Character Meaning
- 'r' open for reading (default)
- 'w' open for writing, truncating the file first
- 'x' open for exclusive creation, failing if the file already exists
- 'a' open for writing, appending to the end of the file if it exists
- 'b' binary mode
- 't' text mode (default)
- '+' open a disk file for updating (reading and writing)
- 'U' universal newlines mode (deprecated)
Truncate---删除。
不删除时的更新写入，还可以指定写入时的位置---开头还是结尾。
可以认为：sep默认模式是空格， end的默认模式是
