[Bug]: DataFrame API Series.agg with a list drops the Series name for mean, std, var #40572

What happened?

DeferredSeries.agg with a list loses the Series name when the list includes one of the hand-implemented aggregations (mean, std, var, skew, kurt, nunique, ...). Those results are wrapped with pd.Series(x, index=[base_func]), which has no name, so agg(['mean']) comes back unnamed and agg(['sum', 'mean']) loses the name when the rows are concatenated.

import apache_beam as beam
import pandas as pd
from apache_beam.dataframe.convert import to_dataframe, to_pcollection

values = [3.0, 1.0, 2.0, 5.0, 4.0]
print(pd.Series(values, name='x').agg(['sum', 'mean']))

with beam.Pipeline() as p:
  df = to_dataframe(p | beam.Create([beam.Row(x=v) for v in values]))
  _ = (
      to_pcollection(df.x.agg(['sum', 'mean']), yield_elements='pandas')
      | beam.Map(print))

pandas:

sum     15.0
mean     3.0
Name: x, dtype: float64

Beam:

sum     15.0
mean     3.0
dtype: float64

agg(['sum']) and agg(['sum', 'max']) keep the name. The existing test_series_agg_modes uses an unnamed Series, so it doesn't catch this.

Seen on Beam 2.77.0 and on master (a9f808e), pandas 2.3.3, Python 3.12.

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

Issue Components

• Component: Python SDK