Pandas: Rolling mean using only the last update based on another column

Question

I would like to perform a rolling mean while the mean excludes duplicates found in another column. Let me provide an example dataframe:

Date            Warehose       Value
10-01-1998      London          10
13-01-1998      London          13
15-01-1998      New York        37
12-02-1998      London          21
20-02-1998      New York        39
21-02-1998      New York        17

In this example, let's say I like to perform 30-day rolling mean of Value but taking into account only the last update of the Warehouse location. The resulting dataframe is expected to be:

 Date         Value     Rolling_Mean
02-01-1998      10           10
05-01-1998      13           13
15-01-1998      37           20
12-02-1998      21           29           
20-02-1998      39           30 
21-02-1998      17           19

The data I have is relatively big so as efficient as possible is appreciated.

Answer 1

It's a bit tricky. As rolling.apply works on Series only and you need both "Wharehose" and "Value" to perform the computation, you need to access the complete dataframe using a function (and a "global" variable, which is not super clean IMO):

df['Date'] = pd.to_datetime(df['Date'], dayfirst=True)
df2 = df.set_index('Date')

def agg(s):
    return (df2.loc[s.index]
               .drop_duplicates(subset='Warehose', keep='last')
               ['Value'].mean()
           )

df['Rolling_Mean'] = (df.sort_values(by='Date')
                        .rolling('30d', on='Date')
                        ['Value']
                        .apply(agg, raw=False)
                      )

output:

        Date  Warehose  Value  Rolling_Mean
0 1998-01-10    London     10          10.0
1 1998-01-13    London     13          13.0
2 1998-01-15  New York     37          25.0
3 1998-02-12    London     21          29.0
4 1998-02-20  New York     39          30.0
5 1998-02-21  New York     17          19.0