Python pandas dataframe - daily data - get first and last day for every year

Question

I have a Python pandas dataframe with daily data that looks like:

            Date       Open       High        Low      Close  Adj Close   Volume
0     2009-12-31  31.709999  31.840000  31.299999  31.309999  23.547892  1957700
1     2010-01-04  31.480000  31.840000  31.330000  31.469999  23.668222  3472500
2     2010-01-05  31.549999  31.770000  31.400000  31.639999  23.796082  3458700
3     2010-01-06  31.600000  31.889999  31.430000  31.559999  23.735907  3745800
4     2010-01-07  31.549999  31.700001  31.049999  31.230000  23.487726  7232100

How can I save the first and last entry for every year? If the last entry for a year is let's say October 31st, I would like to keep this one as the last entry for the year.

Answer 1

1. Sort by Date if the rows are not already in sorted order i.e., something like:

   df = df.sort_values(by='Date')

2. Group by year and only retain the first and last elements of each group:

   df.groupby(pd.DatetimeIndex(df.Date).to_period('Y')).nth([0,-1])

Output using example data frame from question:

            Date       Open       High        Low      Close  Adj Close  Volume
Date                                                                            
2009  2009-12-31  31.709999  31.840000  31.299999  31.309999  23.547892  1957700
2010  2010-01-04  31.480000  31.840000  31.330000  31.469999  23.668222  3472500
2010  2010-01-07  31.549999  31.700001  31.049999  31.230000  23.487726  7232100

Note: If there is only one entry per year like there is in the example, (2009), that row will only be in the output once, not twice, but if used on real data that would not be an issue anyway.