old_contents: class Buffer:
  
      def __init__(self, file_name):
          self.file_name = file_name
  
new_contents: class Buffer:
  
      def __init__(self, file_name):
          self.file_name = file_name
          with open(file_name, "r") as f:
              self.lines = []
              for line in f:
                  self.lines.append(line)
  
subject: read file content into buffer