you can have multiple pythons enabled at the same time so that those applications which you run which make assumptions about python2 being available work. (i’m looking at you gsutil and gcloud)

elmo(~)% pyenv global 3.7.1 2.7.15
elmo(~)% pyenv versions
  system
* 2.7.15 (set by /Users/sulrich/.pyenv/version)
  2.7.15/envs/foo
  2.7.5
* 3.7.1 (set by /Users/sulrich/.pyenv/version)
  3.7.2
  foo
elmo(~)% which python2
/Users/sulrich/.pyenv/shims/python2
elmo(~)% which python3
/Users/sulrich/.pyenv/shims/python3
elmo(~)% which python
/Users/sulrich/.pyenv/shims/python
elmo(~)% python --version
Python 3.7.1