Refactoring Patterns Skill

Systematic refactoring with safety checks and verification.

When to Use

• Extracting methods, classes, or modules
• Renaming identifiers safely across codebase
• Moving code between files or modules
• Simplifying complex code
• Modernizing legacy patterns

Reference Documents

• Extract Patterns - Extract method/class/module patterns
• Rename Patterns - Safe renaming across codebase
• Move Patterns - Moving code between files/modules
• Simplify Patterns - Reducing complexity patterns

Core Principles

1. Never Refactor Without Tests

BEFORE refactoring:
1. Ensure tests exist for code being changed
2. Run tests to confirm they pass
3. Note coverage percentage

DURING refactoring:
4. Run tests after each small change
5. Keep changes atomic and reversible

AFTER refactoring:
6. Run full test suite
7. Verify coverage didn't decrease

2. Small, Incremental Changes

BAD:  One massive PR that changes everything
GOOD: Series of small PRs, each working state

Commit 1: Add new class
Commit 2: Add tests for new class
Commit 3: Migrate first usage
Commit 4: Migrate remaining usages
Commit 5: Remove old code

3. Preserve Behavior

Refactoring changes structure, not behavior.

# BEFORE
def calculate_total(items):
    total = 0
    for item in items:
        total += item.price * item.quantity
    return total

# AFTER (same behavior, different structure)
def calculate_total(items):
    return sum(item.price * item.quantity for item in items)

Refactoring Workflow

Step 1: Identify the Smell

Code Smell	Refactoring
Long method	Extract Method
Large class	Extract Class
Duplicated code	Extract Method/Class
Long parameter list	Introduce Parameter Object
Data clumps	Extract Class
Primitive obsession	Replace Primitive with Object
Switch statements	Replace with Polymorphism
Parallel inheritance	Collapse Hierarchy

Step 2: Ensure Test Coverage

# Check current coverage
pytest --cov=module_to_refactor --cov-report=term-missing

# If coverage < 80%, add tests first
pytest tests/test_module.py -v

Step 3: Apply Refactoring

Execute refactoring in small steps:

Extract Method Example:
1. [ ] Identify code to extract
2. [ ] Create new method with extracted code
3. [ ] Replace original code with method call
4. [ ] Run tests
5. [ ] Commit

Step 4: Verify

# Run tests
pytest

# Check for regressions
git diff HEAD~1 --stat

# Review changes
git diff HEAD~1 path/to/file.py

Common Refactorings

Extract Method

# BEFORE
def process_order(order):
    # Validate order
    if not order.items:
        raise ValueError("Order has no items")
    if order.total <= 0:
        raise ValueError("Order total must be positive")

    # Calculate discount
    discount = 0
    if order.customer.is_premium:
        discount = order.total * 0.1

    # Process payment
    payment = Payment(amount=order.total - discount)
    payment.process()

# AFTER
def process_order(order):
    validate_order(order)
    discount = calculate_discount(order)
    process_payment(order, discount)

def validate_order(order):
    if not order.items:
        raise ValueError("Order has no items")
    if order.total <= 0:
        raise ValueError("Order total must be positive")

def calculate_discount(order):
    if order.customer.is_premium:
        return order.total * 0.1
    return 0

def process_payment(order, discount):
    payment = Payment(amount=order.total - discount)
    payment.process()

Extract Class

# BEFORE
class Order:
    def __init__(self):
        self.items = []
        self.shipping_address_line1 = ""
        self.shipping_address_line2 = ""
        self.shipping_city = ""
        self.shipping_state = ""
        self.shipping_zip = ""
        self.shipping_country = ""

    def format_shipping_address(self):
        return f"{self.shipping_address_line1}\n{self.shipping_city}, {self.shipping_state}"

# AFTER
class Address:
    def __init__(self, line1, line2, city, state, zip_code, country):
        self.line1 = line1
        self.line2 = line2
        self.city = city
        self.state = state
        self.zip_code = zip_code
        self.country = country

    def format(self):
        return f"{self.line1}\n{self.city}, {self.state}"

class Order:
    def __init__(self):
        self.items = []
        self.shipping_address = None

Rename

# Safe rename using IDE/tools
# In VS Code: F2 on symbol
# In PyCharm: Shift+F6

# Manual rename with verification
grep -rn "old_name" --include="*.py"
# Replace all occurrences
sed -i 's/old_name/new_name/g' path/to/files/*.py
# Run tests
pytest

Safety Checklist

Before refactoring:

• Tests exist and pass
• Coverage is adequate (>80%)
• Changes are scoped and understood
• Rollback plan exists

During refactoring:

• Make one change at a time
• Run tests after each change
• Commit working states

After refactoring:

• All tests pass
• Coverage maintained or improved
• No behavior changes (unless intentional)
• Code review completed

Quick Reference

Refactoring	When to Use	Risk
Rename	Unclear names	Low
Extract Method	Long methods	Low
Inline Method	Unnecessary indirection	Low
Extract Class	Large class	Medium
Move Method	Method in wrong class	Medium
Replace Conditional with Polymorphism	Complex conditionals	High
Change Method Signature	API changes	High