> ## Documentation Index
> Fetch the complete documentation index at: https://fastapi.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Dependency Management

> Set up isolated virtual environments, install packages with pip or uv, and manage dependencies with pyproject.toml

## 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

```bash theme={null}
# 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.

```bash theme={null}
# 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.

```bash theme={null}
# 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`

```toml theme={null}
[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:

```bash theme={null}
# Sync environment with dependencies defined in pyproject.toml
uv sync
```