Missing Series name when using count() on groupby object · Issue #6265 · pandas-dev/pandas (original) (raw)

Consider this very basic group by operation:

df = pd.DataFrame({'col1': ['a', 'b', 'a', 'c'], 'col2': [1, 3, 2, 5]}) df_grp = df.groupby('col1')['col2']

When aggregating using mean, min, ... (everything but count) the resulting Series has col2 as its name:

df_grp.mean()

col1 a 1.5 b 3.0 c 5.0 Name: col2, dtype: float64

However, if I use count for aggregation, the name is not set:

df_grp.count()

col1 a 2 b 1 c 1 dtype: int64

Not a big problem for a simple case like that, but I stumbled over that while working with a MultiIndex that needed to be reindexed and led to a KeyError due to that missing column name.