Universal Templating Skill

Comprehensive guide to template format selection, design patterns, generation workflows, and best practices across Handlebars, Cookiecutter, Copier, Maven, and Harness templates.

Template Format Matrix

Handlebars

Use Cases:

• Simple variable substitution
• Email templates
• Document generation
• Configuration files
• Quick string templating

Syntax:

Hello {{name}},
{{#if premium}}Welcome to premium!{{/if}}
{{#each items}}- {{this}}{{/each}}

Strengths:

• Minimal learning curve
• Fast execution
• Great for config files
• Small file size
• No external dependencies

Weaknesses:

• Limited logic capabilities
• No native loops/conditionals
• Requires helpers for complex operations

Best For: Configuration file templating, simple document generation

---

Cookiecutter

Use Cases:

• Interactive project scaffolding
• Multi-step wizard templates
• Python package templates
• Post-generation hooks

Syntax:

{
  "project_name": "{{ cookiecutter.project_name }}",
  "author": "{{ cookiecutter.author_name }}"
}

Strengths:

• Interactive CLI prompts
• Python ecosystem integration
• Post-generation hooks
• Conditional rendering
• JSON-based config

Weaknesses:

• Python dependency required
• Jinja2 templates (verbose)
• Less flexible validation
• Community templates vary in quality

Best For: Python projects, quick prototypes, community templates

---

Copier

Use Cases:

• Modern project scaffolding
• Template versioning and updates
• Multi-template composition
• Complex validation rules

Syntax:

_templates_suffix: .jinja
_copy_without_render:
  - "*.png"
  - "*.jpg"

project_name:
  type: str
  help: What is your project name?
  default: my_project

Strengths:

• Powerful Jinja2 templating
• Template versioning
• Update existing projects
• Composite templates
• Advanced validation
• Excellent documentation

Weaknesses:

• Python dependency
• Steeper learning curve
• Larger footprint
• Development active (API changes possible)

Best For: Enterprise templates, versioned scaffolding, complex projects

---

Maven

Use Cases:

• Java/JVM project archetypes
• Enterprise Java scaffolding
• Build system integration
• Dependency management

Syntax:

<archetype>
  <groupId>org.apache.maven.archetypes</groupId>
  <artifactId>maven-archetype-quickstart</artifactId>
</archetype>

Strengths:

• Native Maven integration
• Build tool awareness
• Dependency management
• Enterprise adoption
• IDEs have built-in support

Weaknesses:

• Java/JVM only
• XML-heavy
• Complex archetype internals
• Verbose setup

Best For: Java/JVM projects, Maven-based builds

---

Harness Templates

Use Cases:

• CI/CD pipeline steps
• Reusable stage definitions
• Pipeline patterns
• Deployment strategies

Syntax:

template:
  name: Deploy Service
  type: StepGroup
  spec:
    steps:
      - step:
          name: Deploy K8s
          identifier: deploy_k8s
          type: K8sDeploy
          spec:
            service: <+input>

Strengths:

• Native Harness integration
• Expression language support
• Runtime inputs
• Pipeline-aware
• Built-in approval flows

Weaknesses:

• Harness-specific only
• YAML complexity
• Requires Harness setup
• Limited reusability outside Harness

Best For: Harness pipelines, deployment templates

---

Format Selection Decision Tree

START: Need to generate what?
│
├─ Configuration files
│  ├─ Simple substitution → Handlebars
│  └─ Complex validation → Copier
│
├─ Project scaffold
│  ├─ Python project → Cookiecutter
│  ├─ Enterprise/versioned → Copier
│  └─ Java/JVM → Maven
│
├─ CI/CD pipeline
│  ├─ Harness platform → Harness Templates
│  └─ Other CI → Handlebars + custom
│
├─ Document/email
│  └─ Handlebars
│
└─ Reusable components
   ├─ Code snippets → Handlebars
   └─ Full modules → Copier

---

Generation Workflow Steps

Step 1: Template Planning

Inputs:

• Target audience (users, developers, automation)
• Use cases and scenarios
• Complexity level (simple, moderate, advanced)
• Maintenance burden tolerance
• Integration requirements

Deliverables:

• Template specification document
• Format selection justification
• Variable naming convention document
• Example instantiation

Questions to Answer:

1. What will be generated?
2. Who uses it (users, scripts, tools)?
3. How often will it change?
4. Will it need versioning?
5. What validation is needed?

---

Step 2: Variable Definition

Essential Variables:

project_name         - Primary identifier
author_name         - Creator/maintainer
organization        - Company/org name
description         - Brief description
license             - License type (MIT, Apache, etc.)
target_framework    - Framework/language version

Optional Variables (by use case):

// Python projects
python_version      - Target Python version
package_name        - PyPI package name
django_version      - Django version (if applicable)

// Java projects
java_version        - JDK version
groupId            - Maven group ID
artifactId         - Maven artifact ID

// Cloud projects
aws_region         - AWS region
kubernetes_cluster - K8s cluster name
docker_registry    - Container registry

---

Step 3: Variable Naming Conventions

Naming Rules:

1. Format: snake_case (all formats support this)

2. Prefixes:

   • generated_* - Files/content created by template
   • input_* - User input required
   • computed_* - Derived from other variables
   • optional_* - Optional user input

3. Examples:

   ✓ project_name
   ✓ author_email
   ✓ generated_version
   ✓ target_framework
   ✗ ProjectName (avoid PascalCase)
   ✗ PROJECT_NAME (avoid SCREAMING_SNAKE_CASE)
   

