snippet.rb

module RedirectPuts
  class Interceptor
    def initialize io
      @io = io
    end
    
    private
    
    def puts stuff
      @io.puts stuff
    end
  end
  
  def redirect_puts io, &block
    Interceptor.new(io).instance_eval &block
  end
end

include RedirectPuts

require 'stringio'

io = StringIO.new
i = 3

puts '1'
redirect_puts(io) do
  puts i
end
puts '2'

io.rewind
puts io.read