Today I found myself needing to work on an old Python 3.5 codebase, which refused to install its requirements in any later Python version. My first idea was to install the Python 3.5 package from the deadsnakes repos, but they’re only available for LTS releases, which my currently installed OS - Ubuntu 21.04, aka Hirsute Hippo - is not one of.

Here’s (approximately) what I did to get a usable and fairly self-contained python 3.5 environment.

Build python 3.5:

cd
git clone git@github.com:python/cpython.git
cd cpython
git checkout 3.5
./configure --prefix=$HOME/python35
make
make install

Create a python 3.5 virtualenv within my project. I initially tried this with the virtualenv package and had some minor issues that didn’t occur when using the venv module instead:

cd ~/my_crufty_old_project
~/python35/bin/python3.5 -m venv venv
. venv/bin/activate

Upgrade pip and install my project’s requirements:

pip install -U pip
pip install -r requirements.pip