YAML (YAML Ain’t Markup Language)

What is YAML?

YAML (YAML Ain’t Markup Language) is a human-readable data serialization format, commonly used for

configuration files and
data exchange.

Basic Syntax

Key-Value Pairs


key: value
name: John Doe
age: 30
is_student: false

Strings


# Single line
name: John Doe
 
# Multi-line (preserves newlines)
description: |
  This is a
  multi-line
  string
 
# Multi-line (folds newlines)
# same as
# description: "This will be a single line string"
description: >
  This will be
  a single line
  string

Numbers


integer: 42
float: 3.14
scientific: 1.2e+34
infinity: .inf
not_a_number: .nan

Booleans


true_value: true
false_value: false
yes: yes    # Also true
no: no      # Also false

Null Values


null_value: null
empty: ~    # Also null

Collections

Lists/Arrays


# Block style
fruits:
  - Apple
  - Banana
  - Orange
 
# Inline style
fruits: [Apple, Banana, Orange]

Dictionaries/Maps


# Block style
person:
  name: John
  age: 30
  city: New York
 
# Inline style
person: {name: John, age: 30, city: New York}

Nested Structures


users:
  - name: John
    age: 30
    hobbies:
      - reading
      - swimming
  - name: Jane
    age: 25
    hobbies: [hiking, photography]

Advanced Features

Anchors and Aliases (Reuse)


defaults: &defaults
  adapter: postgres
  host: localhost
 
development:
  <<: *defaults
  database: dev_db
 
test:
  <<: *defaults
  database: test_db

Multi-document Files


---
# Document 1
name: John
age: 30
---
# Document 2
name: Jane
age: 25

Comments


# This is a comment
key: value  # Inline comment

Special Syntax

Environment Variables


# Some YAML parsers support this
database_url: ${DATABASE_URL:-sqlite:///default.db}

Type Casting (YAML 1.2)


# Some parsers support explicit types
string: !!str 123
integer: !!int "123"
float: !!float "3.14"
boolean: !!bool "true"

Common Use Cases

Docker Compose YAML and JSON

YAML Format


version: '3.8'
services:
  web:
    image: nginx:alpine
    ports:
      - "80:80"
    environment:
      - NODE_ENV=production
    volumes:
      - ./app:/app
 
  database:
    image: postgres:13
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass

JSON Format


{
  "version": "3.8",
  "services": {
    "web": {
      "image": "nginx:alpine",
      "ports": [
        "80:80"
      ],
      "environment": [
        "NODE_ENV=production"
      ],
      "volumes": [
        "./app:/app"
      ]
    },
    "database": {
      "image": "postgres:13",
      "environment": {
        "POSTGRES_DB": "myapp",
        "POSTGRES_USER": "user",
        "POSTGRES_PASSWORD": "pass"
      }
    }
  }
}

CI/CD Pipeline YAML and JSON

YAML Format


name: CI/CD Pipeline
on: [push, pull_request]
 
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Run tests
        run: npm test

JSON Format


{
  "name": "CI/CD Pipeline",
  "on": ["push", "pull_request"],
  "jobs": {
    "test": {
      "runs-on": "ubuntu-latest",
      "steps": [
        {
          "uses": "actions/checkout@v2"
        },
        {
          "name": "Run tests",
          "run": "npm test"
        }
      ]
    }
  }
}

Best Practices

1. Indentation


# Good (2 spaces)
key:
  nested:
    - item1
    - item2
 
# Bad (mixing tabs and spaces)
key:
    nested:
      - item1
      - item2

2. Quotes


# Use quotes when needed
special_chars: "value with: colon"
boolean_string: "true"
numeric_string: "123"

3. Readability


# Good - clear structure
server:
  host: localhost
  port: 8080
  ssl:
    enabled: true
    cert: /path/to/cert
 
# Bad - too compact
server: {host: localhost, port: 8080, ssl: {enabled: true, cert: /path/to/cert}}

Common Pitfalls

1. Boolean Interpretation


# These might be interpreted as booleans
yes: yes    # → true
no: no      # → false
on: on      # → true
off: off    # → false
 
# Use quotes to force string
yes: "yes"  # → "yes"

2. Floating Point Numbers


# This might be interpreted as time
version: 3.10  # → 3.1 (float)
version: "3.10" # → "3.10" (string)

3. Special Characters


# Use quotes for special characters
special: "value with: colon & asterisk *"

Validation Tools

Online Validators

Command Line


# Install yamllint
pip install yamllint
 
# Validate file
yamllint file.yaml
 
# Validate with specific rules
yamllint -d relaxed file.yaml

YAML vs JSON

Feature	YAML	JSON
Readability	High	Medium
Comments	Yes	No
Multi-line strings	Yes	No
References/Anchors	Yes	No
Strict syntax	Less strict	Very strict
File size	Smaller	Larger


# YAML
person:
  name: John
  age: 30
  hobbies: [reading, swimming]


// JSON
{
  "person": {
    "name": "John",
    "age": 30,
    "hobbies": ["reading", "swimming"]
  }
}

Remember: YAML is a superset of JSON - all valid JSON is valid YAML!

Key Features of YAML:

Human-readable: YAML is designed to be easy to read and write.
Hierarchical structure: Data is represented in a tree structure, which makes it easy to represent complex data.
Indentation-based: YAML relies on indentation to represent the structure, so there are no brackets or other delimiters.
Supports complex data types: YAML supports scalar values (strings, numbers), lists, and maps (key-value pairs).