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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