Pandas better way of counting groups within a group?

Question

I saw a couple of related questions (1, 2, 3), but they don't produce the output that I'm looking for. I have a dataframe like this:

df = pd.DataFrame({'Country': ['USA','USA','Canada','Canada'],
                   'City': ['Los Angeles','NYC','Toronto','Toronto'],
                   'Value': [100,200,300,400]})
df

  country city        value
0 USA     Los Angeles 100
1 USA     NYC         200
2 Canada  Toronto     300
3 Canada  Toronto     400

And I want to count the number of unique cities for each country. I'm currently doing this:

df.groupby('country').city.unique().apply(len)
Country
Canada  1
USA     2

Is this a reasonable way, or is there something better? Thank you.