Support at/iat indexers in cudf.pandas

[mroeschke]

Description

closes #16112

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

Does this cover the case of a new column that doesn't exist? I'd like to see a test that more clearly resembles the patterns in the minimal repro of #16112.

[mroeschke]

Yup, added a unit test in a43454c to mirror the issue. (It appears that iat.__setitem__ in pandas doesn't support the type of behavior demonstrated in the issue)

[mroeschke]

Huh, thanks for encouraging a unit test with the issue. Even though the test passed locally the CI didn't agree.

Left some investigation notes in #16112 (comment) that I may come back to later

[bdice]

@mroeschke There's one failing test:

=========================== short test summary info ============================
FAILED python/cudf/cudf_pandas_tests/test_cudf_pandas.py::test_at_setitem_empty - AssertionError: Attributes of DataFrame.iloc[:, 0] (column name="name") are different

Attribute "dtype" are different
[left]:  object
[right]: float64
FAILED python/cudf/cudf_pandas_tests/test_profiler.py::test_profiler_fast_slow_name_mismatch - AttributeError: '_iLocIndexer' object has no attribute '_convert_key'

Once that's fixed up I can approve.

[mroeschke]

Thanks @bdice all green here

[mroeschke]

/merge