Consider this pandas data frame:
import pandas as pd
df = pd.DataFrame(columns=['city','pop'])
df['city'] = ['faa','foo','faa','faa','foo']
df['pop'] = [10,20,15,30,60]
  city  pop
0  faa   10
1  foo   20
2  faa   15
3  faa   30
4  foo   60
How to sum the pop of each city? I'd like to obtain
   city  pop
0   faa   55
1   foo   80
