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Virtual environments

How we keep a project's Python isolated — from the system Python and from every other project. The rule underneath all of it: never install into, or upgrade, the system Python. The OS depends on it; a project must never touch it.

Leave the system Python alone

No sudo pip install, no upgrading the system interpreter for a project. If a project needs a different version or a package, that goes in a virtual environment — never the system one. Breaking system Python can break the OS.

Project-based isolation

Every project runs against its own isolated environment. There are three ways that happens, depending on where the project runs:

A virtualenv living in the repo. Simplest for day-to-day local work.

python -m venv .venv          # create it (once)
source .venv/bin/activate     # activate for this shell
pip install -r requirements.txt

Keep .venv/ gitignored — it's per-machine, never committed.

Wrap the venv in a make target so every dev (and CI) sets up identically — no "did you activate it?" drift.

VENV := .venv
PY := $(VENV)/bin/python

$(VENV): requirements.txt
    python -m venv $(VENV)
    $(PY) -m pip install -r requirements.txt

.PHONY: run
run: $(VENV)
    $(PY) -m yourapp

make run creates the venv if missing, installs deps, and runs — all against the isolated interpreter, no manual activation.

The container is the isolation — its own filesystem, its own interpreter, nothing shared with the host. You don't need a .venv inside an image; install straight into the container's Python.

FROM python:3.12-slim
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .

This is how things run in production — see the Deploy guide for the full container standard (uid 1337, mounts, layer caching).

Which one?

Local .venv for quick iteration, Makefile when you want repeatable setup across the team, Docker for anything that ships. They're not exclusive — a project often has a .venv for local dev and a Dockerfile for deploy.

Local dev with pyenv

For local work you also need the right Python version, not just isolated deps. We target Python 3.10+, and pyenv installs and switches versions per-project without touching the system Python.

  • Per-project pinning: a .python-version file in a repo makes pyenv auto-select that interpreter when you cd in — everyone on the project runs the same one.
  • Pairs with venvs: combined with pyenv-virtualenv, each project gets both an isolated version and isolated deps.

Install

Follow the official pyenv installation for the installer and build dependencies — no point reproducing it here. Then add the shell init below to your ~/.zshrc (or ~/.bashrc) and restart your shell.

Shell init

Without these lines pyenv's shims aren't on PATH, so pyenv and auto-version-switching won't work:

# pyenv — Python version management
export PYENV_ROOT="$HOME/.pyenv"
export PATH="$PYENV_ROOT/bin:$PATH"
eval "$(pyenv init --path)"          # (1)!
eval "$(pyenv init -)"               # (2)!
eval "$(pyenv virtualenv-init -)"    # (3)!
  1. Puts the shims dir on PATH, so python resolves to the pyenv-selected version instead of the system one.
  2. Shell integration — command rehashing and completion.
  3. Auto-activates a project's virtualenv on cd — only if you use pyenv-virtualenv. Drop this line if you don't.

Everyday use

pyenv install 3.10.14      # install a version (one-time)
pyenv install --list       # see available versions
cd <project>
pyenv local 3.10.14        # writes .python-version -> auto-selects here
python --version           # confirms the pinned version
  • pyenv local <ver> per project — commit the .python-version so the team matches.
  • pyenv global <ver> for your default outside any project.

Shell quality-of-life extras

A couple of optional lines worth having alongside pyenv:

# flake8 with our shared config (max line 120)
alias flake8='flake8 --config ~/.config/flake8'

# local bins on PATH (pip --user installs, npm globals)
export PATH="$HOME/.local/bin:$PATH"
export PATH="$HOME/.npm-global/bin:$PATH"
  • The flake8 alias keeps everyone linting with the same config (our 120 max-line, etc. — see Standards).
  • The .local/bin / npm-global PATH lines stop "command not found" after a pip install --user or a global npm install.

More shell setup

This is the Python-env slice. The fuller dev shell setup — the WSL/Windows clipboard bridges, per-repo git-identity aliases, and the gl graph log — is on the Workflow page.