Aggregation over Partition - pandas Dataframe

I am looking for the best way to aggregate values based on a particular partition , an equivalent of

SUM(TotalCost) OVER(PARTITION BY ShopName) Earnings  ( SQL server)

I am able to do this by the following steps in Pandas , but looking for a native approach which I am sure should exist

TempDF= DF.groupby(by=['ShopName'])['TotalCost'].sum()

TempDF= TempDF.reset_index() 

NewDF=pd.merge(DF , TempDF, how='inner', on='ShopName')

Thanks a lot for reading through !