Вот вам go Стю, надеюсь, это поможет.
import pandas as pd
dataLasMonth = {'Brand': ['Honda Civic','Toyota Corolla','Ford Focus','Audi A4'],
'Rank': [1,2,3,4]
}
dfLasMonth = pd.DataFrame(dataLasMonth, columns = ['Brand', 'Rank'])
#print (dfLasMonth)
dataThisMonth = {'Brand': ['Honda Civic','Toyota Corolla','Ford Focus','Audi A4'],
'Rank': [3,1,5,10]
}
dfThisMonth = pd.DataFrame(dataThisMonth, columns = ['Brand', 'Rank'])
#print (dfThisMonth)
dfMerged = dfLasMonth.merge(dfThisMonth,on='Brand' )
dfMerged['diff'] = dfMerged['Rank_x'] - dfMerged['Rank_y']
dfMerged