Extend Ruby Hash with Useful Methods

hash_extend.rb

class Hash
  def except(*blacklist)
    {}.tap do |h|
      (keys - blacklist).each { |k| h[k] = self[k] }
    end
  end

  def only(*whitelist)
    {}.tap do |h|
      (keys & whitelist).each { |k| h[k] = self[k] }
    end
  end
end