Re: [Visualpython-users] Subclassing cvisual classes is cool

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

On Sun, 2004-08-01 at 10:05, Arthur wrote:
> Jonathan, Bruce - 
> 
> Thanks for the efforts that brought us VPython 3.0!
> 
> What I am most excited about is the ability to now subclasss cvisual classes
> directly in Python.  

Well, you might find that that is dangerous since most of the
cvisual.foo objects are incomplete, and the balance of what is
implemented in C++ vs what is implemented in Python is subject to
change.  

Note that cvisual.foo objects are written entirely in C++, and don't
export the complete API specified by Visual's documentation.   The
visual.primitives.foo objects extend the cvisual.foo objects to complete
the API in some of its more "Pythonic" ways (like variable-length
keyword argument constructors, for example).

> Since the facility is undocumented at this point, I
> thought I'd bring it to the attention of the community of VPython users.
> 
> A trivial example, of a complete scene with a sphere that defaults to being
> blue, instead of the VPython standard white:
> 
> import cvisual
> import time
> import atexit
> 
> def __waitclose():
>     while not cvisual.allclosed(): time.sleep(0.05)
> atexit.register(__waitclose)
> 
> scene = cvisual.display()
> 
> class bluesphere (cvisual.sphere):
>         def __init__( self,*args,**keywords):
>                 cvisual.sphere.__init__(self)
>                 self.color=(0,0,1)
>                 self.complete_init( self, self,
>                 1, scene, None)
> bluesphere()

>>> obj2 = bluesphere()
>>> obj2.pos = cvisual.vector(1,1,1)
>>> # The sphere didn't move!

Therefore, it is a much better idea to inherit from the
visual.primitives.foo or visual.ui.display objects, manipulate
**keywords as you see fit, and then call the parent's __init__()
method.  Here is a much simpler implementation of the "force to blue for
sphere's only" policy that doesn't rely on any internal API's, and
includes the complete API for Visual's sphere objects:

import visual.primitives

class bluesphere( visual.primitives.sphere):
    def __init__(self, **keywords):
        keywords['color'] = (0,0,1)
	visual.primitives.sphere.__init__(self, **keywords)


> I would hope that an eventual documentation of the API for subclassing from
> cvisual (and perhaps some refinement of it) will unleash the VPtyhon
> community's creative exploration of the possibilities inherent in this
> facility.
>  


It hasn't been documented yet, because the *big* change will be the
ability to prototype totally new renderable objects using subclassing +
PyOpenGL.  That stuff isn't ready yet, but it is coming.

What's ready now and here to stay is that Visual's classes are all
first-class citizens in the Python class system (say that three times
fast ;).  Anything you expect to do with any other Python class can be
done with these.

-Jonathan