TST: groupby.first/last retains categorical dtype (#43153)

Author: debnathshoham

pandas/tests/groupby/test_categorical.py

@@ -1741,3 +1741,15 @@ def test_groupby_categorical_indices_unused_categories():
     assert result.keys() == expected.keys()
     for key in result.keys():
         tm.assert_numpy_array_equal(result[key], expected[key])
+
+
+@pytest.mark.parametrize("func", ["first", "last"])
+def test_groupby_last_first_preserve_categoricaldtype(func):
+    # GH#33090
+    df = DataFrame({"a": [1, 2, 3]})
+    df["b"] = df["a"].astype("category")
+    result = getattr(df.groupby("a")["b"], func)()
+    expected = Series(
+        Categorical([1, 2, 3]), name="b", index=Index([1, 2, 3], name="a")
+    )
+    tm.assert_series_equal(expected, result)