code: def copy_to_file(self, name, fp_dest, callback=None):
          
          assert compat.is_native(name)
  
          def _write_to_file(data):
              # print("_write_to_file() {} bytes.".format(len(data)))
              fp_dest.write(data)
              if callback:
                  callback(data)
  
          self.ftp.retrbinary(
              "RETR {}".format(name), _write_to_file, FtpTarget.DEFAULT_BLOCKSIZE
          )
docs: Write cur_dir/name to file-like `fp_dest`.
  
          Args:
              name (str): file name, located in self.curdir
              fp_dest (file-like): must support write() method
              callback (function, optional):
                  Called like `func(buf)` for every written chunk