python - How to group dataframe by hour using timestamp with Pandas

Question

I have the following dataframe structure that is indexed with a timestamp:

    neg neu norm    pol pos date
time                        
1520353341  0.000   1.000   0.0000  0.000000    0.000   
1520353342  0.121   0.879   -0.2960 0.347851    0.000   
1520353342  0.217   0.783   -0.6124 0.465833    0.000   

I create a date from the timestamp:

data_frame['date'] = [datetime.datetime.fromtimestamp(d) for d in data_frame.time]

Result:

    neg neu norm    pol pos date
time                        
1520353341  0.000   1.000   0.0000  0.000000    0.000   2018-03-06 10:22:21
1520353342  0.121   0.879   -0.2960 0.347851    0.000   2018-03-06 10:22:22
1520353342  0.217   0.783   -0.6124 0.465833    0.000   2018-03-06 10:22:22

I want to group by hour, while getting the mean for all the values, except the timestamp, that should be the hour from where the group started. So this is the result I want to archive:

    neg neu norm    pol pos
time                    
1520352000  0.027989    0.893233    0.122535    0.221079    0.078779
1520355600  0.028861    0.899321    0.103698    0.209353    0.071811

The closest I have gotten so far has been with this answer:

data = data.groupby(data.date.dt.hour).mean()

Results:

    neg neu norm    pol pos
date                    
0   0.027989    0.893233    0.122535    0.221079    0.078779
1   0.028861    0.899321    0.103698    0.209353    0.071811

But I cant figure out how to keep the timestamp that takes in account he hour where the grouby started.

Answer 1

I came across this gem, pd.DataFrame.resample, after I posted my round-to-hour solution.

# Construct example dataframe
times = pd.date_range('1/1/2018', periods=5, freq='25min')
values = [4,8,3,4,1]
df = pd.DataFrame({'val':values}, index=times)

# Resample by hour and calculate medians
df.resample('H').median()

Or you can use groupby with Grouper if you don't want times as index:

df = pd.DataFrame({'val':values, 'times':times})
df.groupby(pd.Grouper(level='times', freq='H')).median()