py_binary doesn't automatically pick a Python runtime matching the requested version

[emcfarlane]

Description of the problem / feature request:

Running py_test() and py_binary() rules under bazel doesn't select python3 when only srcs_version="PY3", default_python_version="PY3" attributes are set on the rules. How do you specify python2/3 builds?

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I wrote an integration test: emcfarlane@5a6feef

bazel.log

What operating system are you running Bazel on?

Mac

What's the output of bazel info release?

release 0.10.0-homebrew

What's the output of git remote get-url origin ; git rev-parse master ; git rev-parse HEAD ?

git@github.com:afking/bazel.git
d3e102c9fb62f4416099dc9f097a5f207aa7d442
5a6feef7953d2fe664e7a3444a36c9a875416913

Have you found anything relevant by searching the web?

#3871
#3517
bazel-contrib/rules_python#62

[buchgr]

@lberki can you take this please?

[brandjon]

A proper solution would probably involve allowing py_runtime to be used to specify both py2 and py3 interpreters, and allowing individual py_binary rules to declare their runtimes (probably by exposing the hidden py_interpreter attribute).

But in the meantime, a quicker and simpler solution would probably be to make the default behavior (where py_interpreter / --python_top is not given) simply choose shell commands "python2" or "python3" depending on the python mode.