如果用户没有使用Python datetime设置，如何将微秒设置为零？

import datetime

now = datetime.datetime.now()
now_str = now.strftime("%Y-%m-%d %H:%M:%S")
now_str = now_str[:-3] + "000"
new_time = datetime.datetime.strptime(now_str, "%Y-%m-%d %H:%M:%S.%f")

import datetime

now = datetime.datetime.now()
new_time = now.replace(microsecond=0)