I have tried the following code and it works however it shows excess columns that I don't require. This is the output showing the extra columns: 
import pandas as pd
df = pd.read_csv("data.csv")
df = df.groupby(['City1', 'City2']).sum('PassengerTrips')
df['Vacancy'] = 1-df['PassengerTrips'] / df['Seats']
df = df.groupby(['City1','City2']).max('Vacancy')
df = df.sort_values('Vacancy', ascending =False)
print('The 10 routes with the highest proportion of vacant seats:')
print(df[:11])
I have tried to add the following code in after sorting the vacancy values however it gives me an error:
df = df[['City1', 'City2', 'Vacancy']]