median = data.groupby('locality_name')['days_exposition'].median()    # список медиан времени размещения
                                                                     # по городам
median_list = median.apply(pd.Series)

def day_median(row):
    locality= row['locality_name']
    days_exposition = row['days_exposition']

    for i in range(0, len(list_of_cities_for_days)):
        if locality in list_of_cities_for_days.index[i]:
            return list_of_cities_for_days[i][0]
        return days_exposition

#data['days_exposition_test'] = data.apply(day_median, axis = 1)
data.loc[data['days_exposition'].isnull(),'days_exposition'] = data.apply(day_median, axis = 1)