Refactoring Patterns

Safe techniques for restructuring code without changing its behavior. Every refactoring follows: test green -> refactor -> test green.

Golden Rule

Never refactor and change behavior in the same commit. Refactoring = same behavior, different structure. Feature work = different behavior. Mixing them makes bugs untraceable.

Pre-Refactoring Checklist

Before any refactoring:

• Tests exist and pass for the code being refactored
• You can describe what the code does WITHOUT reading it line by line
• The refactoring has a clear motivation (not "it could be cleaner")
• The scope is bounded — you know exactly which files/functions change

Core Refactorings

1. Extract Function

When: A code block does one identifiable thing, or you need to add a comment explaining what a block does.

# BEFORE
def process_order(order):
    # Validate order
    if not order.items:
        raise ValueError("Empty order")
    if order.total < 0:
        raise ValueError("Negative total")
    if not order.customer_id:
        raise ValueError("Missing customer")

    # Apply discounts
    total = order.total
    if order.is_member:
        total *= 0.9
    if len(order.items) > 10:
        total *= 0.95

    return total

# AFTER
def process_order(order):
    validate_order(order)
    return apply_discounts(order)

def validate_order(order):
    if not order.items:
        raise ValueError("Empty order")
    if order.total < 0:
        raise ValueError("Negative total")
    if not order.customer_id:
        raise ValueError("Missing customer")

def apply_discounts(order):
    total = order.total
    if order.is_member:
        total *= 0.9
    if len(order.items) > 10:
        total *= 0.95
    return total

Verification: Run tests. Output must be identical.

2. Inline Function

When: A function's body is as clear as its name, or it's only called once and adds indirection without value.

# BEFORE
def is_valid_age(age):
    return age >= 0

def process(age):
    if is_valid_age(age):
        ...

# AFTER (if is_valid_age is only used once and obvious)
def process(age):
    if age >= 0:
        ...

When NOT to inline: If the function is called from multiple places, or if the name adds clarity that the body doesn't.

3. Rename

When: A name doesn't describe what the thing does, or uses abbreviations/jargon.

# BEFORE
def proc(d, f=True):
    ...
x = get_data()
tmp = transform(x)

# AFTER
def process_invoice(invoice_data, *, validate=True):
    ...
raw_invoices = fetch_invoices()
normalized_invoices = normalize(raw_invoices)

Rules:

• Search for ALL usages before renaming (Grep across entire codebase)
• Update imports, tests, documentation, and config files
• If it's a public API, this is a BREAKING CHANGE — requires version bump

4. Move

When: A function/class is in the wrong module — it's more closely related to another module's concerns.

Process:

1. Grep for all imports of the function/class
2. Move to new location
3. Update all import statements
4. Add re-export from old location if it's a public API (temporary, with deprecation warning)
5. Run tests

5. Simplify Conditionals

When: Nested if/else chains, complex boolean expressions, or repeated condition checks.

# BEFORE: Nested guards
def get_price(product, user):
    if product is not None:
        if product.is_available:
            if user is not None:
                if user.is_member:
                    return product.price * 0.9
                else:
                    return product.price
            else:
                return product.price
        else:
            return None
    else:
        return None

# AFTER: Early returns (guard clauses)
def get_price(product, user):
    if product is None or not product.is_available:
        return None

    if user is not None and user.is_member:
        return product.price * 0.9

    return product.price

6. Decompose Large Functions

When: A function is longer than ~30 lines or has multiple levels of abstraction.

Process:

1. Identify logical sections (often marked by comments or blank lines)
2. Extract each section into a named function
3. The parent function should read like a table of contents
4. Each extracted function should work at one level of abstraction

7. Replace Magic Values

# BEFORE
if response.status_code == 429:
    time.sleep(60)

# AFTER
RATE_LIMIT_STATUS = 429
DEFAULT_RETRY_DELAY_SECONDS = 60

if response.status_code == RATE_LIMIT_STATUS:
    time.sleep(DEFAULT_RETRY_DELAY_SECONDS)

Code Smells That Signal Refactoring

Smell	Refactoring
Long function (>30 lines)	Extract Function, Decompose
Deeply nested conditionals	Guard Clauses, Extract Function
Duplicated code blocks	Extract Function, parameterize
Feature envy (method uses another class's data more than its own)	Move Method
Long parameter list (>4 params)	Introduce Parameter Object
Comments explaining "what" (not "why")	Rename, Extract Function
Boolean parameters	Split into two functions
Dead code	Delete it

Refactoring Safety

Test-First Verification

# 1. Confirm tests pass BEFORE refactoring
python -m pytest tests/ -x --tb=short

# 2. Make the refactoring change

# 3. Confirm tests STILL pass
python -m pytest tests/ -x --tb=short

# 4. Verify no behavioral change
git diff  # Review: structure changes only, no logic changes

When to Stop

• FORBIDDEN: "While I'm here, I'll also..." — scope creep
• FORBIDDEN: Refactoring code you don't have tests for
• FORBIDDEN: Refactoring and adding features in the same commit
• FORBIDDEN: Premature abstraction — three similar blocks is NOT a pattern until you see the fourth

Scale Guidelines

Scope	Approach
Single function	Refactor inline, same commit
Single file	Refactor in dedicated commit
Multiple files	Plan first, dedicated branch
Module/package boundary	ADR required, phased approach