Managing Python Versions Effectively

After recently upgrading my homebrew, links to python2 and pip2 broke! I had no way to run different versions of Python. This left me searching for a more effective way to manage Python versions and dependencies.

I discovered pyenv, a great tool for specifying exact Python versions. It also integrates nicely with virtual environments and Jupyter!

Setting Up Pyenv

Install pyven
```
brew install pyenv
```

Setup .bash_profile to configure and initialize pyenv

# pyenv for running specific python version
export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"
# initialize pyenv and virtualenv in shell
if command -v pyenv 1>/dev/null 2>&1; then  
eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"
fi
# show virtualenv changes in prompt
export PYENV_VIRTUALENV_DISABLE_PROMPT=1

Using Pyenv

Install a specific python version

$ pyenv install 2.7.10
$ pyenv install 3.5.0
$ pyenv versions

You can similarly uninstall a specific version using uninstall.

pyenv introduces three scopes for using python versions. It uses the first found in order of specificity: shell, local, and global (else defaults to system python).

Set the global Python version
```
$ pyenv global 2.7.10
```
Set the local version
```
$ pyenv local 3.10
# to remove
$ pyenv local --unset 
```
This creates a .python-version file in the directory. Pyenv automatically activates the local python version for all subdirectories:
Shell overrides the python version for this shell session
```
$ pyenv shell 2.7.10
$ pyenv shell --unset
```

Virtual Environments

Install virtualenv plugin for pyenv.

Use Homebrew: brew install pyenv-virtualenv

Working with Virtual Environments

Create a new virtual environment

$ pyenv virtualenv 2.7.10 my-project-2.7.10

This stores the virtualenv in a new directory my-project-2.7.10 under $(pyenv root)/versions/—similar to virtualenvwrapper.

Activate or deactivate

$ pyenv activate <name>
$ pyenv deactivate

Pyenv and Jupyter

How do you setup Jupyter kernels for Python 2 and Python 3?

activate shell for each python version pyenv activate 2.7.14
Install Jupyter pip install jupyter
Install Kernel python -m ipykernel install --user

Et voila!