dealloc function in python extend c module

[Shen, Yu-Teh]

I create my extend type something like http://www.python.org/doc/current/extending/newtypes.html.
And my type has a member which is a pointer point to my allocate
memory ( no ref count).
ex:
---------------------------------------------------
typedef struct {
PyObject_HEAD
/* Type-specific fields go here. */
mytype *p;
} noddy_NoddyObject;

And i write dealloc function like this:
---------------------------------------------------
static void
Noddy_dealloc(Noddy* self)
{
delete p;
self->ob_type->tp_free((PyObject*)self);
}

And I found it's strange that it didn't call dealloc when there is no
reference.

ex:
a = Noddy()
b = a
del a
del b
# i think there is no ref to the object, and it should call dealloc
but it didn't call!!!

could anyone tell me what happened?

Thanks a lot!!

 

[Philip Semanchuk]

I create my extend type something like http://www.python.org/doc/current/extending/newtypes.html
.
And my type has a member which is a pointer point to my allocate
memory ( no ref count).
ex:
---------------------------------------------------
typedef struct {
PyObject_HEAD
/* Type-specific fields go here. */
mytype *p;
} noddy_NoddyObject;

And i write dealloc function like this:
---------------------------------------------------
static void
Noddy_dealloc(Noddy* self)
{
delete p;
self->ob_type->tp_free((PyObject*)self);
}

And I found it's strange that it didn't call dealloc when there is no
reference.

ex:
a = Noddy()
b = a
del a
del b
# i think there is no ref to the object, and it should call dealloc
but it didn't call!!!

could anyone tell me what happened?

Hi Shen,
I'm no expert on Python memory management, but since no once else has
answered your question I'll tell you what I *think* is happening.

Python doesn't delete objects as soon as they're dereferenced. It
merely marks them as safe for garbage collection (GC). If GC never
happens (and it might not in a small test program), your dealloc
function won't run.

Now some more knowledgeable person will probably correct me. =)

You can control the garbage collector manually with Python's gc
module. You will probably never want to control the garbage collector
manually in ordinary programs, but it can be useful for testing.


Hope this helps
Philip

 

[Gabriel Genellina]

En Thu, 02 Jul 2009 14:22:53 -0300, Philip Semanchuk

And you set the tp_dealloc field in the type structure to Noddy_dealloc,
did you?

You can use sys.getrefcount(a) to check how many references it has.
Remember that it returns one more than the value you expect (because the
function itself holds a temporary reference to its argument)

Hi Shen,
I'm no expert on Python memory management, but since no once else has
answered your question I'll tell you what I *think* is happening.

Python doesn't delete objects as soon as they're dereferenced.

Nope. CPython *does* destroy objects as soon as their reference count
reaches zero. It does not rely on garbage collection for that.

It merely marks them as safe for garbage collection (GC). If GC never
happens (and it might not in a small test program), your dealloc
function won't run.

The garbage collector is only used to recover memory from object cycles
(in the CPython implementation; Jython *does* use garbage collection for
"normal" object destruction too)

Now some more knowledgeable person will probably correct me. =)

And now some more knowledgeable person will probably correct me. =)

 

[Philip Semanchuk]

En Thu, 02 Jul 2009 14:22:53 -0300, Philip Semanchuk


Nope. CPython *does* destroy objects as soon as their reference count
reaches zero. It does not rely on garbage collection for that.


The garbage collector is only used to recover memory from object
cycles
(in the CPython implementation; Jython *does* use garbage collection
for
"normal" object destruction too)

Wow, that last part was the only part I got right. =) Thanks for
straightening me out, Gabriel.