Pandas - cumsum by month?

Question

I have a dataframe that looks like this:

Date          n
2014-02-27    4
2014-02-28    5
2014-03-01    1
2014-03-02    6
2014-03-03    7

I\

Answer 1

If you're doing timeseries work, I recommend using a DatetimeIndex. In this example, you can use a TimeGrouper to group by month (which groups by year-month, like in a resample):

In [11]: g = df.groupby(pd.TimeGrouper('M'))

In [12]: g['n'].cumsum()
Out[12]: 
Date
2014-02-27     4
2014-02-28     9
2014-03-01     1
2014-03-02     7
2014-03-03    14
dtype: int64

In [13]: df['csn'] = g['n'].cumsum()

Note: If you're not already using a DatetimeIndex, pass over the to_datetime function and set the index:

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