Re: [Visualpython-users] Automation

[Bruce S. <ba...@an...>]

--On Wednesday, December 27, 2000 8:26 AM -0800 Matthew Grimm 
<mat...@ya...> wrote:

>   Store the data (pos, axis, color, etc) for
>   each individual component of a complex item
>   in a database

Clearly a database issue, nothing particularly to do with Visual.

>   Pull the data from the database and feed it
>   into Visual

If as you say Visual is running on a different computer, you have the 
problem of using a socket or some such mechanism for moving the data, and 
your Visual program has to be checking that new data has arrived. It is my 
understanding that Python has modules for doing this kind of 
intercommunication, though I've never done anything of this kind myself. 
Again, there isn't anything here particularly germane to Visual.

>   Automate the process of refreshing the scene
>   when changes are made to the data

There are three different issues: modify an existing object (change axis or 
color, for example), create a new object, or delete an old object. Only 
deletion involves "nuances of Visual's behavior". There is no direct delete 
of a Visual object. Deletion is a two-step process:

1) Make the object invisible.
2) Assign the name of the object to something else, so that there is no 
longer a valid reference to the object. Example:

     ball = sphere()
     .....
     ball.visible = 0  # make invisible
     ball = None  # no Python name is associated with the sphere any more

Python automatically does "garbage collection" -- that is, Python 
periodically checks to see whether any objects are no longer referenced, 
and removes them from memory, freeing up this memory for other uses. When 
you set ball = None, the number of names referencing the original sphere 
drops to zero, and the memory set aside for keeping track of the sphere 
becomes available to be reclaimed.

If however you set ball = None before making the sphere invisible, Visual 
itself still refers to the sphere (since Visual has to keep displaying it, 
and allowing the user to navigate around it), and Python can't delete the 
sphere. Nor can you delete the sphere, because you no longer have a name of 
the sphere to be able to say ball.visible = 0 (this statement would give an 
error, since "None" doesn't have a visible attribute).

Bruce Sherwood