如何在熊猫中获得上个月的平均值

import pandas as pd
import datetime

# 假设数据存储在名为data的DataFrame中，包含日期和数值两列
data['日期'] = pd.to_datetime(data['日期'])  # 将日期列转换为日期类型
data.set_index('日期', inplace=True)  # 将日期列设置为索引

today = datetime.date.today()
first_day_of_month = today.replace(day=1)
last_day_of_previous_month = first_day_of_month - datetime.timedelta(days=1)
first_day_of_previous_month = last_day_of_previous_month.replace(day=1)

previous_month_data = data.loc[first_day_of_previous_month:last_day_of_previous_month]

average_value = previous_month_data['数值'].mean()