1. 显示昨天（前一天）的日期

from datetime import date, timedelta

yesterday_date = (date.today() + timedelta(days = -1)).strftime("%Y-%m-%d") #输出：2019-11-21
yesterday_date = (date.today() + timedelta(days = -1)).strftime("%Y%m%d") #输出：20191121

2. 显示第前 'n' 天，将 timedelta的参数 'days' 设置为 '-n'

#例如，n=10
n = 10
before_n_days = (date.today() + timedelta(days= -n)).strftime("%Y-%m-%d")