Engineering Craft › Refactoring

Inline Function / Variable

Replacing an unnecessary indirection with its body.

Also known as: inline function, inline variable, inline method

Inlining is the opposite of extracting: you replace a function call or a variable with its body or value, because the indirection isn’t helping.

Inline a variable

# before
base = order.price * order.quantity
return base

# after
return order.price * order.quantity

A temporary variable that’s used once and adds no explanation can go.

Inline a function

# before
def is_adult(user):
    return user.age >= 18

def can_vote(user):
    return is_adult(user)         # a pointless wrapper

# after
def can_vote(user):
    return user.age >= 18

When to inline

• The body is as clear as the name, or clearer.
• A function is a thin wrapper that only calls another.
• You’ve gone too far with extraction, and tracing the code means jumping through many tiny functions.
• It’s a first step before restructuring: inline everything into one place, then re-extract better pieces.
• The variable name adds nothing (result = x; return result).

When not to

• The name explains something the code doesn’t. is_eligible_for_discount(user) can hide a complicated condition.
• The function is used in many places, or part of a public interface.
• Overridden methods in subclasses, or functions passed as callbacks, can’t simply be inlined.

Safety

• Inline with your editor’s tool, which updates every call (automated refactoring).
• Beware of side effects and evaluation order: inlining an expression with side effects into several places changes how often it runs.
• Run the tests before and after (refactoring).

Extract and inline are two tools for the same goal: code that’s easy to read (readability).