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

[Bruce S. <Bru...@nc...>]

I think I see. Thanks for the further analysis. So two wrongs don't make 
(or shouldn't have made) a right.

Bruce Sherwood

Jonathan Brandmeyer wrote:

> 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
> 
> 
> 
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