我有一個(gè)代表顧客登記（訪問(wèn)）餐廳的數(shù)據(jù)框。year簡(jiǎn)直就是在餐廳辦理入住的那一年。average_checkin我想要做的是在我的初始數(shù)據(jù)框中添加一列df，表示每年餐廳的平均訪問(wèn)次數(shù)。data = {        'restaurant_id':  ['--1UhMGODdWsrMastO9DZw', '--1UhMGODdWsrMastO9DZw','--1UhMGODdWsrMastO9DZw','--1UhMGODdWsrMastO9DZw','--1UhMGODdWsrMastO9DZw','--1UhMGODdWsrMastO9DZw','--6MefnULPED_I942VcFNA','--6MefnULPED_I942VcFNA','--6MefnULPED_I942VcFNA','--6MefnULPED_I942VcFNA'],        'year': ['2016','2016','2016','2016','2017','2017','2011','2011','2012','2012'],        }df = pd.DataFrame (data, columns = ['restaurant_id','year'])# here i count the total number of checkins a restaurant hadd = df.groupby('restaurant_id')['year'].count().to_dict()df['nb_checkin'] = df['restaurant_id'].map(d)mean_checkin= df.groupby(['restaurant_id','year']).agg({'nb_checkin':[np.mean]})mean_checkin.columns = ['mean_checkin']mean_checkin.reset_index()# the values in mean_checkin makes no sens#I need to merge it with df to add that new column我對(duì) pandas 庫(kù)還是個(gè)新手，我嘗試過(guò)類似的方法，但我的結(jié)果毫無(wú)意義。我的語(yǔ)法有問(wèn)題嗎？如果需要任何澄清，請(qǐng)?jiān)儐?wèn)。