openFile <filename_string> [ mode:<mode_string> ] 语法
语法默认是，rt - read-only text - file must exist 只读文件，文件必须存在。这个是关键点。
r - read-only text - file must exist 只读文件 文件必须存在。
rb - read-only binary - file must exist 只读二进制文件，文件必须存在。
r+ - read/write text - file must exist 读写文件 文件必须存在
a - write-only text, append writes - file must exist --只写文件 文件必须存在。
at - write-only text, append writes - file must exist 只写文件 文件必须存在
ab - write-only binary, append writes - file must exist 只写二进制文件 文件必须存在
a+ - read/write text, append writes - file must exist 读写文件 附上写 文件必须存在
w - write-only text - deletes file contents if it exists 只写文件删除文件内容 如果他存在
wt - write-only text - deletes file contents if it exists
wb - write-only binary - deletes file contents if it exists
w+ - read/write text - deletes file contents if it exists