Python C API and references

[lallous]

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