Engineering Craft › Refactoring

Extract Function

Pulling a block of code into a well-named function.

Also known as: extract method, extract function refactoring

Extract function is a refactoring where you take a block of code, move it into a new function, and replace the original with a call. It’s the most used refactoring there is, and the standard cure for long methods.

Before:

def print_invoice(order):
    total = 0
    for item in order.items:
        total += item.price * item.qty
    if order.customer.is_member:
        total *= 0.9
    print(f"{order.customer.name}: {total}")

After:

def calculate_total(order):
    total = sum(i.price * i.qty for i in order.items)
    return total * 0.9 if order.customer.is_member else total

def print_invoice(order):
    print(f"{order.customer.name}: {calculate_total(order)}")

When to do it

• A block needs a comment to explain it: turn the comment into a function name.
• A function does several things.
• The same code appears in two places (DRY).
• Deeply nested code could use a named inner step (deep nesting).

How to do it safely

1. Make sure you have tests (or write a quick one) (refactoring).
2. Select the block and use your editor’s extract function command (IDE refactoring tools). It figures out parameters and return values.
3. Name it for what it does, not how (naming): calculate_total, not loop_items.
4. Run the tests.

By hand, check what the block reads (it becomes parameters) and what it changes (it becomes return values).

Pitfalls

• Too many parameters. If the new function needs six arguments, the extraction cut across a boundary. Consider extracting a class or grouping values (introduce parameter object).
• Meaningless names like part2.
• Tiny functions that each need the others’ details can scatter the logic.
• Behavior changes. Extraction should not change what the code does.