Refactoring

Behavior-preserving code transformation protected by tests.

Core Principle

Tests must pass before AND after every change.

When to Use

• Reducing duplication (DRY)
• Extracting methods/classes
• Improving naming and readability
• Simplifying complex conditionals
• Preparing code for new features

MCP Workflow

# 1. Understand current structure
serena.get_symbols_overview(relative_path="target/file")

# 2. Check all usages before change
serena.find_referencing_symbols(name_path="TargetClass/method")

# 3. Verify test coverage
serena.search_for_pattern("test.*TargetClass", relative_path="test/")

# 4. Get target code
serena.find_symbol(name_path="TargetClass/method", include_body=True)

# 5. Safe rename
serena.rename_symbol(name_path="oldName", relative_path="file", new_name="newName")

Workflow

1. Prepare

- Ensure tests exist and pass
- Identify refactoring scope
- Plan incremental steps

2. Refactor (small steps)

For each change:
  1. Make one refactoring move
  2. Run tests immediately
  3. If PASS: commit
  4. If FAIL: revert, try smaller step

3. Verify

- Run full test suite
- Review final structure
- Update docs if needed

Common Patterns

Extract Method

When: Function too long or has multiple responsibilities

BEFORE:
processOrder(order):
    # validate (10 lines)
    # calculate shipping (10 lines)
    # save (5 lines)

AFTER:
processOrder(order):
    validateOrder(order)
    shipping = calculateShipping(order)
    repository.save(order.copy(shipping=shipping))

Extract Class

When: Class has too many responsibilities

OrderService with 6 methods
    -> OrderService (orchestration)
    -> OrderCalculator (calculations)
    -> OrderNotifier (notifications)

Replace Conditional with Polymorphism

When: Repeated switch/if-else on type

BEFORE:
switch customer.type:
    case "REGULAR": return total * 0.05
    case "PREMIUM": return total * 0.10

AFTER:
interface CustomerType { calculateDiscount(total) }
class RegularCustomer implements CustomerType { ... }
class PremiumCustomer implements CustomerType { ... }

Simplify Conditionals

BEFORE:
if a:
    if b:
        if c:
            return true
return false

AFTER:
return a and b and c

Output Format

## Refactoring: [Target]

| Item | Value |
|------|-------|
| Target | ClassName.method |
| Type | Extract / Rename / Simplify |
| Usages | N references |
| Tests | X tests covering |

### Steps
1. [Description] - Tests: PASS
2. [Description] - Tests: PASS

### Result
Files: N | Changes: +X/-Y | Tests: PASS

Safety Checklist

• Tests exist and pass before starting
• Each step is independently committable
• No behavior changes
• All references updated
• Tests still pass after completion

When NOT to Refactor

• No test coverage (add tests first)
• Under time pressure (do minimal fix)
• Code is about to be deleted
• Not sure about desired outcome