Dependency Management - Backend Development with FastAPI

1. Virtual Environments (venv)

A virtual environment is a self-contained directory tree that contains a Python installation for a particular version of Python, plus a number of additional packages.

Why Isolate Environments?

By default, standard python installations share global libraries. If Project A needs fastapi 0.95 and Project B needs fastapi 0.110, a global environment will crash. Virtual environments solve this by isolating packages per project directory.

Creating and Activating an Environment

# 1. Create the environment (creates a folder named .venv)
python -m venv .venv

# 2. Activate the environment
# macOS / Linux:
source .venv/bin/activate

# Windows (PowerShell):
.venv\Scripts\Activate.ps1

Once activated, your terminal prompt will display (.venv), indicating that any python or pip commands will run inside the isolated environment.

2. Package Installation with Pip

pip is the default package installer for Python.

# Install a package
pip install fastapi

# Save installed dependencies
pip freeze > requirements.txt

# Install dependencies from a file
pip install -r requirements.txt

3. High-Performance Package Management with `uv`

uv is an ultra-fast Python package installer and resolver written in Rust by Astral. It serves as a drop-in replacement for standard pip and virtualenv tools, running 10x to 100x faster.

# Install uv globally (if not installed)
curl -LsSf https://astral.sh/uv/install.sh | sh

# Create a virtual environment with uv (instant!)
uv venv

# Install packages with uv
uv pip install fastapi uvicorn

4. Declarative Dependency Management: `pyproject.toml`

Modern Python projects use a single configuration file named pyproject.toml to define dependencies, project metadata, and tool configuration, replacing legacy files like requirements.txt and setup.py.

Example `pyproject.toml`

[project]
name = "my-fastapi-app"
version = "0.1.0"
description = "A production-ready FastAPI backend"
dependencies = [
    "fastapi>=0.110.0",
    "uvicorn[standard]>=0.28.0",
    "pydantic[email]>=2.6.0",
]

[tool.uv]
dev-dependencies = [
    "pytest>=8.0.0",
    "black>=24.0.0",
]

Using uv, you can install these dependencies instantly:

# Sync environment with dependencies defined in pyproject.toml
uv sync

​1. Virtual Environments (venv)

​Why Isolate Environments?

​Creating and Activating an Environment

​2. Package Installation with Pip

​3. High-Performance Package Management with uv

​4. Declarative Dependency Management: pyproject.toml

​Example pyproject.toml