code: def make_file_exist(self, filename=None):
          
          if filename is None:
              path_to_file = FilePath(self)
              path_to_file.make_file_exist()
              return path_to_file
          else:
              self.make_directory_exist()
              path_to_file = self.touch_file(filename)
              return FilePath(path_to_file)
docs: Make the directory exist, then touch the file
  
          If the filename is None, then use self.name as filename