- one of "r", "r+", "w", "w+", "a", "a+"
+ a string consisting of "r", "w", or "a", optionally followed
+ by "b" or "t", optionally followed by "+".
+ :"b":
+ binary mode: write() accepts bytes, read() returns bytes.
+ :"t":
+ text mode (default): write() accepts strings, read() returns strings.