refactoring-patternslisted

# 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. ```python # 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: