SFrame如何根据列A的限制条件来求列B的性质

train_data: SFrame

 

train_data[train_data['zipcode']=='98039']

 

train_data[train_data['zipcode']=='98039']['price']

 

求zipcode为98039时，平均price

train_data[train_data['zipcode']=='98039']['price'].mean()