UitbreidenOS Refactor Skill
When to activate
- A function is too long and should be split into smaller, named functions
- Code is duplicated across 2+ places and should be extracted
- A variable or function name no longer reflects what it does
- A file has grown too large and should be split into modules
- Nested conditionals are hard to read and should be simplified
- You want to refactor safely with tests catching regressions
When NOT to use
- Architectural rewrites (changing frameworks, replacing DBs) — too much scope
- Refactoring untested code without first adding a test harness — risky
- Performance optimisation — profile first, then use the perf skill
- Changes that alter external behaviour — that's a feature, not a refactor
Instructions
Core principle: refactor in small steps, test at each step
The biggest risk in refactoring is breaking something that used to work. The safest pattern:
1. Run tests → confirm green
2. Make one small refactor
3. Run tests → confirm still green
4. Commit
5. Repeat
Never do multiple refactors in the same commit. Each commit should be a single, reversible step.
Refactor types and when to use each
Extract function — a block of code does one identifiable thing:
# Before — too much in one place
def process_order(order):
# 20 lines calculating total
subtotal = sum(item.price * item.qty for item in order.items)
tax = subtotal * 0.08
shipping = 4.99 if subtotal < 50 else 0
total = subtotal + tax + shipping
# 15 lines sending confirmation
msg = f"Order {order.id} confirmed. Total: ${total:.2f}"
send_email(order.email, "Order Confirmed", msg)
# After — named, testable pieces
def calculate_total(order) -> float:
subtotal = sum(item.price * item.qty for item in order.items)
tax = subtotal * 0.08
shipping = 4.99 if subtotal < 50 else 0
return subtotal + tax + shipping
def send_order_confirmation(order, total: float) -> None:
msg = f"Order {order.id} confirmed. Total: ${total:.2f}"
send_email(order.email, "Order Confirmed", msg)
def process_order(order):
total = calculate_total(order)
send_order_confirmation(order, total)
Extract class — a group of functions share state and belong together:
# Before — functions operating on the same dict
def get_user_name(user_dict): ...
def get_user_email(user_dict): ...
def update_user_email(user_dict, email): ...
# After — cohesive class
class User:
def __init__(self, data): self._data = data
@property
def name(self): return self._data['name']
@property
def email(self): return self._data['email']
def update_email(self, email): self._data['email'] = email
Rename — name no longer matches meaning:
# Before
def get_data(x): ... # what data? what is x?
# After
def fetch_user_by_id(user_id: int) -> User: ...
Simplify conditionals — nested ifs are hard to reason about:
# Before — pyramid of doom
if user:
if user.is_active:
if user.has_permission("read"):
return get_data()
else:
raise PermissionError()
else:
raise UserInactiveError()
else:
raise UserNotFoundError()
# After — guard clauses (fail fast)
if not user:
raise UserNotFoundError()
if not user.is_active:
raise UserInactiveError()
if not user.has_permission("read"):
raise PermissionError()
return get_data()
Remove duplication — same logic appears in 2+ places:
# Before — discount logic duplicated in two places
def checkout_price(items, user):
total = sum(i.price for i in items)
if user.is_premium:
total *= 0.9
return total
def cart_preview_price(items, user):
total = sum(i.price for i in items)
if user.is_premium:
total *= 0.9
return total
# After — single source of truth
def apply_user_discount(price: float, user) -> float:
return price * 0.9 if user.is_premium else price
def checkout_price(items, user):
return apply_user_discount(sum(i.price for i in items), user)
def cart_preview_price(items, user):
return apply_user_discount(sum(i.price for i in items), user)
Split large file — file has multiple unrelated concerns:
# Before: one 800-line auth.py with models, services, routes, and utils
# After: split into clear modules
auth/
├── __init__.py
├── models.py # User, Session data models
├── service.py # Business logic: login, logout, refresh
├── router.py # HTTP routes and request/response handling
└── utils.py # JWT encoding/decoding, password hashing
Invoking the skill
/refactor
Code: [paste code or give file path]
Goal: [extract function / rename / simplify / split / remove duplication]
Constraint: [keep the public API the same / must stay in one file / etc]
Or:
/refactor
The `process_payment()` function in src/billing/payments.py is 120 lines.
Extract the validation logic and the Stripe API call into separate functions.
Tests are in tests/test_payments.py — run them to verify nothing breaks.
Output format
Claude produces:
- Refactor plan — what will change and why
- Step-by-step diffs — one small change at a time
- Test command — what to run after each step
- Final state — the complete refactored code
Example
Before:
async function handleSignup(req: Request, res: Response) {
if (!req.body.email || !req.body.password || req.body.password.length < 8) {
return res.status(400).json({ error: 'Invalid input' })
}
const existing = await db.user.findUnique({ where: { email: req.body.email } })
if (existing) {
return res.status(409).json({ error: 'Email taken' })
}
const hash = await bcrypt.hash(req.body.password, 10)
const user = await db.user.create({ data: { email: req.body.email, passwordHash: hash } })
const token = jwt.sign({ userId: user.id }, process.env.JWT_SECRET!, { expiresIn: '7d' })
res.status(201).json({ token })
}
Refactor plan:
- Extract
validateSignupInput()→ testable, reusable - Extract
createUser()service function → DB logic out of route handler - Extract
generateAuthToken()→ reusable for login too
After:
function validateSignupInput(body: unknown): { email: string; password: string } {
if (!body.email || !body.password || body.password.length < 8)
throw new ValidationError('Email and password (min 8 chars) required')
return { email: body.email, password: body.password }
}
async function createUser(email: string, password: string): Promise<User> {
const existing = await db.user.findUnique({ where: { email } })
if (existing) throw new ConflictError('Email already registered')
const passwordHash = await bcrypt.hash(password, 10)
return db.user.create({ data: { email, passwordHash } })
}
function generateAuthToken(userId: string): string {
return jwt.sign({ userId }, process.env.JWT_SECRET!, { expiresIn: '7d' })
}
async function handleSignup(req: Request, res: Response) {
const { email, password } = validateSignupInput(req.body)
const user = await createUser(email, password)
const token = generateAuthToken(user.id)
res.status(201).json({ token })
}