Raise error for string types in nsmallest and nlargest (#13946)

closes #13945 

This PR contains changes that raises an error message exactly matching pandas for `nsmallest` and `nlargest`.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #13946

Author: galipremsagar

python/cudf/cudf/core/indexed_frame.py

@@ -2465,6 +2465,20 @@ def _n_largest_or_smallest(self, largest, n, columns, keep):
         if isinstance(columns, str):
             columns = [columns]
 
+        method = "nlargest" if largest else "nsmallest"
+        for col in columns:
+            if isinstance(self._data[col], cudf.core.column.StringColumn):
+                if isinstance(self, cudf.DataFrame):
+                    error_msg = (
+                        f"Column '{col}' has dtype {self._data[col].dtype}, "
+                        f"cannot use method '{method}' with this dtype"
+                    )
+                else:
+                    error_msg = (
+                        f"Cannot use method '{method}' with "
+                        f"dtype {self._data[col].dtype}"
+                    )
+                raise TypeError(error_msg)
         if len(self) == 0:
             return self
 

python/cudf/cudf/tests/test_dataframe.py

@@ -10316,3 +10316,16 @@ def test_dataframe_reindex_with_index_names(index_data, name):
     expected = pdf.reindex(index_data)
 
     assert_eq(actual, expected)
+
+
+@pytest.mark.parametrize("attr", ["nlargest", "nsmallest"])
+def test_dataframe_nlargest_nsmallest_str_error(attr):
+    gdf = cudf.DataFrame({"a": [1, 2, 3, 4], "b": ["a", "b", "c", "d"]})
+    pdf = gdf.to_pandas()
+
+    assert_exceptions_equal(
+        getattr(gdf, attr),
+        getattr(pdf, attr),
+        ([], {"n": 1, "columns": ["a", "b"]}),
+        ([], {"n": 1, "columns": ["a", "b"]}),
+    )

python/cudf/cudf/tests/test_series.py

@@ -2244,3 +2244,13 @@ def test_series_typecast_to_object():
         assert new_series[0] == "1970-01-01 00:00:00.000000001"
         new_series = actual.astype(np.dtype("object"))
         assert new_series[0] == "1970-01-01 00:00:00.000000001"
+
+
+@pytest.mark.parametrize("attr", ["nlargest", "nsmallest"])
+def test_series_nlargest_nsmallest_str_error(attr):
+    gs = cudf.Series(["a", "b", "c", "d", "e"])
+    ps = gs.to_pandas()
+
+    assert_exceptions_equal(
+        getattr(gs, attr), getattr(ps, attr), ([], {"n": 1}), ([], {"n": 1})
+    )