Class Methods & Constants

def self.method_name — a method on the CLASS, not an instance

Every method you've written so far runs on an instance: priya.price_label. Sometimes the logic doesn't belong to any one walker — it's about walkers in general: finding the cheapest one, building one from raw data, counting how many exist. Prefix def with self. and the method attaches to the class itself instead:

class Walker
  def self.cheapest(walkers)
    walkers.min_by(&:price_per_30_min_cents)
  end
end

Walker.cheapest([priya, sam])   # called on the CLASS, not an instance

walkers.min_by(&:price_per_30_min_cents) is the &:symbol block shorthand from module 02 — it's equivalent to walkers.min_by { |w| w.price_per_30_min_cents }. Compare this to Swift's static func or Python's @classmethod — different keywords, identical idea: a method that belongs to the type, not to one instance of it.