[FEA] Cannot select struct typed columns in Dask cuDF

[beckernick]

Today, in cuDF Python I can manipulate struct columns. But, I can't with Dask. One hurdle appears to be that we cannot do column selection on a struct typed column.

I believe this is best framed as a feature request, as this is a new type.

import cudf
import dask_cudf
​
df = cudf.DataFrame(
    {"col": [
        {"a":5, "b":10},
        {"a":3, "b":7},
        {"a":-3, "b":11}
    ]}
)
ddf = dask_cudf.from_cudf(df, 2)
ddf.col
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/tmp/ipykernel_22770/1289136161.py in <module>
     10 )
     11 ddf = dask_cudf.from_cudf(df, 2)
---> 12 ddf.col

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask/dataframe/core.py in __getattr__(self, key)
   3991     def __getattr__(self, key):
   3992         if key in self.columns:
-> 3993             return self[key]
   3994         else:
   3995             raise AttributeError("'DataFrame' object has no attribute %r" % key)

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask/dataframe/core.py in __getitem__(self, key)
   3912             dsk = partitionwise_graph(operator.getitem, name, self, key)
   3913             graph = HighLevelGraph.from_collections(name, dsk, dependencies=[self])
-> 3914             return new_dd_object(graph, name, meta, self.divisions)
   3915         elif isinstance(key, slice):
   3916             from pandas.api.types import is_float_dtype

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask/dataframe/core.py in new_dd_object(dsk, name, meta, divisions, parent_meta)
   6802     """
   6803     if has_parallel_type(meta):
-> 6804         return get_parallel_type(meta)(dsk, name, meta, divisions)
   6805     elif is_arraylike(meta) and meta.shape:
   6806         import dask.array as da

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask_cudf/core.py in __init__(self, dsk, name, meta, divisions)
     63         self.dask = dsk
     64         self._name = name
---> 65         meta = dask_make_meta(meta)
     66         if not isinstance(meta, self._partition_type):
     67             raise TypeError(

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask/dataframe/dispatch.py in make_meta(x, index, parent_meta)
    125 
    126     try:
--> 127         return make_meta_dispatch(x, index=index)
    128     except TypeError:
    129         if parent_meta is not None:

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask/utils.py in __call__(self, arg, *args, **kwargs)
    573         """
    574         meth = self.dispatch(type(arg))
--> 575         return meth(arg, *args, **kwargs)
    576 
    577     @property

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/dask_cudf/backends.py in make_meta_cudf(x, index)
    130 @make_meta_dispatch.register((cudf.Series, cudf.DataFrame))
    131 def make_meta_cudf(x, index=None):
--> 132     return x.head(0)
    133 
    134 

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/series.py in head(self, n)
   1169         dtype: object
   1170         """
-> 1171         return self.iloc[:n]
   1172 
   1173     def tail(self, n=5):

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/indexing.py in __getitem__(self, arg)
     91         if isinstance(arg, tuple):
     92             arg = list(arg)
---> 93         data = self._sr._column[arg]
     94 
     95         if (

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/column/struct.py in __getitem__(self, args)
     82 
     83     def __getitem__(self, args):
---> 84         result = super().__getitem__(args)
     85         if isinstance(result, dict):
     86             return {

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/column/column.py in __getitem__(self, arg)
    511         elif isinstance(arg, slice):
    512             start, stop, stride = arg.indices(len(self))
--> 513             return self.slice(start, stop, stride)
    514         else:
    515             arg = as_column(arg)

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/column/column.py in slice(self, start, stop, stride)
    495             stop = stop + len(self)
    496         if (stride > 0 and start >= stop) or (stride < 0 and start <= stop):
--> 497             return column_empty(0, self.dtype, masked=True)
    498         # compute mask slice
    499         if stride == 1:

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/column/column.py in column_empty(row_count, dtype, masked)
   1365         mask = None
   1366 
-> 1367     return build_column(
   1368         data, dtype, mask=mask, size=row_count, children=children
   1369     )

/raid/nicholasb/miniconda3/envs/rapids-21.08/lib/python3.8/site-packages/cudf/core/column/column.py in build_column(data, dtype, size, mask, offset, null_count, children)
   1474         if size is None:
   1475             raise TypeError("Must specify size")
-> 1476         return cudf.core.column.StructColumn(
   1477             data=data,
   1478             dtype=dtype,

cudf/_lib/column.pyx in cudf._lib.column.Column.__init__()

cudf/_lib/column.pyx in cudf._lib.column.Column.set_base_mask()

ValueError: The Buffer for mask is smaller than expected, got 0 bytes, expected 64 bytes.

Note: The mask is expected to be sized according to the base allocation as opposed to the offsetted or sized allocation.

This does not happen for list typed columns.

conda list | grep "cudf\|dask\|pandas\|numpy\|arrow"
arrow-cpp                 4.0.1           py38hf0991f3_4_cuda    conda-forge
arrow-cpp-proc            3.0.0                      cuda    conda-forge
cudf                      21.08.00a210706 cuda_11.2_py38_g3ee264c4c3_244    rapidsai-nightly
cudf_kafka                21.08.00a210706 py38_g3ee264c4c3_244    rapidsai-nightly
dask                      2021.6.2           pyhd8ed1ab_0    conda-forge
dask-core                 2021.6.2           pyhd8ed1ab_0    conda-forge
dask-cuda                 21.08.00a210706         py38_32    rapidsai-nightly
dask-cudf                 21.08.00a210706 py38_g3ee264c4c3_244    rapidsai-nightly
geopandas                 0.9.0              pyhd8ed1ab_1    conda-forge
geopandas-base            0.9.0              pyhd8ed1ab_1    conda-forge
libcudf                   21.08.00a210706 cuda11.2_g3ee264c4c3_244    rapidsai-nightly
libcudf_kafka             21.08.00a210706 g3ee264c4c3_244    rapidsai-nightly
numpy                     1.21.0           py38h9894fe3_0    conda-forge
pandas                    1.2.5            py38h1abd341_0    conda-forge
pyarrow                   4.0.1           py38hb53058b_4_cuda    conda-forge