---

Step 4: Content Structure Design

Standard Project Structure:

{project_name}/
├── README.md              # Template instructions
├── {project_name}/        # Main package/app
│   ├── __init__.py        # (if applicable)
│   ├── main.py
│   └── config.py
├── tests/                 # Test directory
│   ├── __init__.py
│   └── test_main.py
├── docs/                  # Documentation
│   └── API.md
├── .gitignore
├── LICENSE
├── requirements.txt       # (Python)
├── setup.py              # (Python)
├── package.json          # (Node.js)
└── {{cookiecutter.var}}/ # Template variables

---

Step 5: Conditional Rendering

When to Use:

• Optional features
• Different project types
• Target-specific configurations
• License-based files

Handlebars Example:

{{#if include_docker}}
FROM python:3.11
COPY . /app
{{/if}}

Cookiecutter/Copier Example:

{%- if use_docker %}
# Docker configuration
{%- endif %}

---

Step 6: Validation & Constraints

Input Validation:

• Email format checking
• Version number validation
• Project name uniqueness checks
• Path validation

Copier Example:

project_name:
  type: str
  help: Project name (lowercase, alphanumeric + underscore)
  regex: "^[a-z_][a-z0-9_]*$"

python_version:
  type: str
  default: "3.11"
  help: Python version (3.9, 3.10, 3.11, 3.12)
  choices:
    - "3.9"
    - "3.10"
    - "3.11"
    - "3.12"

---

Step 7: Post-Generation Hooks

Cookiecutter/Copier Hooks:

# hooks/post_gen_project.py
import os
from pathlib import Path

# Initialize git repository
os.system("git init")

# Create virtual environment
os.system("python -m venv venv")

# Install dependencies
os.system("pip install -r requirements.txt")

# Generate API docs
os.system("python generate_docs.py")

---

Step 8: Documentation

Required Documentation:

1. README.md - How to use template
2. VARIABLES.md - All available variables
3. EXAMPLES.md - Example instantiations
4. TROUBLESHOOTING.md - Common issues

---

Best Practices for Template Design

1. Variable Defaults

Good Defaults:

# Clear, sensible defaults
author_name: "Your Name"
license: "MIT"
python_version: "3.11"  # Latest stable
include_docker: false   # Opt-in for complexity
include_tests: true     # Always good to start with tests

Bad Defaults:

# Unclear or empty
author_name: ""
unknown_var: "???"
version: "1.0.0"  # Should be context-aware

---

2. DRY Principle (Don't Repeat Yourself)

Template Variables Once:

Define: project_name = "my_project"
Use in:
  - Directory name
  - README title
  - Setup.py name
  - Docker image name

---

3. File Organization

Group Related Files:

template/
├── [project_name]/        # Project source (stays as-is)
├── [project_name]_docs/   # Docs structure
├── [project_name]_config/ # Config templates
└── tests/                 # Test templates

---

4. Template Readability

Use Clear Comments:

{# This file is generated from {{template_name}} #}
{# Last updated: {{generated_date}} #}
{# For questions, see: {{docs_url}} #}

---

5. Version Management

Template Versioning:

# In template metadata
version: "1.0.0"
harness_compatibility: "1.4+"
minimum_python: "3.9"

---

6. Error Handling

Clear Error Messages:

# Instead of: ValueError
# Use:
if not re.match(r"^[a-z_][a-z0-9_]*$", project_name):
    raise ValueError(
        f"Project name '{project_name}' is invalid.\n"
        f"Must start with lowercase letter or underscore,\n"
        f"followed by lowercase letters, numbers, or underscores."
    )

---

Template Generation Workflow

End-to-End Generation Process

1. USER SELECTION
   ├─ Choose template
   ├─ Select format (if flexible)
   └─ Provide variables

2. VALIDATION
   ├─ Validate all inputs
   ├─ Check constraints
   └─ Generate variable report

3. PRE-PROCESSING
   ├─ Compute derived variables
   ├─ Expand conditionals
   └─ Build file tree

4. GENERATION
   ├─ Render templates
   ├─ Copy static files
   ├─ Create directory structure
   └─ Handle special files

5. POST-PROCESSING
   ├─ Run hooks
   ├─ Initialize git/vcs
   ├─ Install dependencies
   └─ Generate documentation

6. VALIDATION
   ├─ Check generated files
   ├─ Verify structure
   ├─ Test basic functionality
   └─ Generate report

7. OUTPUT
   ├─ Display summary
   ├─ Provide next steps
   └─ Save manifest

---

Harness Expression Language in Templates

Available Context Variables

Harness Expressions:
├─ <+input.VARIABLE_NAME>        # Inputs
├─ <+pipeline.PROPERTY>          # Pipeline-level
├─ <+stage.PROPERTY>             # Stage-level
├─ <+steps.STEP_ID.PROPERTY>     # Step outputs
├─ <+env.PROPERTY>               # Environment variables
├─ <+secrets.getValue("NAME")>   # Secret references
└─ <+execution.PROPERTY>         # Execution context

Template Examples with Expressions

template:
  name: Deploy Service
  type: Step
  spec:
    service:
      name: <+input.service_name>
    environment:
      name: <+input.environment>
    variables:
      version: <+input.artifact_version>
      deploy_timeout: <+input.timeout_minutes>
      approval_required: <+input.requires_approval>

---

Related Documentation

• Handlebars.js
• Cookiecutter
• Copier
• Maven Archetypes
• Harness Templates