install python3 in virtual-env on macOS 10.12.1

output.sh

######
## - the problem: on macOS 10.12.1, trying to install a virtual-env specifying python 3 does not work
## - cause of problem seems to be an old version of `virtualenv`
## - this is a work-around...
######

## if you need to install python 3...

## ensure you can run brew doctor with no errors/warnings

$ brew doctor

Your system is ready to brew.

$ brew install python3

## lots of output

$ which python3

/usr/local/bin/python3

## my old `virtualenv` seems to be the problem
## don't remember how I originally installed `virtualenv`, 
##   but this is what I've been using successfully for a _long_ time with python 2:

$ which virtualenv

/usr/local/bin/virtualenv

$ virtualenv --version

1.11.2

## I could `sudo pip install --upgrade virtualenv`, but I try never to sudo my system for installs
## so, the work-around (though really, i need to look into a non-sudo way of updating this `virtualenv`)...

$ virtualenv --python=/usr/bin/python2.7 --prompt=[env_py2] --no-site-packages /path/to/stuff/env_py2

$ source ./env_py2/bin/activate

[env_py2] $ 

[env_py2] $ pip install --upgrade pip

## output

[env_py2] $ pip install virtualenv

## output

[env_py2] $ which virtualenv

/path/to/stuff/env_py2/bin/virtualenv

[env_py2] $ virtualenv --version

15.1.0

## big difference!

## using the newer local `virtualenv`, _now_ the install of a virtual-env with python 3 will work

[env_py2] $ virtualenv --python=/usr/local/bin/python3 --prompt=[env_py3] --no-site-packages /path/to/stuff/env_py3

Running virtualenv with interpreter /usr/local/bin/python3
...
Installing setuptools, pip, wheel...done.

## yay!

[env_py2] $ deactivate

$

$ source ./env_py3/bin/activate

[env_py3] $

[env_py3] $ python
Python 3.6.0 (default, Dec 24 2016, 08:01:42) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 

## nice!