Can you do a df.dtypes ? In the example below type is Int as it works fine.
import pandas as pd
##group by 1 columns
df = pd.DataFrame({' data': [4610, 4611, 4612, 4613], 'Category': [2, 2, 7, 7]})
print df.groupby('Category'). mean()
##Mutiple columns to group by
df1 = pd.DataFrame({' data': [4610, 4611, 4612, 4613], 'Category': [2, 2, 7, 7], 'Category2' : ['A','B','A','B']})
key=['Category','Category2']
print df1.groupby( key).mean()
Category Category2
2 A 4610
B 4611
7 A 4612
B 4613