[BUG] `pylibcudf.Column.from_iterable_of_py(<nested_list>, dtype=plc.TypeId.LIST)` erroneously raises

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#21,092 opened on Jan 20, 2026 0 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty: 2/5
Estimated time: 1-3 hours
Newbie friendliness: 72/100

Issue type: Bug
Clarity: Clearly specified
Activity status: Stale
Tech stack: python
Domain: data

Research direction

Start in pylibcudf/column.pyx at Column.from_iterable_of_py and inspect the related DataType handling in pylibcudf/types.pyx. Reproduce the nested-list example with an explicit LIST dtype, then verify that it follows the same inferred inner-type behavior as the call without dtype and no longer raises NotImplementedError.

Written by the indexing model from the issue text.

Description

bug pylibcudf

Steps/Code to reproduce bug

In [1]: import pylibcudf as plc

In [2]: plc.Column.from_iterable_of_py([[1, 2, 3]])  # works
Out[2]: <pylibcudf.column.Column at 0x77d150dcbd60>

In [3]: plc.Column.from_iterable_of_py([[1, 2, 3]], dtype=plc.DataType(plc.TypeId.LIST))
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
File ~/cudf/python/pylibcudf/pylibcudf/types.pyx:151, in pylibcudf.types.DataType._python_typecode.__get__()

KeyError: <type_id.LIST: 24>

During handling of the above exception, another exception occurred:

NotImplementedError                       Traceback (most recent call last)
Cell In[3], line 1
----> 1 plc.Column.from_iterable_of_py([[1, 2, 3]], dtype=plc.DataType(plc.TypeId.LIST))

File ~/cudf/python/pylibcudf/pylibcudf/column.pyx:1238, in pylibcudf.column.Column.from_iterable_of_py()

File ~/cudf/python/pylibcudf/pylibcudf/types.pyx:153, in pylibcudf.types.DataType._python_typecode.__get__()

NotImplementedError: No Python typecode for DataType <type_id.LIST: 24>

I believe here we should still be "flattening" and inferring the inner type in the else clause if a plc.DataType(plc.TypeId.LIST) is passed

        if dtype is None:
            depth, py_dtype = _infer_list_depth_and_dtype(obj)
            dtype = DataType.from_py(py_dtype)
        else:
            depth, _ = _infer_list_depth_and_dtype(obj)

Expected behavior
Same as Out[2]

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