title: AttributeError: 'compound' object has no attribute '_origin' in vpython.py
body: <p><strong>What needs to be done to fix this problem?</strong></p>
  
  <p>At <a href="https://www.glowscript.org/#/user/murray.garth/folder/Public/program/Eyeballs" rel="nofollow noreferrer">https://www.glowscript.org/#/user/murray.garth/folder/Public/program/Eyeballs</a> I found a GlowScript example that i tried to use as a python3 script. </p>
  
  <p><a href="https://i.stack.imgur.com/QscGF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QscGF.png" alt="enter image description here"></a></p>
  
  <p>I modified the header to</p>
  
  <pre><code>#https://www.glowscript.org/#/user/murray.garth/folder/Public/program/Eyeballs
  #GlowScript 2.1 VPython
  from vpython import *
  </code></pre>
  
  <p>and changed the true/false references to uppercase. </p>
  
  <p>running </p>
  
  <pre><code>python3 eyeballs.py 
  </code></pre>
  
  <p>starts a static image
  <a href="https://i.stack.imgur.com/Ly4yM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ly4yM.png" alt="enter image description here"></a></p>
  
  <p>and then gives the error message:</p>
  
  <pre><code>compound event return
  compound event return
  compound event return
  Traceback (most recent call last):
    File "eyeballs.py", line 39, in &lt;module&gt;
      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'
  </code></pre>
  
  <p>According to <a href="https://www.glowscript.org/docs/VPythonDocs/compound.html" rel="nofollow noreferrer">https://www.glowscript.org/docs/VPythonDocs/compound.html</a>
  the syntax for compound_to_world is:</p>
  
  <pre><code>world_pos = c.compound_to_world(v) 
  </code></pre>
  
  <p>Which seems to be o.k. to me.</p>
  
  <p>The environment is macports python3</p>
  
  <pre><code>python3 --version
  Python 3.7.4
  </code></pre>
  
  <p>I had installed vpython with</p>
  
  <pre><code>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)
  </code></pre>