Pandas Groupby Agg Function Does Not Reduce

Question

I am using an aggregation function that I have used in my work for a long time now. The idea is that if the Series passed to the function is of length 1 (i.e. the group only

Answer 1

This is a misfeature in DataFrame. If the aggregator returns a list for the first group, it will fail with the error you mention; if it returns a non-list (non-Series) for the first group, it will work fine. The broken code is in groupby.py:

def _aggregate_series_pure_python(self, obj, func):

    group_index, _, ngroups = self.group_info

    counts = np.zeros(ngroups, dtype=int)
    result = None

    splitter = get_splitter(obj, group_index, ngroups, axis=self.axis)

    for label, group in splitter:
        res = func(group)
        if result is None:
            if (isinstance(res, (Series, Index, np.ndarray)) or
                    isinstance(res, list)):
                raise ValueError('Function does not reduce')
            result = np.empty(ngroups, dtype='O')

        counts[label] = group.shape[0]
        result[label] = res

Notice that if result is None and isinstance(res, list. Your options are:

1. Fake out groupby().agg(), so it doesn't see a list for the first group, or

2. Do the aggregation yourself, using code like that above but without the erroneous test.