attr_accessor parsed as a method variable when setting with a get

base.rb

class Base
  attr_accessor :configuration

  def initialize(configuration)
    @configuration = configuration
  end
  
  def do_a_thing()
    # specifying the parenthesis below resolves your problem
    configuration = configuration() + 1
  end
end

puts Base.new(2).do_a_thing #=> 3