Dataset.groupby summary methods

[jhamman]

This may just be a documentation issue but the summary apply and combine methods for the Dataset.GroupBy object seem to be missing.

In [146]:

foo_values = np.random.RandomState(0).rand(3, 4)
times = pd.date_range('2000-01-01', periods=3)
ds = xray.Dataset({'time': ('time', times),
                   'foo': (['time', 'space'], foo_values)})

ds.groupby('time').mean()  #replace time with time.month after #121 is adressed
# ds.groupby('time').apply(np.mean)  # also Errors here

 ---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-146-eec1e73cff23> in <module>()
      3 ds = xray.Dataset({'time': ('time', times),
      4                    'foo': (['time', 'space'], foo_values)})
----> 5 ds.groupby('time').mean()
      6 ds.groupby('time').apply(np.mean)

AttributeError: 'DatasetGroupBy' object has no attribute 'mean'

Adding this functionality, if not already present, seems like a really nice addition to the package.

[shoyer]

The problem is that it's ambiguous which array you would like to summarize. I suppose we could default to using all noncoordinates? That would be similar to how pandas does its DataFrame.groupby methods.

[jhamman]

Right, although it doesn't have to be ambiguous. Why not just summarize each group along the group_dim?

[shoyer]

We currently don't implement methods like ds.mean(), which it would make sense to implement before adding the groupby version. But I do think it would be perfectly consistent to say that the mean of a dataset is the dataset given by taking the mean of every variable in the dataset.