pandas dataframe groupby：只有正数的总和/数

Question

I have a dataframe ('frame') on which I want to aggregate by Country and Date:

aggregated=pd.DataFrame(frame.groupby(['Country','Date']).CaseID.count())

aggregated["Total duration"]=frame.groupby(['Country','Date']).Hours.sum()

aggregated["Mean duration"]=frame.groupby(['Country','Date']).Hours.mean()

I want to compute the above figures (total duration, mean duration, etc.) only for the positive 'Hours' numbers in 'frame'. How can I do that?

Thanks!

Sample "frame"

import pandas as pd
Line1 = {"Country": "USA", "Date":"01 jan", "Hours":4}
Line2 = {"Country": "USA", "Date":"01 jan", "Hours":3}
Line3 = {"Country": "USA", "Date":"01 jan", "Hours":-999}
Line4 = {"Country": "Japan", "Date":"01 jan", "Hours":3}
pd.DataFrame([Line1,Line2,Line3,Line4])

Answer 1

怎么样 -

frame[frame["Hours"] > 0].groupby(['Country','Date'])

Answer 2

Not as elegant as above, but deals differently some corner cases. df stands for frame from original question.

>>> df.groupby(['Country','Date']).agg(lambda x: x[x>0].mean())
                Hours
Country Date
Japan   01 jan    3.0
USA     01 jan    3.5
>>> df.ix[3, 'Hours'] = -1
>>> df.groupby(['Country','Date']).agg(lambda x: x[x>0].mean())
                Hours
Country Date
Japan   01 jan    NaN
USA     01 jan    3.5