Understanding Software Dependencies: What They Are and Why They Matter

Once upon a time, businesses had two choices: build custom software from scratch or buy off-the-shelf solutions. Today, software development is a whole new world. Most applications rely on third-party and open-source components, forming complex dependency stacks that make modern software possible.

But what exactly are dependencies, and why should developers care about managing them? Let’s break it down.

What Are Software Dependencies?

Think of dependencies like the ingredients in a recipe. If you're baking a cake, you need flour, eggs, sugar, and butter. Likewise, software relies on external code—such as libraries, frameworks, APIs, and services—to function properly.

A dependency is any external code your software needs to work. If one piece of software calls another to perform a task, it depends on it. Simple, right?

Why Managing Dependencies Matters

Keeping your dependencies in check isn’t just a good habit—it’s crucial for your project's security, performance, and maintainability. Here’s why:

✅ Security – Updates fix vulnerabilities and protect your app from attacks.
✅ Bug Fixes – Patching dependencies resolves known issues, keeping your app stable.
✅ Performance – Newer versions often run faster and more efficiently.
✅ Compatibility – Updates ensure your software works with the latest tools and frameworks.
✅ New Features – Stay ahead with improved functionality and enhancements.
✅ Maintenance – Align with actively supported versions to avoid outdated tech.
✅ Avoiding Technical Debt – Keep your project agile and easy to update in the future.

Skipping updates might seem harmless today, but outdated dependencies can create major headaches later.

Types of Dependencies (And Why They Matter)

Not all dependencies serve the same purpose. Some are core to your project, while others assist with testing, documentation, or development workflows. Let's explore the key categories:

1. Core Dependencies – The Backbone of Your Project

These are the must-have dependencies. Without them, your project won’t function. They provide essential functionality that your app relies on.

📌 Examples:

• FastAPI – A fast, modern web framework for Python.
• gRPC – A high-performance RPC framework.

2. Feature-Specific Dependencies – Optional Add-ons

Not every user needs every feature. Feature-specific dependencies allow users to install only what they need, reducing installation size and potential conflicts.

📌 Example:

• Cargo Features – A mechanism in Rust that allows optional dependencies.

3. Build Dependencies – Behind-the-Scenes Tools

These dependencies are needed only during the build process to compile, package, or transform code. They don’t affect the final application but are critical for creating deployable versions of the software.

📌 Examples:

• Cargo – Rust’s build system and package manager.
• Setuptools – A tool for packaging Python projects.

4. Development Dependencies – Tools for a Smoother Workflow

These are essential during development but unnecessary in production. They help with code formatting, linting, debugging, and automation.

📌 Examples:

• Pre-commit – Runs checks before committing code.
• Ruff – A fast Python linter.

5. Test Dependencies – Ensuring Code Quality

Testing is non-negotiable. These dependencies help automate, validate, and maintain code quality without affecting the final product.

📌 Example:

• Pytest – A testing framework for Python.

6. Documentation Dependencies – Keeping Docs Clean and Readable

Good documentation is key to a successful project. These dependencies help generate and maintain clear, user-friendly documentation.

📌 Example:

• MkDocs Material – A tool for building project documentation.

Micro-Tutorial: Handling Extra Dependencies in pyproject.toml with UV

Here's a quick way to neatly organize and install build, core and additional optional dependencies using pyproject.toml:

Step 1: Define in pyproject.toml

[build-system]  # build
requires = ["setuptools>=65.0"]
build-backend = "setuptools.build_meta"

[project]  # core
...  # additional config for the project 
dependencies = [
    "fastapi[standard]"
]

[project.optional-dependencies]
server = ["uvicorn"]  # this is a feature dependency
test = ["pytest>=7.0"]
docs = ["mkdocs-material>=9.5"]
dev = ["ruff", "pre-commit"]

Step 2: Install

To install the base dependencies along with specific extras:

pip install .[test,docs]

Or, for development dependencies:

pip install .[dev]

👉 Your Turn: How do you manage dependencies in your projects? Share your best practices in the comments! 🚀