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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