Add support for Python 3.x

[mrry]

Currently we only support Python 2.7, but we should support Python 3.

[girving]

Main things this involves: print -> print(), handle __floordiv__ / __truediv__ / __div__ correctly.

[cdnsteve]

👍 to this issue

[dvbuntu]

👍

[ashwin31]

👍

[joestepp]

👍

[girving]

We're working on it.

[kevinaloys]

Python 3 is a must have. 👍

[mgcdanny]

How do we contribute towards python3 support? Are there any specific tickets open? Is there a python3 development branch?

[cdnsteve]

@mgcdanny seems they require contributors to sign an agreement first.
https://github.com/tensorflow/tensorflow/blob/master/CONTRIBUTING.md

[MikalaiDrabovich]

👍

[girving]

I'm running futurize on the code at the moment; once that's done it'll be easier to parallelize the remaining work. So far futurize --stage1 is checked in and I'm working through futurize --stage2 now (checking each of our divisions very carefully :)).

Unfortunately our contribution process needs a bit of improvement (we're working on streamlining it), but I'll see if there are natural chunks of work to break off and ask for help in fixing once the initial futurize push is done. Before that it's tricky to parallelize.

The only non-obvious questions (so far) are what to do with unicode and where. Cc'ing @mrry since he was taking a look at that. Most of the "names" for things, such as names for ops and names for tensors in a graph, are already restricted to be roughly alphanumeric, so hopefully we should be able to leave them as C++ string while still accepting unicode input from Python.

[girving]

Also, question for people that have done such conversions before / recently: is six still the recommended way to make code transparently support both? In particular, we need stuff like xrange and iteritems as symbols that exist in both 2.7 and 3.x.