AttributeError: 'compound' object has no attribute '_origin' in vpython.py

Question

What needs to be done to fix this problem?

At https://www.glowscript.org/#/user/murray.garth/folder/Public/program/Eyeballs I found a GlowScript example that i tried to use as a python3 script.

I modified the header to

#https://www.glowscript.org/#/user/murray.garth/folder/Public/program/Eyeballs
#GlowScript 2.1 VPython
from vpython import *

and changed the true/false references to uppercase.

running

python3 eyeballs.py 

starts a static image

and then gives the error message:

compound event return
compound event return
compound event return
Traceback (most recent call last):
  File "eyeballs.py", line 39, in <module>
    world_pos = Head.compound_to_world( vRightEye.pos )
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/vpython/vpython.py", line 1553, in compound_to_world
    v = v-self._origin
AttributeError: 'compound' object has no attribute '_origin'

According to https://www.glowscript.org/docs/VPythonDocs/compound.html the syntax for compound_to_world is:

world_pos = c.compound_to_world(v) 

Which seems to be o.k. to me.

The environment is macports python3

python3 --version
Python 3.7.4

I had installed vpython with

pip install vpython
pip --version
pip 18.1 from /opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip (python 3.7)

Answer 1

This was a tough one. There are some 1.2 million questions on python on stackoverflow. If you search for

[python]"object has no attribute" 

you get some 12066 results. So that's probably why this question didn'get much attention.

I tried out the issue with a smaller example according to https://www.glowscript.org/docs/VPythonDocs/compound.html

from vpython import *

handle = cylinder( size=vec(1,.2,.2),color=vec(0.72,0.42,0) )
head = box( size=vec(.2,.6,.2), pos=vec(1.1,0,0),color=color.gray(.6) )

hammer = compound([handle, head])
hammer.axis = vec(1,1,0)

world_pos = hammer.compound_to_world(hammer.axis) 

giving the error mentioned in the question:

compound event return
Traceback (most recent call last):
  File "hammer.py", line 10, in <module>
    print (hammer.origin)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/vpython/vpython.py", line 1536, in origin
    return self._origin
AttributeError: 'compound' object has no attribute '_origin'

The relevant source code lines of vpython.py are:

@property
def origin(self):
    return self._origin

@origin.setter
def origin(self,value): # compound origin cannot be reset
   if not self._constructing:
      raise AttributeError('The compound "origin" attribute is read-only; change "pos" instead.')
   self._origin = value

def world_to_compound(self, v):
        v = v-self._pos
        x_axis = self._axis.hat
        y_axis = self._up.hat
        z_axis = x_axis.cross(y_axis)
        ox = self._size0.x/self._size.x # _size0 is the original size
        oy = self._size0.y/self._size.y
        oz = self._size0.z/self._size.z
        return self._origin + vector(v.dot(x_axis)*ox, v.dot(y_axis)*oy, v.dot(z_axis)*oz)

def compound_to_world(self, v):
        v = v-self._origin
        x_axis = self._axis.hat
        y_axis = self._up.hat
        z_axis = x_axis.cross(y_axis)
        ox = self._size.x/self._size0.x # _size0 is the original size
        oy = self._size.y/self._size0.y
        oz = self._size.z/self._size0.z
        return self._pos + v.x*ox*x_axis + v.y*oy*y_axis + v.z*oz*z_axis

and indeed a few lines further up the constructor does not set any origin. So adding a default origin:

class compound(standardAttributes):
    compound_idx = 0 # same numbering scheme as in GlowScript

    def __init__(self, objList, **args):
        self._origin = vector(0,0,0)

makes the syntax error go away.

from vpython import *

handle = cylinder( size=vec(1,.2,.2),                   color=vec(0.72,0.42,0) )

head = box( size=vec(.2,.6,.2), pos=vec(1.1,0,0),              color=color.gray(.6) )

hammer = compound([handle, head])
hammer.axis = vec(1,1,0)

print (hammer.origin)

world_pos = hammer.compound_to_world(hammer.axis)
print (world_pos)

then gives the result:

compound event return
<0, 0, 0>
<0.6, 1.41421, 0>

and the eyeballs.py code works as expected:

I do not know there to report this bug but I posted a message to the vpython-users group

Answer 2

Thanks for reporting this bug, which I'll report as an issue in the vpython repository. As you say, a workaround is to specify an origin.