How to Manage Python Versions and Use pyenv

<> In Python and programming languages, there are many cases where you need to install multiple versions side by side to match compatibility with libraries, frameworks, and dependencies you use. What can be utilized in such cases are called “version management tools” or “version managers”, and for Python, “pyenv” is the one. </>

Let’s install pyenv on Ubuntu 16.04 to enable Python version management. It can be done similarly on Ubuntu 18.04 and WSL.

Install it:

git clone https://github.com/pyenv/pyenv.git ~/.pyenv

Add to ~/.bashrc:

export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"

if command -v pyenv 1>/dev/null 2>&1; then
  eval "$(pyenv init -)"
fi

Here’s a note: As follows, if echo $BASH_ENV outputs ~/.bashrc, it seems better not to add the above code to /.bashrc. Instead, add it to /.bash_profile.

In my environment, I haven’t created ~/.bash_profile, and BASH_ENV is also undefined, so I ignored it and added it to .bashrc and use it, but I haven’t had any problems.

General warning: There are some systems where the BASH_ENV variable is configured to point to .bashrc. On such systems you should almost certainly put the abovementioned line eval ”$(pyenv init -)” into .bash_profile, and not into .bashrc. Otherwise you may observe strange behaviour, such as pyenv getting into an infinite loop.

Restart:

bash

As follows, before installing Python with pyenv, it says to install dependency programs.

Install Python build dependencies before attempting to install a new Python version.

Those who already have a Python environment should be fine, but those who haven’t installed Python itself should install these. I haven’t actually verified it, but I think running roughly the following code should be fine.

sudo apt-get install -y python3-pip python3-dev # for Python 3.n
pip3 install --upgrade pip

You can install any version of Python as follows:

pyenv install 3.6.7
pyenv install 2.7.12

Specify the Python version to use with pyenv global. This automatically overrides the python and python3 commands with pyenv.

$ pyenv global 2.7.12 3.6.7
$ pyenv versions
  system
* 2.7.12 (set by /Users/yyuu/.pyenv/version)
* 3.6.7 (set by /Users/yyuu/.pyenv/version)
$ python --version
Python 2.7.12
$ python2.7 --version
Python 2.7.12
$ python3.6 --version
Python 3.6.7

Commands that were failing before changing versions with pyenv…

$ python3 --version
Python 3.5.2

$ epy 'regex_match("hoge","hoge")'
  File "/mnt/c/pg/epy.py", line 28
    if ( re.match(rf"{regex}", text, re.M) ) : return True
                            ^
SyntaxError: invalid syntax

Now succeed without errors.

$ python3.6 --version
Python 3.6.7
$ python3 --version
Python 3.6.7
$ epy 'regex_match("hoge","hoge")'
$