# Buggy code
def _reduce(df):
    # See note above about setting `as_index`
    groupby_args["as_index"] = True
    if other_len > 1:
        by = list(df.columns[0:other_len])
    else:
        by = df.columns[0]
    result = map_func(df.groupby(by=by, axis=axis, **groupby_args), **map_args)
    if (
        not isinstance(result.index, pandas.MultiIndex)
        and result.index.name is not None
        and "_modin_groupby_" in result.index.name
    ):
        result.index.name = result.index.name[len("_modin_groupby_") :]
    return result


# Traceback (ValueError)
Traceback (most recent call last):
File "/localdisk/gashiman/omniscripts/taxi/taxibench_pandas.py", line 190, in <module>
query(query_df)
File "/localdisk/gashiman/omniscripts/taxi/taxibench_pandas.py", line 95, in q3
return transformed.groupby(['passenger_count','pickup_datetime'])[['passenger_count','pickup_datetime']].count()['passenger_count']
File "/nfs/site/proj/scripting_tools/gashiman/modin/modin/pandas/groupby.py", line 249, in __getitem__
return SeriesGroupBy(self._default_to_pandas(lambda df: df.__getitem__(key)))
File "/nfs/site/proj/scripting_tools/gashiman/modin/modin/pandas/groupby.py", line 535, in _default_to_pandas
return self._df._default_to_pandas(groupby_on_multiple_columns)
File "/nfs/site/proj/scripting_tools/gashiman/modin/modin/pandas/base.py", line 227, in _default_to_pandas
result = op(pandas_obj, *args, **kwargs)
File "/nfs/site/proj/scripting_tools/gashiman/modin/modin/pandas/groupby.py", line 533, in groupby_on_multiple_columns
return f(df.groupby(by=by, axis=self._axis, **self._kwargs), **kwargs)
File "/nfs/site/home/gashiman/.local/lib/python3.7/site-packages/pandas/core/generic.py", line 7894, in groupby
**kwargs
File "/nfs/site/home/gashiman/.local/lib/python3.7/site-packages/pandas/core/groupby/groupby.py", line 2522, in groupby
return klass(obj, by, **kwds)
File "/nfs/site/home/gashiman/.local/lib/python3.7/site-packages/pandas/core/groupby/groupby.py", line 391, in __init__
mutated=self.mutated,
File "/nfs/site/home/gashiman/.local/lib/python3.7/site-packages/pandas/core/groupby/grouper.py", line 652, in _get_grouper
if not isinstance(gpr, Grouping)
File "/nfs/site/home/gashiman/.local/lib/python3.7/site-packages/pandas/core/groupby/grouper.py", line 347, in __init__
raise ValueError("Grouper for '{}' not 1-dimensional".format(t))
ValueError: Grouper for '<class 'pandas.core.frame.DataFrame'>' not 1-dimensional

# Fixed code
def _reduce(df):
    other_len = len(df.index.names)
    df = df.reset_index(drop=False)
    # See note above about setting `as_index`
    groupby_args["as_index"] = as_index
    if other_len > 1:
        by_part = list(df.columns[0:other_len])
    else:
        by_part = df.columns[0]
    result = reduce_func(
        df.groupby(by=by_part, axis=axis, **groupby_args), **reduce_args
    )
    if (
        not isinstance(result.index, pandas.MultiIndex)
        and result.index.name is not None
        and "_modin_groupby_" in result.index.name
    ):
        result.index.name = result.index.name[len("_modin_groupby_") :]
    # Avoid inserting data after the first partition or if the data did not come
    # from this query compiler.
    if not as_index and (first_column not in df.columns or drop_by):
        return result.drop(columns=by_part)
    return result