Remove test_groupby_agg_extension (marked xfail) (#54272)

* Remove test_groupby_agg_extension (marked xfail)

Remove special `test_groupby_agg_extension` in `test_json.py` as we can now pass the test generally with these fixes to `GroupBy.first` and `GroupBy.last`.

Signed-off-by: Michael Tiemann <[email protected]>

* Update v2.1.0.rst

Fix sort order.

Signed-off-by: Michael Tiemann <[email protected]>

* Apply suggestions from code review

Incorporate suggestions from @mroeschke (consolidating docs and fixing a malformed return statement).

Signed-off-by: Michael Tiemann <[email protected]>

Co-authored-by: Matthew Roeschke <[email protected]>

---------

Signed-off-by: Michael Tiemann <[email protected]>
Co-authored-by: Matthew Roeschke <[email protected]>

Author: MichaelTiemannOSC

doc/source/whatsnew/v2.1.0.rst

@@ -688,6 +688,7 @@ Other
 - Bug in :meth:`DataFrame.reindex` with a ``fill_value`` that should be inferred with a :class:`ExtensionDtype` incorrectly inferring ``object`` dtype (:issue:`52586`)
 - Bug in :meth:`DataFrame.shift` and :meth:`Series.shift` and :meth:`DataFrameGroupBy.shift` when passing both "freq" and "fill_value" silently ignoring "fill_value" instead of raising ``ValueError`` (:issue:`53832`)
 - Bug in :meth:`DataFrame.shift` with ``axis=1`` on a :class:`DataFrame` with a single :class:`ExtensionDtype` column giving incorrect results (:issue:`53832`)
+- Bug in :meth:`GroupBy.first` and :meth:`GroupBy.last` where an empty group would return ``np.nan`` instead of a an ExtensionArray's NA value (:issue:`39098`)
 - Bug in :meth:`Index.sort_values` when a ``key`` is passed (:issue:`52764`)
 - Bug in :meth:`Series.align`, :meth:`DataFrame.align`, :meth:`Series.reindex`, :meth:`DataFrame.reindex`, :meth:`Series.interpolate`, :meth:`DataFrame.interpolate`, incorrectly failing to raise with method="asfreq" (:issue:`53620`)
 - Bug in :meth:`Series.argsort` failing to raise when an invalid ``axis`` is passed (:issue:`54257`)

pandas/core/groupby/groupby.py

@@ -3279,7 +3279,7 @@ def first(x: Series):
                 """Helper function for first item that isn't NA."""
                 arr = x.array[notna(x.array)]
                 if not len(arr):
-                    return np.nan
+                    return x.array.dtype.na_value
                 return arr[0]
 
             if isinstance(obj, DataFrame):
@@ -3338,7 +3338,7 @@ def last(x: Series):
                 """Helper function for last item that isn't NA."""
                 arr = x.array[notna(x.array)]
                 if not len(arr):
-                    return np.nan
+                    return x.array.dtype.na_value
                 return arr[-1]
 
             if isinstance(obj, DataFrame):

pandas/tests/extension/json/test_json.py

@@ -356,10 +356,6 @@ def test_groupby_extension_no_sort(self):
         """
         super().test_groupby_extension_no_sort()
 
-    @pytest.mark.xfail(reason="GH#39098: Converts agg result to object")
-    def test_groupby_agg_extension(self, data_for_grouping):
-        super().test_groupby_agg_extension(data_for_grouping)
-
 
 class TestArithmeticOps(BaseJSON, base.BaseArithmeticOpsTests):
     def test_arith_frame_with_scalar(self, data, all_arithmetic_operators, request):