private attr_writer

private_attr_writer.rb

class Klass
  def initialize(number)
    self.number = number
  end
  
  attr_reader :number
  
  private
  
  attr_writer :number
end

Klass.instance_methods false # => [:number]
Klass.new(123).number # => 123

begin
  klass = Klass.new 123
  klass.number = 456
rescue
  $! # => #<NoMethodError: private method `number=' called for #<Klass:0x007fe0fb887c80 @number=123>>
end