Want to use plugins like Deoplete and other Python plugins on OSX, you need a working installation of both Python2 and Python3 managed by Homebrew (ie, system default Python isn’t enough).
brew install python
brew install python3
pip2 install neovim --upgrade
pip3 install neovim --upgrade
To complete this, add this to your ~/.config/nvim/init.vim:
let g:python2_host_prog = '/usr/local/bin/python'
let g:python3_host_prog = '/usr/local/bin/python3'
And of course, you have to have Neovim installed for this. The repo homebrew-neovim has all the info on how to do this.
brew install neovim/neovim/neovim
Troubleshooting
You can use the :HealthCheck command to see issues with your Python providers.