python 判断 linux user/group是否存在，并获取uid/gid

import pwd
import grp
user='nginx'
group='nginx'
usernames = [x[0] for x in pwd.getpwall()]
if user in usernames:
    print(pwd.getpwnam(user).pw_uid)
groupnames=[x[0] for x in grp.getgrall()]
if group in groupnames:
    print(grp.getgrnam(group).gr_gid)