I need to be able to switch back and forth between Python 2 and 3. How do I do that using Homebrew as I don’t want to mess with path and get into trouble. Right now I have 2.7 installed through Homebrew.

I would use pyenv You can install it:

$ brew install pyenv

To enable pyenv in your Bash shell, you need to run:

$ eval "$(pyenv init -)"

To do this automatically for Bash upon startup, add that line to your ~/.bash_profile. ¹

Usage:

Once you have installed pyenv and activated it, you can install different versions of python and choose which one you can use. Example:

$ pyenv install 2.7.5

You can check the versions you have installed with:

$ pyenv versions

And you can switch between python versions with the command:

$ pyenv global 3.3.1

Also you can set a python version for the current directory with:

$ pyenv local 3.5.2

You can check by running python --version:

$ python --version
Python 3.5.2

¹ _{Homebrew used to instruct you to do this upon installation of pyenv, but the message was removed. For Zsh and other shells, the precise steps may be different.}

You can have both versions installed at the same time.

For Homebrew >=1.5.0:

Since 1st March 2018 the python formula will be upgraded to Python 3.x, while a new python@2 formula will be added for Python 2.7, specifically.

See changes announcement here or the final doc about using Homebrew for Python here.

For older Homebrew:

For Python 2.x:

brew install python

For Python 3.x:

brew install python3

Now, you will have both the versions installed in your machine. When you want to use version 2, use the python executable. When you want to use version 3, use the python3 executable.

Currently Homebrew provides two different formulas for Python 2 and 3. brew install python installs python3, and brew install python@2 installs python2. More details in Homebrew docs:

https://docs.brew.sh/Homebrew-and-Python

If you currently have 2.x installed via Homebrew, Homebrew will give you a message such as:

Error: python 2.7.14 is already installed
To upgrade to 3.6.5, run `brew upgrade python`

If you run:

brew upgrade python

you should be able to do:

python --version

and

python3 --version

To see what versions of Python 2.x and 3.x installed.

Alternatively, you probably can just enter “python3” to run your most current version of python3.x and “python” or “python2” to run the latest installed 2.x version.

There are ways to use both , but the simplest solution today is to use pyenv. pyenv allows easy switching between versions. Here is what I did to set up:

STEP1:

Remove all pythons from your mac

 brew uninstall --ignore-dependencies --force python
 sudo rm -rf ~/miniconda3/
 sudo rm -rf ~/.conda/

Remove the following from ~/.bash_profile

export PATH="/Users/ishandutta2007/miniconda3/bin:$PATH"

and also the following from ~/.bashrc

export PYTHONPATH=/usr/local/lib/python2.7/site-packages:$PYTHONPATH
export PYTHONPATH=/usr/local/lib/python2.7/site-packages/google:$PYTHONPATH
alias python="/usr/bin/python"

STEP2:

Install pyenv and the python versions you need

brew update
brew install pyenv
pyenv install 2.7
pyenv install 3.7.0

STEP3:

add pyenv init to bash_profile or bashrc

echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(pyenv init -)"\nfi' >> ~/.bash_profile

STEP4:

Check what got installed

pyenv versions

• system (set by /Users/ishandutta2007/.pyenv/version)

  2.7

  3.7.0

STEP5:

Choose a default

pyenv global 3.7.0

When a project needs older version, just go its root folder and run

pyenv local 2.7

Use asdf !

Ballad of asdf

Once upon a time there was a programming language
There were many versions of it
So people wrote a version manager for it
To switch between versions for projects
Different, old, new.

Then there came more programming languages
So there came more version managers
And many commands for them

I installed a lot of them
I learnt a lot of commands

Then I said, just one more version manager
Which I will write instead

So, there came another version manager
asdf version manager – https://github.com/asdf-vm/asdf

A version manager so extendable
for which anyone can create a plugin
To support their favourite language
No more installing more version managers
Or learning more commands

https://github.com/asdf-vm/asdf
https://github.com/tuvistavie/asdf-python
https://github.com/asdf-vm/asdf-plugins

I thought I had the same requirement – to move between Python versions – but I achieved all I needed with only Python3.6 by building from source instead of using homebrew.

git clone https://git.<theThingYouWantToInstall>

Depending on the repo, check if there is MAKE file already setup for this option.

I was able to just go to https://www.python.org/downloads/mac-osx/ and download the latest python. It installed along side current python in my system.

Okay, I was struggling with my brew installation of Python3, because I didn’t have pip3

sudo pip3 command not found

and so I did

brew uninstall --force --ignore-dependencies python3

and installed the regular Python 3.6.2 from official distribution and then I had pip3 and all components were ok.