python-design-patterns

Python Design Patterns

Write maintainable Python code using fundamental design principles. These patterns help you build systems that are easy to understand, test, and modify.

When to Use This Skill

• Designing new components or services
• Refactoring complex or tangled code
• Deciding whether to create an abstraction
• Choosing between inheritance and composition
• Evaluating code complexity and coupling
• Planning modular architectures

Core Concepts

1. KISS (Keep It Simple)

Choose the simplest solution that works. Complexity must be justified by concrete requirements.

2. Single Responsibility (SRP)

Each unit should have one reason to change. Separate concerns into focused components.

3. Composition Over Inheritance

Build behavior by combining objects, not extending classes.

4. Rule of Three

Wait until you have three instances before abstracting. Duplication is often better than premature abstraction.

Quick Start

# Simple beats clever
# Instead of a factory/registry pattern:
FORMATTERS = {"json": JsonFormatter, "csv": CsvFormatter}

def get_formatter(name: str) -> Formatter:
    return FORMATTERS[name]()

Fundamental Patterns

Pattern 1: KISS - Keep It Simple

Before adding complexity, ask: does a simpler solution work?

# Over-engineered: Factory with registration
class OutputFormatterFactory:
    _formatters: dict[str, type[Formatter]] = {}

    @classmethod
    def register(cls, name: str):
        def decorator(formatter_cls):
            cls._formatters[name] = formatter_cls
            return formatter_cls
        return decorator

    @classmethod
    def create(cls, name: str) -> Formatter:
        return cls._formatters[name]()

@OutputFormatterFactory.register("json")
class JsonFormatter(Formatter):
    ...

# Simple: Just use a dictionary
FORMATTERS = {
    "json": JsonFormatter,
    "csv": CsvFormatter,
    "xml": XmlFormatter,
}

def get_formatter(name: str) -> Formatter:
    """Get formatter by name."""
    if name not in FORMATTERS:
        raise ValueError(f"Unknown format: {name}")
    return FORMATTERS[name]()