Add guide on using multiple Python versions for local development (GoogleCloudPlatform#1621)

* Add guide on using multiple Python versions for local development
* Add note about why these instructions differ from the recommended GCP setup.

Author: tswast

CONTRIBUTING.md

@@ -29,6 +29,10 @@ be able to accept your pull requests.
 1. Ensure that your code has an appropriate set of unit tests which all pass.
 1. Submit a pull request.
 
+## Setting up a development environment
+
+* [Mac development environment guide](MAC_SETUP.md)
+
 ## Authoring, testing, and contributing samples
 
 See [AUTHORING_GUIDE.md](AUTHORING_GUIDE.md).

MAC_SETUP.md

@@ -0,0 +1,113 @@
+# Setting up a Mac development environment with pyenv and pyenv-virtualenv
+
+In this guide, you'll set up a local Python development environment with
+multiple Python versions, managed by [pyenv](https://github.com/pyenv/pyenv).
+
+This guide differs from the [Google Cloud Python development
+instructions](https://cloud.google.com/python/setup) because developers of
+samples and libraries need to be able to use multiple versions of Python to
+test their code.
+
+## Before you begin
+
+1.  [Optional] Install [homebrew](https://brew.sh/).
+
+## Installing pyenv and pyenv-virtualenv
+
+1.  Install [pyenv](https://github.com/pyenv/pyenv).
+
+    I (tswast@) use [homebrew](https://brew.sh/) to install it.
+
+    ```
+    brew update
+    brew install pyenv
+    ```
+
+1.  Install the [pyenv-virtualenv](https://github.com/pyenv/pyenv-virtualenv)
+    plugin.
+
+    ```
+    brew install pyenv-virtualenv
+    ```
+
+1.  Append the following to your `~/.bashrc`:
+
+    ```
+    eval "$(pyenv init -)"
+    eval "$(pyenv virtualenv-init -)"`
+    ```
+
+    Note that this also works with ZSH.
+
+1.  Reload your shell.
+
+    ```
+    source ~/.bashrc
+    ```
+
+1.  Verify that you are now using the pyenv Python shim.
+
+    ```
+    $ which python
+    /Users/tswast/.pyenv/shims/python
+    ```
+
+## Installing multiple Python versions
+
+
+1.  See the available Python versions with
+
+    ```
+    pyenv install --list
+    ```
+
+    The Python versions are at the top of the long list. If the Python
+    version you want isn't listed, you may need to upgrade your pyenv with
+    homebrew.
+
+    ```
+    brew update
+    brew upgrade pyenv
+    ```
+
+1. Compile the necessary Python versions with pyenv. Use the latest release
+   of the versions you wish to test against.
+
+    As of August 8, 2018 my (tswast@) Python versions are:
+
+    *  2.7.15 (latest 2.7.x release)
+    *  3.5.4 (latest 3.5.x release)
+    *  3.6.4 (latest 3.6.x release)
+    *  3.7.0 (latest 3.7.x release)
+
+## Using pyenv and pyenv-virtualenv to manage your Python versions
+
+1.  Change to the desired source directory.
+
+    ```
+    cd ~/src/python-docs-samples
+    ```
+
+1.  Create a virtualenv using `pyenv virtualenv`.
+
+    ```
+    pyenv virtualenv 3.6.4 python-docs-samples
+    ```
+
+    This creates a virtualenv folder within `~/.pyenv/versions/`.
+
+1.  Set the local Python version(s) with `pyenv local`
+
+    ```
+    # pyenv local [name of virtualenv] [list of python versions to use]
+    pyenv local python-docs-samples 3.6.4 3.7.0 3.5.4 2.7.15
+    ```
+
+1.  Now, when you `cd` into the source directory or a subdirectory within it,
+    pyenv will make your virtualenv the default Python. Since you specified
+    more than one version, it will also add binaries like `python36` and
+    `python27` to your PATH, which nox uses when picking Python interpreters.
+
+1.  Add `.python-version` to your [global gitignore
+    file](https://help.github.com/articles/ignoring-files/#create-a-global-gitignore),
+    so it wont be committed into the repository.