Flexibility without Dependency Injection

gistfile1.rb

class TaxCode
  GENERATORS = {
    :us => lambda { |id| "US-#{id}" },
    :br => lambda { |id| "#{id + 9}-BRA" }
  }

  def self.generate(code, id)
    gen = GENERATORS[code] || (raise ArgumentError "No generator for country #{code}")
    gen.call(id)
  end
end

class User

  attr_accessor :country_code
  attr_accessor :id

  def tax_code
    TaxCode.generate(country_code, id)
  end
end

u = User.new
u.id=1234
u.country_code = :br
u.tax_code