python open mode

• 必须其一: r, w, a: read(默认), write, append

• 可选: b, t, +, U: binary, text(默认), reading and writing, universal newlines mode

• rt == r, wt == w, at == a

• r+

  >>> with open('aa', 'r+') as f:
  ...     print(f.tell())
  ...
  0
  >>> with open('aa', 'r+') as f:
  ...     f.readline()
  ...     print(f.tell())
  ...
  '0
  '
  2
  

  • read执行在前, write执行在后: 添加到最后一行
  • write执行在前: 写入并替换第一行内容

• w+ == w

• a+ == a