py_val);
> Py_DECREF(py_val)
>
> Right?

Yes. Here you've created the reference in the first place, so you
should Py_DECREF it before you exit. Right after the
PyObject_SetAttrString call is a good place to do this.

>
> If so, take sysmodule.c:
>
> if (PyObject_SetAttrString(builtins, "_", Py_None) != 0)
> return NULL;
>
> Shouldn't they also call Py_DECREF(Py_None) ?

No, I don't think so. I assume you're looking at the sys_displayhook
function? This function doesn't create new references to Py_None
(well, except when it's about to return Py_None to the caller), so at
this point it doesn't own any reference to Py_None: it's not
responsible for decrementing the reference count.

> Same logic applies to PyDict_SetItemString()

Yes.

--
Mark
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gagsl-py2 at yahoo

Nov 13, 2009, 4:07 AM

Post #4 of 4 (64 views)
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Re: Python C API and references [In reply to]

En Thu, 12 Nov 2009 06:23:54 -0300, lallous <lallous [at] lgwm> escribió:

> Everytime I use PyObject_SetAttrString(obj, attr_name, py_val) and I
> don't need the reference to py_val I should decrement the reference
> after this call?

If you own a reference to py_val, and you don't need it anymore, you must
decrement it. It doesn't matter if you call PyObject_SetAttrString or
whatever, except when the called function says it "steals" a reference.

> So for example:
>
> PyObject *py_val = PyInt_FromLong(5)
> PyObject_SetAttrString(py_obj, "val", py_val);
> Py_DECREF(py_val)
>
> Right?

Yes, because PyInt_FromLong returns a new reference, and you own it.

> If so, take sysmodule.c:
>
> if (PyObject_SetAttrString(builtins, "_", Py_None) != 0)
> return NULL;
>
> Shouldn't they also call Py_DECREF(Py_None) ?

No, because the reference count of Py_None was not incremented previously;
the code doesn't own a reference to Py_None at that time. It's not the
same as the example above.

> Same logic applies to PyDict_SetItemString() and the reference should be
> decrement after setting the item (ofcourse if the value is not needed).

Yes, same as your first example.

--
Gabriel Genellina

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