官方的文档解释的很清楚：
getpass.
getuser
(
)
Return the “login name” of the user.
This function checks the environment variables LOGNAME, USER, LNAME and USERNAME, in order, and returns the value of the first one which is set to a non-empty string. If none are set, the login name from the password database is returned on systems which support the pwd module, otherwise, an exception is raised.
返回你当前系统的用户名，比如你当前的用户名是administrator，那么返回字符串adminstrator。
