Python virtual environment in MacOS

On mac osx, default python is version 2, to support system pre-installed packages.

To try python3 environment, one option is using virtual environment, which allows installing different packages in isolated environment.

ref: https://packaging.python.org/guides/installing-using-pip-and-virtual-environments/

With python2, install virtualenv

With python3, built-in module is    'venv'

    $ python3 -m pip install --user --upgrade pip     # update pip3

    $ python3 -m pip --version    # check version 

## assume venv for python3

# reuse  ~/.virtualenv  as the root for all new environments

    cd ~/.virtualenv

# (one time only) create new environment for python3

    $ python3 -m venv  py3

## use of environment from now on

# 1) using environment

    $ source py3/bin/activate

    $ which python        # should be within env now

# now can install diff python3 packages for testing

# 2) leaving venv

    $ deactivate