YAML

YAML (YAML Ain't Markup Language) is a data serialization language designed for human readability. It uses indentation (spaces only, never tabs) to express nested structure and omits the brackets, braces, and quotation marks that make JSON verbose. YAML is a superset of JSON — any valid JSON is valid YAML — and is the standard format for Kubernetes manifests, GitHub Actions workflows, Docker Compose files, and Ansible playbooks.

Syntax Example

# Comments are supported in YAML
server:
  host: localhost
  port: 8080        # integers inferred from context
  tls: true
  tags:
    - web
    - primary

database:
  url: postgres://localhost/mydb
  pool:
    min: 2
    max: 10

description: |
  This is a multi-line
  literal block scalar.
  Newlines are preserved.

summary: >
  This text will be
  folded into one line.

Common Pitfalls

The Norway Problem (YAML 1.1)

In YAML 1.1 (used by many parsers), no, NO, yes, YES, on, off are parsed as booleans:

countries:
  - US
  - NO   # Parsed as false in YAML 1.1!
  - SE

Fix: quote values that might be misinterpreted: "NO".

Octal Numbers

In YAML 1.1, leading zeros indicate octal: 0777 becomes 511 (decimal). Quote these values.

Indentation

YAML uses spaces only — tabs cause parse errors. Mixing tabs and spaces is a common mistake for developers familiar with Python or Makefiles.

YAML-Specific Features

# Anchors and aliases (reuse values)
defaults: &defaults
  timeout: 30
  retries: 3

production:
  <<: *defaults    # Merge anchor
  host: prod.example.com

# Explicit types
port: !!int "8080"
enabled: !!bool "yes"
data: !!null ""

YAML vs JSON

YAML is for human-authored files. JSON is for machine-generated data and APIs. Never use yaml.load() in Python — use yaml.safe_load() to prevent arbitrary code execution via YAML deserialization.

Format and validate YAML with the YAML Formatter Tool.