Fix compatibility with Python 2

[lrq3000]

This PR fixes a compatibility issue with Python 2: decorating with just @widgets.register will transform the decorated class into a function, whereas using @widgets.register() will correctly return a class. This fixes the following error:

TypeError: Error when calling the metaclass bases function() argument 1 must be code, not str

With this fix I could successfully run the following simple example:

import ipyvolume
ipyvolume.examples.ball(rmax=3, rmin=2.5, shape=64, lighting=True)

BTW it would be nice I think to add the mybinder examples in the doc directly, or even the readme, so that new users can easily test out the widget for themselves :-) Otherwise in the doc there is no other example that does not require a 3rd-party library (bokeh etc).

[coveralls]

Coverage remained the same at 61.763% when pulling 2f1f89af93a810ca32ad4058d23a654ae9049cd8 on lrq3000:py2fix into 6e1f088a94c60ff85f87cb9c66a996e1facd6745 on maartenbreddels:master.

[lrq3000]

Also for reference (about the decorator):

• https://bugs.python.org/issue6829

[maartenbreddels]

Hi there, thanks for your contribution! I cannot reproduce this actually. The decorator might be a bit confusing, since going from ipywidget 6 to 7 it does not take a name anymore, but a widget instead. If you installed ipyvolume with ipywidget 6 this may happen actually!. A binder link in the docs would be great, I saw some auto generated once, I'll look into it.

[lrq3000]

@maartenbreddels mmm that's weird, did you try with Python >= 2.7.13 ? Because it's known to work with Python >= 3 since the class management changed, so this issue affects only Python 2. I have ipywidget v7.1.1, I had to update else the widget would not even load with the time old issue of "javascript can't be loaded" or something like that.

[maartenbreddels]

it's 2.7.14, on OSX, using miniconda, and ipyvolume works with that. I'm almost sure it is ipywidget 6 you are using, maybe you have it installed in ~/.local? can you check the output of python -c "import ipywidgets as w; print(w.__version__, w.__file__)" ?

[lrq3000]

Hey @maartenbreddels , thank you for your suggestion, here is the output:

('7.1.1', 'C:\\Anaconda2\\lib\\site-packages\\ipywidgets\\__init__.pyc')

The path is where I initially applied my patch to make ipyvolume works. Also my Python version is 2.7.12, maybe there was an update between .12 and .14 that renders Py2 compatible with such decorator?

[maartenbreddels]

Ok, I need to check this on windows, I only have OSX and Linux now, but I'll try to boot up a virtual machine soon!