How to group pandas DataFrame entries by date in a non-unique column

Question

A Pandas DataFrame contains column named "date" that contains non-unique datetime values. I can group the lines in this frame using:

data.groupby(data['date']) 

However, this splits the data by the datetime values. I would like to group these data by the year stored in the "date" column. This page shows how to group by year in cases where the time stamp is used as an index, which is not true in my case.

How do I achieve this grouping?

Answer 1

I'm using pandas 0.16.2. This has better performance on my large dataset:

data.groupby(data.date.dt.year) 

Using the dt option and playing around with weekofyear, dayofweek etc. becomes far easier.

Answer 2

ecatmur's solution will work fine. This will be better performance on large datasets, though:

data.groupby(data['date'].map(lambda x: x.year))