Re: [Visualpython-users] What does 'visible' actually do ..

[Jonathan B. <jbr...@ea...>]

On Fri, 2004-02-20 at 02:16, Alan Littleford wrote:
> Hi all,
> 
>    Just started looking at VPython and instantly got confused by the
> reference manual (!):
> 
> In one section I read
> 
> "visible If false (0), object is not displayed; e.g. ball.visible = 0
> Use ball.visible = 1 to make the ball visible again."
> 
> Whereas in another I read
> 
> "To delete a Visual object just make it invisible: ball.visible = 0
> 
> Technical detail: If you later re-use the name ball, for example by
> creating a new object and naming it ball, Python will be free to release
> the memory used by the object formerly named ball"
> 
> So which is it? I tried a test and created a frame with a number of
> objects, walked the objects attribute setting them to invisible. When I
> looked at the objects attribute again it was empty - certainly implying
> the latter case is correct and once I set something invisible I should
> assume it is gone forever.
> 
> Is this true? How can I set something to be invisible and then make it
> visible again without having to reconstruct the object?
> 
> Tnx
> Alan

The idea is this: when an object is visible, the display that the object
is being rendered in owns at least one reference to the object.  So, as
long as you also own a reference to the object (by having it as a named
variable), it will not be deleted.

Recall that Python's garbage collection is a reference-counting system. 
When the reference count reaches zero, the object's memory is freed.

Does that help?
-Jonathan Brandmeyer