Don't Forget the Fine Print: Licensing Your Python Library

You’ve written some brilliant Python code, packaged it neatly, maybe even set up tests and CI. You’re ready to share it with the world! But wait… how exactly are people allowed to use it? Can they incorporate it into their commercial product? If they modify it, do they have to share their changes? These questions are answered by the open-source license you choose.

Ignoring licensing doesn’t mean your code is free for all; it usually means the opposite! Without an explicit license, default copyright law applies, granting you exclusive rights and generally forbidding others from copying, distributing, or modifying your work. If you want people to use your library, you must choose a license.

Why Licensing Matters

• Provides Clarity: Explicitly tells users what they can and cannot do with your code.
• Legal Protection (for You and Users): Offers a legal framework, reducing ambiguity and potential disputes.
• Sets Expectations: Defines the terms of use and contribution.
• Enables Collaboration & Adoption: Standard open-source licenses encourage reuse and integration into other projects.

Common Open Source Licenses: A Quick Tour

Choosing a license involves balancing your desire for widespread adoption with potential requirements you want to place on downstream users. Here are a few common categories and examples:

1. Permissive Licenses: These place minimal restrictions on how others can use your code. They generally allow usage in proprietary software without requiring the derivative work to be open-sourced.

• MIT License: Very popular, simple, and permissive. Essentially requires only that the original copyright and license notice be preserved in copies of the software. Allows sublicensing and use in proprietary works.
• Apache License 2.0: Also permissive and popular. Similar to MIT but more explicit about granting patent rights from contributors to users. It also requires noting any significant changes made to the original code.
• BSD Licenses (2-Clause and 3-Clause): Similar to MIT in permissiveness. The 3-clause version has a non-endorsement clause (preventing use of the original authors’ names to endorse derivative products).

2. Copyleft Licenses: These licenses are designed to ensure that derivative works also remain open source under the same (or compatible) license. They often have a “viral” effect.

• GNU General Public License (GPLv3): The most well-known strong copyleft license. If you include GPLv3 code in your project, your entire project must typically also be licensed under GPLv3 (or a compatible license), and you must make the source code available. This is intended to keep software free and open.
• GNU Lesser General Public License (LGPLv3): A weaker copyleft license often used for libraries. It allows linking with non-GPL code (including proprietary applications) but requires that modifications to the LGPL-licensed library itself remain under LGPL and that users can replace the LGPL component.
• GNU Affero General Public License (AGPLv3): Similar to GPL, but adds a requirement for network services: if you run a modified version on a server and let users interact with it over a network, you must provide them access to the modified source code.

There are many other licenses (Mozilla Public License, Eclipse Public License, etc.), each with nuances. Resources like choosealicense.com and the Open Source Initiative (OSI) provide excellent guides.

The Tangled Web: License Compatibility

Here’s a critical point often missed: your dependencies have licenses too! When you include another library, you must comply with its license, and the licenses must be compatible.

• The GPL Challenge: The most common compatibility issue arises with GPL. You generally cannot release a project under a permissive license like MIT if it incorporates or directly depends on GPL-licensed code. The GPL’s copyleft nature typically requires the combined work to be GPL-licensed as well.
• Permissive Compatibility: Permissive licenses (MIT, Apache 2.0, BSD) are generally compatible with each other.
• LGPL Linking: LGPL libraries are specifically designed to be linkable by proprietary code, offering a middle ground.

Before adding a dependency, always check its license and consider its compatibility with your chosen license and your project’s goals. Tools and resources exist to help analyze compatibility, but careful manual review is often necessary.

Meeting Your Obligations: Compliance Best Practices

Choosing a license isn’t enough; you need to follow its rules:

1. Include the License Text: Place the full text of your chosen license in a file named LICENSE (or LICENSE.txt, LICENSE.md) in your project’s root directory.
2. Copyright Notice: Include a copyright notice (e.g., Copyright (c) 2024 Your Name) in your license file and often as a comment header in source files.
3. Attribution/Notices: Some licenses (like Apache 2.0) require you to preserve or provide specific notices, especially if you modify the licensed code. If your dependencies require attribution, you might need a NOTICE file.
4. State Changes (Apache 2.0): If you modify Apache-licensed files, you must indicate that changes were made.
5. Source Code Distribution (Copyleft): If using GPL or similar, ensure you have a mechanism to provide the corresponding source code upon request if you distribute binaries.
6. Declare in pyproject.toml: Use the license field in the [project] table. It’s good practice to use the standard SPDX license identifier if possible (e.g., license = "MIT" or license = {text = "Apache-2.0"}). If using a file, license = { file = "LICENSE" } is standard.

Choose Early, Choose Wisely

Licensing should be one of the first decisions you make for an open-source project. It sets the foundation for how your project interacts with the broader community.

Disclaimer: I am not a lawyer cannot provide legal advice. The information here is for educational purposes. If you have complex licensing needs or concerns, consult with a qualified legal professional.

Taking the time to understand and correctly apply an open-source license protects you, clarifies things for your users, and helps ensure your Python library can thrive within the collaborative open-source ecosystem.