pandas read dataframe and sum all value same month and year

[Diego Avesani]

Dear all,

I am reading the following data-frame:

datatime,T,RH,PSFC,DIR,VEL10,PREC,RAD,CC,FOG
2012-01-01 06:00, -0.1,100, 815,313,  2.6,  0.0,   0,  0,0
2012-01-01 07:00, -1.2, 93, 814,314,  4.8,  0.0,   0,  0,0
2012-01-01 08:00,  1.7, 68, 815,308,  7.5,  0.0,  41, 11,0
2012-01-01 09:00,  2.4, 65, 815,308,  7.4,  0.0, 150, 33,0
2012-01-01 10:00,  3.0, 64, 816,305,  8.4,  0.0, 170, 44,0
2012-01-01 11:00,  2.6, 65, 816,303,  6.3,  0.0, 321, 22,0
2012-01-01 12:00,  2.0, 72, 816,278,  1.3,  0.0, 227, 22,0
2012-01-01 13:00, -0.0, 72, 816,124,  0.1,  0.0, 169, 22,0
2012-01-01 14:00, -0.1, 68, 816,331,  1.4,  0.0, 139, 33,0
2012-01-01 15:00, -4.0, 85, 816,170,  0.6,  0.0,  49,  0,0
....
....

I read the data frame as:

 df = pd.read_csv('dati.csv',delimiter=',',header=0,parse_dates=True,na_values=-999)
   df['datatime'] = df['datatime'].map(lambda x: datetime.strptime(str(x), "%Y-%m-%d %H:%M"))
   #
   mask = (df['datatime'] > str(start_date[ii])) & (df['datatime'] <=  str(end_date[ii]))
   df = df.loc[mask]
   df = df.reset_index(drop=True)

I would to create an array with the sum of all the PREC value in the same month.

I have tried with:

df.groupby(pd.TimeGrouper('M')).sum()

But as always, it seems that I have same problems with the indexes. Indeed, I get:
      'an instance of %r' % type(ax).__name__)
TypeError: axis must be a DatetimeIndex, but got an instance of 'Int64Index'

thanks for any kind of help,
Really Really thanks

Diego