ngmthaq/dry-principle

DRY — Don't Repeat Yourself

When to Use

• Use when writing, reviewing, or refactoring code. Enforces the DRY principle: every piece of knowledge or logic must have a single, unambiguous, authoritative representation in the system.
• Apply at all levels: constants, logic, data schemas, configuration, documentation, and tests.

The Principle

"Every piece of knowledge must have a single, unambiguous, authoritative representation within a system." — Andy Hunt & Dave Thomas, The Pragmatic Programmer

Duplication is the root of many bugs: when logic lives in two places, they inevitably drift apart. DRY is not just about avoiding copy-paste — it's about single sources of truth.

Common Violations and Fixes

1. Duplicated Logic

Bad:

// In checkout flow
const tax = price * 0.08;

// In order summary
const tax = subtotal * 0.08;

// In invoice generator
const tax = amount * 0.08;

Good:

const TAX_RATE = 0.08;
function calculateTax(amount) {
  return amount * TAX_RATE;
}

2. Copy-Pasted Conditionals

Bad:

# In create_user()
if len(password) < 8 or not re.search(r'[A-Z]', password):
    raise ValueError("Invalid password")

# In reset_password()
if len(password) < 8 or not re.search(r'[A-Z]', password):
    raise ValueError("Invalid password")

Good:

def validate_password(password: str) -> None:
    if len(password) < 8 or not re.search(r'[A-Z]', password):
        raise ValueError("Invalid password")

# Both callers use validate_password()

3. Duplicated Data Shapes / Schema

Bad:

// In API handler
type User = { id: string; email: string; role: string };

// In database layer
type UserRecord = { id: string; email: string; role: string };

// In tests
const mockUser = { id: "1", email: "[email protected]", role: "admin" };

Good:

// types/user.ts — single source of truth
export type User = { id: string; email: string; role: string };
// All layers import from here

4. Magic Numbers / Strings

Bad:

if user.trial_days > 14:
    expire_account(user)

if signup_date + timedelta(days=14) < today:
    send_expiry_warning(user)

Good:

TRIAL_PERIOD_DAYS = 14

if user.trial_days > TRIAL_PERIOD_DAYS:
    expire_account(user)

if signup_date + timedelta(days=TRIAL_PERIOD_DAYS) < today:
    send_expiry_warning(user)

5. Duplicated Test Setup

Bad:

it("creates order", () => {
  const user = { id: 1, name: "Alice", role: "customer" };
  // ...
});
it("cancels order", () => {
  const user = { id: 1, name: "Alice", role: "customer" };
  // ...
});

Good:

const makeUser = (overrides = {}) => ({
  id: 1, name: 'Alice', role: 'customer', ...overrides
});

it('creates order', () => { const user = makeUser(); ... });
it('cancels order', () => { const user = makeUser(); ... });

DRY vs. Wrong Abstraction

DRY does not mean "merge anything that looks similar." Two pieces of code that look the same but represent different concepts should stay separate. Premature abstraction is its own problem (see YAGNI).

Ask: "Would these two pieces of code always need to change together?"

• Yes → Extract to a shared abstraction.
• No → They may be coincidentally similar. Leave them separate.

Enforcement Rules

1. Before copy-pasting code, stop — extract a function, constant, or module first.
2. Before hardcoding a value, ask if it appears (or could appear) elsewhere — if so, name it as a constant.
3. When adding a new field to a data shape, verify there's only one definition of that shape in the codebase.
4. During code review, flag duplications with: "DRY violation: this logic already exists in [location]."
5. Configuration and environment values belong in one place (e.g., a config file or env layer), not scattered across the codebase.
6. Do not DRY prematurely — wait until you see duplication at least twice before abstracting. "Rule of Three": abstract on the third repetition.