random.sample('样本字符',要生成的位数)

该函数的结果类型是list，所以需用"".join将之拼接成字符串

"""生成用户名"""
from selenium import webdriver
import random
from time import sleep

#打开百度注册页面
driver = webdriver.Chrome()
driver.get("https://passport.baidu.com/v2/?reg&tpl=crt&u=http%3A%2F%2Ftest.baidu.com%2F")
sleep(2)

#选中用户名栏
the_elemrnt = driver.find_element_by_id("TANGRAM__PSP_4__userName")
sleep(1)
print(the_elemrnt.get_attribute("placeholder"))

#生成一个6到9位数的用户名
#随机生成用户名
n = random.randint(6, 10)
user = ''.join(random.sample('abcdefghijklmnopqrstuvwxyz123456789', n))
the_elemrnt.send_keys(user)
sleep(1)

#打印用户名
print(the_elemrnt.get_attribute("value"))