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UV ​

Modern manager for Python virtual environments and dependencies.

UV is a lightweight and high-performance tool designed to manage Python virtual environments and project dependencies.

Why Use UV Instead of pip? ​

  • Automatic management: Automatic creation and management of .venv environments
  • Deterministic sync: Eliminates "works on my machine" problems
  • Optimized performance: Built-in cache and offline mode for faster installs

Quick Start ​

sh
# Initialize a new uv project
uv init --app my-project

# Move into the project
cd my-project

# Add FastAPI as a dependency
uv add fastapi --extra standard

# Run the FastAPI dev server
uv run fastapi dev

Project Initialization ​

sh
# Initialize the project
uv init --app my-project

Environment Configuration ​

sh
# Create the environment with Python 3.12
uv venv --python 3.12

Best Practices ​

  • Lock your versions: Use uv lock and uv sync to ensure all team members use the same dependency versions. Always commit the uv.lock file.
  • Optimized cache: UV's local cache speeds up repeated installs and enables offline work. The cache is shared across projects.

VS Code With Many Repos: Isolate Environments Properly ​

If one repo's .venv appears in another repo, it is usually because the same terminal shell is still active. Virtual environment activation is shell state, not repository state.

Why It Happens ​

  • source .venv/bin/activate only affects the current shell process
  • VS Code terminals are persistent tabs
  • If you cd to another repo in that same tab, the old env remains active
sh
# from each project root
uv venv --python 3.12

Create .vscode/settings.json in each Python repo:

json
{
	"python.defaultInterpreterPath": "${workspaceFolder}/.venv/bin/python"
}

Notes:

  • Set this at repository level, not global user settings
  • In multi-root workspaces, keep this setting inside each folder

Daily Workflow ​

Prefer project commands through uv to avoid cross-repo shell pollution:

sh
uv run python -V
uv run pytest

If you manually activate an env:

sh
source .venv/bin/activate
# ...work...
deactivate

Quick checks when switching repos:

sh
echo "$VIRTUAL_ENV"
which python

Optional: Auto Activate/Deactivate With direnv ​

Add .envrc in each project:

sh
source .venv/bin/activate

Then allow it once per repo:

sh
direnv allow .

direnv will load env vars on directory enter and unload them on exit, which is ideal for many repositories.

Resources ​