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A complete guide to writing comments in YAML

This guide offers a thorough introduction to YAML comments, essential for beginners and enthusiasts alike. Learn the syntax, best practices, and practical applications of commenting in YAML files, enhancing readability and maintainability of your configurations and data structures in various applications.

by Arun Kumar
yaml commenting guide

Today, we’re focusing on a seemingly small yet crucial aspect of working with YAML: comments. At first glance, comments might appear as mere sidelines to the primary code, but they play a pivotal role in enhancing understanding, maintenance, and collaboration in YAML files.

In this comprehensive guide, we’ll explore the various facets of YAML comments, from their basic syntax and types to best practices and common use cases.

What are comments in YAML?

Comments in YAML are ways to include notes, explanations, or any human-readable information that shouldn’t be processed by the machine. I personally love using comments to keep track of changes or to explain why I made certain decisions in the configuration.

Syntax of YAML comments

The syntax for adding a comment in YAML is straightforward:

  • A comment begins with a # (hash) symbol.
  • Everything following the # on the same line is treated as a comment.


# This is a comment
key: value  # Inline comment

In this example, # This is a comment and # Inline comment are both ignored by YAML parsers.

Types of comments in YAML

YAML primarily offers one way to write comments, but their usage can be categorized based on their placement:

1. Full line comments

As the name suggests, these comments occupy an entire line.

# Full line comment
key: value

Full line comments in YAML are those that occupy an entire line without any code or commands. They are typically used to provide detailed descriptions or explanations above a section of code. This type of comment is particularly useful for separating different sections of a YAML file or for explaining complex logic that might not be immediately apparent. For instance, before a block of configuration settings, a full line comment can describe what those settings are for.


# Configure database connection settings
  host: localhost
  port: 3306

In this example, the comment # Configure database connection settings clearly indicates that the following lines pertain to database configurations. This makes the YAML file more readable and maintainable, especially for someone new to the project.

2. Inline comments

Inline comments share the line with a code statement.

key: value  # Inline comment

Inline comments in YAML are placed on the same line as a piece of code. They are used to provide specific, brief explanations about the line of code they accompany. This is particularly handy for clarifying the purpose of certain values or parameters that might not be self-explanatory. Inline comments can be invaluable in making your code more understandable without needing to refer to external documentation.


  host: localhost  # Local server host
  port: 8080       # Default port for the server

In this snippet, the inline comments provide immediate context for the host and port configurations. The comment # Local server host clarifies that localhost refers to a local server, and # Default port for the server explains the significance of the port number 8080. These small annotations can greatly enhance the readability and maintainability of the code.

Common use cases for YAML comments

1. Explaining code

Comments are incredibly useful for explaining what a specific piece of YAML code does. This is particularly important in YAML files because they often serve as configuration files, which can be complex and not immediately intuitive to someone who didn’t write them.

For example, in a YAML file configuring a web application, you might have several parameters whose purposes aren’t immediately obvious. Here, comments can clarify what each parameter does, like specifying the role of a certain port number or explaining why a specific timeout duration is set.


  timeout: 30  # Timeout in seconds for server response

2. Documenting changes

In a team environment or even in individual projects, tracking why changes were made to a configuration can be as important as the changes themselves. Comments are a perfect way to annotate these modifications. When you update a YAML file, adding a comment on what was changed and why can be incredibly helpful. This practice aids in maintaining a clear history of the evolution of the file, which is especially beneficial when multiple people are working on the same project.


  connection_limit: 10  # Reduced from 15 to 10 for better resource management

3. Commenting out code

Sometimes, you might want to temporarily disable a part of your YAML configuration without deleting it. This is where commenting out comes into play. By turning a line of code into a comment, you prevent it from being executed or considered by the YAML parser, but you still keep it in the file for future reference or reactivation. This is a common practice when testing different configurations or debugging.


  # - new-user-onboarding  # Temporarily disabled for debugging
  - notifications

In this example, the ‘new-user-onboarding’ feature is commented out, meaning it won’t be active, but it can easily be reinstated by just removing the #.

These use cases showcase how comments in YAML aren’t just for adding contextual notes but are integral to managing, maintaining, and understanding YAML files.

Best practices for using comments in YAML

While comments are flexible, it’s good to follow certain best practices:

1. Clarity

The primary purpose of a comment is to make your code easier to understand. Therefore, clarity is key. Your comments should be concise yet informative enough to convey the necessary message. Avoid vague statements that can confuse readers more than they clarify.

  • Use straightforward language.
  • Be precise in what you are explaining or noting.
  • Avoid unnecessary jargon or overly technical terms, unless they are required for understanding the context.


