YAMLとは？構文、型、そして誰もがハマる落とし穴

# This is a comment — YAML supports them, JSON does not
app:
  name: payment-service
  version: "2.1.0"         # quoted to keep it a string
  port: 8080
  debug: false
  timeout: 30.5
  tags:
    - payments
    - backend
    - critical
  database:
    host: postgres.internal
    port: 5432
    ssl: true
    password: null          # explicitly no value

# Strings — quotes are optional unless value is ambiguous
name: Alice
greeting: "Hello, world"
message: 'single quotes work too'
path: /usr/local/bin           # unquoted — still a string

# Numbers
integer: 42
negative: -7
float: 3.14159
scientific: 6.022e23

# Booleans
ssl_enabled: true
verbose: false

# Null
api_key: null
legacy_field: ~               # tilde is also null in YAML

# YAML 1.1 implicit type coercion — these silently become booleans:
country: NO       # → false  (Norway Problem!)
enabled: yes      # → true
toggle: on        # → true

# The fix: always quote when there's any ambiguity
country: "NO"
enabled: "yes"
version: "1.0"    # also quote version numbers — 1.0 would be a float

# Simple list
languages:
  - Python
  - Go
  - TypeScript

# List of objects (common in Kubernetes)
containers:
  - name: api
    image: my-api:latest
    port: 8080
  - name: sidecar
    image: envoy:v1.28
    port: 9901

# Inline flow style (valid YAML, less readable)
tags: [payments, backend, v2]

# Literal block scalar (|) — preserves newlines exactly
startup_script: |
  #!/bin/bash
  set -e
  echo "Starting service..."
  npm start

# Folded scalar (>) — folds newlines into spaces (good for long descriptions)
description: >
  This service handles payment processing
  for all regions. It connects to Stripe
  and falls back to PayPal.
# Result: "This service handles payment processing for all regions. It connects to Stripe and falls back to PayPal."

# Chomp modifiers
keep_trailing: |+
  line one
  line two

strip_trailing: |-
  line one
  line two

name: CI

on:
  push:
    branches: [main, develop]
  pull_request:

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Set up Node.js
        uses: actions/setup-node@v4
        with:
          node-version: "20"
          cache: npm

      - name: Install dependencies
        run: npm ci

      - name: Run tests
        run: |
          npm run lint
          npm run test -- --coverage
          npm run build

apiVersion: apps/v1
kind: Deployment
metadata:
  name: payment-api
  namespace: production
  labels:
    app: payment-api
    version: "2.1.0"
spec:
  replicas: 3
  selector:
    matchLabels:
      app: payment-api
  template:
    metadata:
      labels:
        app: payment-api
    spec:
      containers:
        - name: api
          image: payment-api:2.1.0
          ports:
            - containerPort: 8080
          env:
            - name: NODE_ENV
              value: production
            - name: DB_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: db-secret
                  key: password
          resources:
            requests:
              memory: "128Mi"
              cpu: "100m"
            limits:
              memory: "512Mi"
              cpu: "500m"

import yaml

# Reading a YAML config file
with open('config.yaml', 'r') as f:
    config = yaml.safe_load(f)

print(config['app']['name'])        # payment-service
print(config['app']['port'])        # 8080 (integer, not string)
print(config['app']['tags'])        # ['payments', 'backend', 'critical']

# Writing Python data to YAML
data = {
    'service': 'auth-api',
    'replicas': 2,
    'endpoints': ['/login', '/logout', '/refresh']
}
with open('output.yaml', 'w') as f:
    yaml.dump(data, f, default_flow_style=False)

import yaml from 'js-yaml';
import fs from 'fs';

// Parse YAML string
const raw = fs.readFileSync('config.yaml', 'utf8');
const config = yaml.load(raw);

console.log(config.app.name);     // payment-service
console.log(config.app.tags);     // ['payments', 'backend', 'critical']

// Dump an object to YAML string
const output = yaml.dump({
  name: 'my-service',
  port: 3000,
  tags: ['api', 'public']
});
console.log(output);