copilot-instructions: Instructions file for GitHub Copilot

Object Calisthenics Instructions

Overview

Object Calisthenics is a set of programming exercises designed to improve code quality through strict constraints. These rules help create more maintainable, testable, and readable object-oriented code.

The Nine Rules

1. Only One Level of Indentation Per Method

Keep methods simple with minimal nesting.

✅ Good:

def process_orders(orders: List[Order]) -> List[ProcessedOrder]:
    return [self._process_single_order(order) for order in orders]

def _process_single_order(self, order: Order) -> ProcessedOrder:
    validated = self._validate_order(order)
    return self._apply_processing(validated)

❌ Bad:

def process_orders(orders: List[Order]) -> List[ProcessedOrder]:
    results = []
    for order in orders:
        if order.is_valid():
            if order.has_items():
                for item in order.items:
                    if item.in_stock():
                        results.append(process_item(item))
    return results

2. Don't Use the ELSE Keyword

Use early returns and guard clauses instead.

✅ Good:

def calculate_discount(customer: Customer, amount: float) -> float:
    if not customer.is_premium():
        return amount

    if amount < 100:
        return amount * 0.95

    return amount * 0.90

❌ Bad:

def calculate_discount(customer: Customer, amount: float) -> float:
    if customer.is_premium():
        if amount >= 100:
            return amount * 0.90
        else:
            return amount * 0.95
    else:
        return amount

3. Wrap All Primitives and Strings

Encapsulate primitives in value objects.

✅ Good:

@dataclass(frozen=True)
class Email:
    value: str

    def __post_init__(self):
        if '@' not in self.value:
            raise ValueError("Invalid email format")

@dataclass(frozen=True)
class CustomerId:
    value: str

class Customer:
    def __init__(self, id: CustomerId, email: Email):
        self.id = id
        self.email = email