Refactoring Ruby: Extract Variable

Let’s take a look at a simple way to add clarity to your Ruby methods: Extract Variable or Introduce Explaining Variable. It's probably one of the least invasive refactoring methods. However, don’t let the simplicity fool you; any clarity or explicitness gained is well worth the change.

When you have complex expressions grouped together in a method, it's best to describe them as clearly and verbosely as necessary using the Extract Variable technique. Let's apply this approach to the Movie#total_price method in the code example below:

class Movie
  TAX_PERCENTAGE = 0.5

  def initialize(price:, discount_percentage: 0)
    @price = price
    @discount_percentage = discount_percentage
  end

  def total_price
    # BAD
    price + (price * TAX_PERCENTAGE) - (price * (discount_percentage / 100.0))
  end

  private

  attr_reader :price, :discount_percentage
end

class Movie
  TAX_PERCENTAGE = 0.5

  def initialize(price:, discount_percentage: 0)
    @price = price
    @discount_percentage = discount_percentage
  end

  def total_price
    # GOOD
    tax = price * TAX_PERCENTAGE
    discount = price * (discount_percentage / 100.0)

    price + tax - discount
  end

  private

  attr_reader :price, :discount_percentage
end

The bad version looks like:

def total_price
  # BAD
  price + (price * TAX_PERCENTAGE) - (price * (discount_percentage / 100.0))
end

While you can deduce that this method returns the total price of a movie, it takes too much effort to understand how it does so.

To improve this method, we'll apply the Extract Variable pattern:

def total_price
  # GOOD
  tax = price * TAX_PERCENTAGE
  discount = price * (discount_percentage / 100.0)

  price + tax - discount
end

In the improved "good" version, we introduce two explaining variables: tax and discount.

Now, at a glance we know the total price of a movie consists of price + tax — discount, and we now have clear, concise descriptions of those complex expressions.

By applying the Extract Variable refactoring pattern, we've made the code more readable and understandable, enhancing its overall clarity.

Comments

[Jordan Brock]

And to take it all the way ...

def total_tax
  @price * TAX_PERCENTAGE
end

def discount
  @price * (@discount_percentage / 100.0)
end

def total_price
  @price + total_tax - discount
end

[Jon Lunsford]

Great point! Yep, a fantastic next step is to use the Extract Method ... method :)

It's interesting how these tiny adjustments can dramatically improve clarity in your code.

[Cherry Ramatis]

Simple and direct to the goal! excelent article

[Jon Lunsford]

Thanks very much!