Introduction

Yes, you can add comments to a requirements.txt file, and it is often a good practice for maintainability. Comments help explain why a package is pinned, which dependency is temporary, or what platform constraint applies. The key is understanding how pip parses lines so comments do not accidentally break installs.

Comment Syntax in Requirements Files

pip treats any text after # as a comment on that line, unless the hash symbol is part of a URL fragment in a direct reference. Standalone comment lines are also supported.

1# Core runtime dependencies
2fastapi==0.116.0
3uvicorn==0.35.0
4
5# Temporary pin due to upstream API change
6httpx==0.28.1
7
8# Testing
9pytest==8.4.1  # required for unit and integration tests

This structure works with pip install -r requirements.txt and keeps intent visible for future maintainers.

Organize Large Dependency Sets

For bigger projects, split dependencies into layered files and add comments describing purpose.

1# requirements.txt
2-r requirements/base.txt
3-r requirements/dev.txt
4
5# requirements/base.txt
6Django==5.1.4
7psycopg[binary]==3.2.9
8
9# requirements/dev.txt
10-r base.txt
11ipython==9.4.0
12mypy==1.16.1

This pattern avoids one giant file and makes production installs cleaner.

Use Comments with Constraints Files

Constraints files do not install packages directly; they only restrict versions. Comments are useful there too.

1# constraints.txt
2# pinned for compatibility with internal plugin
3urllib3==2.5.0
4
5# security baseline
6cryptography==45.0.5

Install with:

pip install -r requirements.txt -c constraints.txt

Documenting why constraints exist saves time during upgrades and incident response.

Environment Markers and Platform Notes

Requirements files support environment markers, which can be combined with comments for clarity.

uvloop==0.21.0 ; platform_system == "Linux"  # Linux event loop acceleration
pywin32==311 ; platform_system == "Windows"  # Windows-only APIs

Markers help keep one dependency spec compatible across multiple operating systems.

Generate and Maintain with Tooling

If you use tools like pip-tools, keep human-written comments in input files and generate lock-style outputs automatically.

pip-compile requirements.in --output-file requirements.txt
pip-sync requirements.txt

In this workflow, comments belong primarily in requirements.in, while requirements.txt is treated as generated output.

Practical Style Guide for Team Repositories

A small style guide improves long-term readability. Group dependencies by purpose, keep comments short, and avoid repeating obvious details that tools already provide. Use one line for the package and a separate line for longer rationale when needed. During dependency upgrades, update both versions and comments in the same change to prevent stale notes. If a package is temporary, add a removal condition so future maintainers know when to delete it. Consistent comment discipline turns requirements files from a raw package list into useful project documentation.

Review Comments During Security Updates

When applying security upgrades, revisit nearby comments instead of updating versions only. Notes about temporary pins, compatibility limits, or incident context may no longer be accurate after a patch cycle. Keeping comments current avoids confusion during future audits and reduces repeated investigation work.

Common Pitfalls

A frequent mistake is placing explanatory text on its own line without #. pip will parse it as a requirement and fail.

Another issue is assuming comments survive regeneration when files are tool-generated. Many generators overwrite content, including notes.

Inline comments can also become misleading during version updates. If a package version changes, update the related comment at the same time.

Finally, avoid putting secrets or internal URLs in comments. Requirements files are often committed to source control.

Summary

• pip supports full-line and inline comments using #.
• Comments are valuable for documenting pins, constraints, and platform rules.
• Split dependencies with -r includes to keep files readable.
• Use constraints files and environment markers with clear notes.
• Keep comments in source files when working with generated requirement locks.