# Bad: Set value
# Good: Set the maximum number of simultaneous connections
max_connections: 50

2. Relevance

Keep your comments relevant and up to date. Outdated comments can be more misleading than having no comments at all. If you modify the code, make sure to check if the associated comments need updating too. This ensures that anyone reading the code understands the current state and purpose of the code.

  • Regularly review comments during code reviews or when updating code.
  • Remove comments that are no longer applicable.
  • Update comments to reflect the current functionality.


# Outdated: Connection timeout in minutes (old version)
# Updated: Connection timeout in seconds (after code update)
timeout: 30

3. Avoid over-commenting

While comments are useful, too many comments can clutter your code and make it difficult to read. Comment only when necessary. If your code is self-explanatory, it might not need a comment at all. The idea is to strike a balance between explaining complex parts and keeping the code clean and readable.

  • Comment on why the code is doing something, rather than how it’s doing it (unless the ‘how’ is not obvious).
  • Avoid stating the obvious. For example, don’t comment on every single line in a straightforward YAML file.
  • Use comments to explain complex logic, configurations, or workarounds that aren’t immediately clear from the code itself.


# Unnecessary: Assign 50 to max_connections
# Necessary: Set this higher for production environments
max_connections: 50

4. Consistency

Maintaining a consistent commenting style throughout your YAML files makes your code more organized and easier to follow. Decide on a style for your comments and stick to it throughout the project. This consistency helps others (and you) to understand and maintain the codebase more efficiently.

  • Decide on full line vs. inline comments and use them consistently.
  • Establish and follow a format for special comments like TODOs, FIXMEs, etc.
  • Keep a similar tone and language style across all comments.


# TODO: Refactor this section to improve performance
# FIXME: Address potential security vulnerability here

By following these best practices, you can ensure that your use of comments in YAML adds value to your code and doesn’t become a source of confusion or clutter.

My feedback

From my experience, comments are a lifesaver, especially when working on complex projects or returning to an old project. They are like breadcrumbs left behind, guiding future-you or others through the thought process behind the code. However, I do find over-commenting to be a bit of an eyesore and prefer a cleaner approach with essential comments only.

Frequently Asked Questions about YAML comments

Here are some frequently asked questions that might help you understand the nuances of commenting in YAML better.

What are YAML comments?

YAML comments are non-executable lines in a YAML file, used to add notes or explanations. They start with the # symbol, and everything following this symbol on the same line is treated as a comment.

Can you have multi-line comments in YAML?

YAML does not support direct multi-line comments like some other languages. Each line of a comment must start with a #. However, you can create a block of comments by prefixing each line in the block with a #.

Are comments in YAML visible in the final output?

No, comments in YAML are ignored by the parser and are not visible in the final output. They are only for the benefit of humans reading the YAML file.

How do you comment out a block of code in YAML?

To comment out a block of code in YAML, you need to prefix each line of the block with a #. Unfortunately, there’s no shortcut to comment out multiple lines at once, as you might find in programming languages like Python or JavaScript.

Can you use comments for documentation purposes in YAML?

Absolutely! Comments are often used to document the structure and purpose of various sections in a YAML file. This practice is particularly useful in large or complex configuration files.

Should comments be used to explain obvious code in YAML?

Generally, it’s better to avoid commenting on very obvious pieces of code. Comments should provide additional insight or explanation that is not immediately apparent from the code itself.

Can YAML comments include special characters?

Yes, YAML comments can include special characters. However, the comment must start with the # symbol, and it’s good practice to leave a space after the # for readability.

Are there any tools to help manage comments in YAML files?

While there aren’t specific tools dedicated to managing comments, most modern code editors and IDEs provide features like syntax highlighting and block commenting, which can help manage comments in YAML files.

Can comments be nested in YAML?

No, YAML does not support nested comments. Once you start a comment with #, everything following it on that line is part of the comment, including other # symbols.

Is there a maximum length for a YAML comment?

There’s no specific maximum length for a YAML comment, but for readability, it’s advisable to keep comments concise and to the point. If a comment is too long, consider breaking it into multiple lines.


Understanding and effectively using comments in YAML can significantly enhance the readability, maintainability, and overall quality of your configuration files. From providing clarity and context to your code, to documenting changes and temporarily disabling code segments, comments in YAML serve crucial functions that go beyond mere annotations. Adhering to best practices, such as maintaining clarity, relevance, and avoiding over-commenting, ensures that your comments are meaningful and useful. Whether you are a beginner or an experienced user, mastering the art of commenting in YAML can make a substantial difference in your work with this versatile language.

Thank you for joining me on this YAML journey. I hope this guide helps you in your coding endeavors. Happy coding, and remember, the # symbol is your friend in YAML!

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