Re: [Visualpython-users] [Fwd: Re: incrementing vectors "problem"]

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

On Sun, 2004-08-22 at 16:38, Bruce Sherwood wrote:
> I ran Jonathan's test routine in pre-Boost and Boost versions of 
> VPython. In the pre-Boost version the behavior is NOT what Gary and 
> Jonathan describe. In the pre-Boost version the two forms of the update 
> statement work exactly the same in Jonathan's test routine. The original 
> Visual deliberately did something like this:
> 
> 	sphere(pos=a) was treated as though it were written as
> 	sphere(pos=vector(a))

Current VPython does this too for all of its built-in attributes.  The
problem stems from the use of user-defined attributes, for which there
is no way to force a copy that will work for all Python data types
(copy.copy() is close, but doesn't work for everything).

Old visual.vector did not provide the in-place operators (+=, *=, and
friends), which forced the interpreter to emulate them incorrectly.

Python programmers must be aware that all Python variables are
references.  Consider this program, which also incorrectly assumes that
'=' makes a copy:

from visual import *
spheres = []
velocity = vector(0,0,0)
spheres.append( sphere( v=velocity))
spheres.append( sphere( v=velocity))


twice = 2
while twice:
    print "before iteration:"
    for i in spheres:
        print i.v

    ctr = 0
    for i in spheres:
        print ctr
        ctr += 1
        print "before:", i.v
        i.v.x += 1
        print "after", i.v

    twice -= 1

-Jonathan