with open('file.txt', 'r') as old_file:
  with open('file.txt', 'r+') as new_file:

    current_line = 0

    # 定位到需要刪除的行
    while current_line < (3 - 1): #(del_line - 1)
      old_file.readline()
      current_line += 1

    # 當前光標在被刪除行的行首，記錄該位置
    seek_point = old_file.tell()

    # 設置光標位置
    new_file.seek(seek_point, 0)

    # 讀需要刪除的行，光標移到下一行行首
    old_file.readline()

    # 被刪除行的下一行讀給 next_line
    next_line = old_file.readline()

    # 連續覆蓋剩余行，后面所有行上移一行
    while next_line:
      new_file.write(next_line)
      next_line = old_file.readline()

    # 寫完最后一行后截斷文件，因為刪除操作，文件整體少了一行，原文件最后一行需要去掉
    new_file.truncate()