Creating variable inside a class

All I want to do, is to create a new variable called data_dfp inside the
class, and be able to set/get it's value from outside

require 'nokogiri'
class MyDocument < Nokogiri::XML::SAX::Document
  attr_accessor :data_dfp

  def start_element(name, attrs = [])
    puts "#{name}"
  end
end

parser = Nokogiri::XML::SAX::Parser.new(MyDocument.new)
parser.data_dfp = 4
puts parser.data_dfp

Error
nokoteste.rb:12:in `<main>': undefined method `data_dfp=' for
#<Nokogiri::XML::SAX::Parser:0x007fcc1413bea0> (NoMethodError)


Whats the right way of doing this?

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