Python Environment Management

Virtual environments, dependency management, and Python version control.

venv (Built-in)

# Create virtual environment
python -m venv .venv
python3 -m venv .venv

# Activate
source .venv/bin/activate          # Linux/macOS
.venv\Scripts\activate             # Windows

# Deactivate
deactivate

# Install packages
pip install requests flask
pip install -r requirements.txt

# Freeze dependencies
pip freeze > requirements.txt

# Upgrade pip
pip install --upgrade pip

requirements.txt

# Pinned versions (production)
flask==3.0.0
requests==2.31.0
sqlalchemy==2.0.23

# Ranges (library development)
flask>=3.0,<4.0
requests~=2.31            # Compatible release (>=2.31, <3.0)

# With extras
fastapi[all]==0.104.0

# From git
git+https://github.com/user/repo.git@main#egg=package

# Separate dev dependencies
pip install -r requirements.txt
pip install -r requirements-dev.txt

Poetry

# Install Poetry
curl -sSL https://install.python-poetry.org | python3 -

# Create new project
poetry new my-project
poetry init  # In existing project

# Add dependencies
poetry add requests flask
poetry add pytest --group dev
poetry add "sqlalchemy>=2.0,<3.0"

# Remove dependency
poetry remove requests

# Install all dependencies
poetry install
poetry install --without dev

# Update dependencies
poetry update
poetry update requests      # Single package

# Show dependencies
poetry show
poetry show --tree

# Run command in virtualenv
poetry run python main.py
poetry run pytest

# Activate shell
poetry shell

# Build & publish
poetry build
poetry publish

# Export to requirements.txt
poetry export -f requirements.txt --output requirements.txt
poetry export -f requirements.txt --without-hashes

pyproject.toml (Poetry)

[tool.poetry]
name = "my-project"
version = "1.0.0"
description = "My project"
authors = ["Name <email@example.com>"]
python = "^3.11"

[tool.poetry.dependencies]
python = "^3.11"
flask = "^3.0"
sqlalchemy = "^2.0"

[tool.poetry.group.dev.dependencies]
pytest = "^8.0"
ruff = "^0.1"
mypy = "^1.7"

[tool.poetry.scripts]
serve = "my_project.main:app"

uv (Fast Python Package Manager)

# Install uv
curl -LsSf https://astral.sh/uv/install.sh | sh

# Create virtual environment
uv venv

# Install packages (10-100x faster than pip)
uv pip install requests flask
uv pip install -r requirements.txt

# Sync from lock file
uv pip sync requirements.txt

# Compile requirements (resolve + lock)
uv pip compile requirements.in -o requirements.txt

# Create project
uv init my-project

# Add dependency
uv add requests
uv add pytest --dev

# Run
uv run python main.py

pyenv (Python Version Management)

# Install pyenv
curl https://pyenv.run | bash

# List available versions
pyenv install --list | grep "3\."

# Install Python version
pyenv install 3.12.1
pyenv install 3.11.7

# Set global default
pyenv global 3.12.1

# Set local version (per directory)
pyenv local 3.11.7    # Creates .python-version

# List installed
pyenv versions

# Uninstall
pyenv uninstall 3.10.0

Pipenv

# Install
pip install pipenv

# Create environment + install
pipenv install
pipenv install requests
pipenv install pytest --dev

# Run in environment
pipenv run python main.py
pipenv shell

# Lock dependencies
pipenv lock

# Install from lock file
pipenv install --deploy

# Show dependency graph
pipenv graph

conda

# Create environment
conda create -n myenv python=3.12
conda create -n myenv python=3.12 numpy pandas

# Activate/deactivate
conda activate myenv
conda deactivate

# Install packages
conda install numpy pandas
conda install -c conda-forge package-name

# List environments
conda env list

# Export environment
conda env export > environment.yml

# Create from file
conda env create -f environment.yml

# Remove environment
conda env remove -n myenv

Common Issues

# "pip not found" after creating venv
python -m pip install --upgrade pip

# Permission errors
pip install --user package-name

# Conflicting versions
pip install --force-reinstall package==1.0

# Clear pip cache
pip cache purge

# Check what's outdated
pip list --outdated

# Verify installation
python -c "import package; print(package.__version__)"

Reference

For comparison and migration guides: references/tools.md