如何在通过python连接到LDAP时指定baseDN?

import ldap

baseDN="ou=unit,o=org.c=xx" # doesn't work
#baseDN="" # works
host="ldaps://test.org.xx:636"
userDN="cn=proxyhlrb,ou=services,o=org,c=xx"
passwd="secret"

server=ldap.initialize(host+"/"+baseDN)

server.bind_s(userDN,passwd,ldap.AUTH_SIMPLE)