Fix for both empty gataframes and object columns with all None values

pyogrio/geopandas.py

@@ -458,11 +458,6 @@ def write_dataframe(
         use_arrow = bool(int(os.environ.get("PYOGRIO_USE_ARROW", "0")))
     path, driver = _get_write_path_driver(path, driver, append=append)
 
-    if use_arrow and (df.empty or len(df) == 0):
-        # with arrow, string columns without data trigger an error, so disable arrow
-        # when writing an empty dataframe.
-        use_arrow = False
-
     geometry_columns = df.columns[df.dtypes == "geometry"]
     if len(geometry_columns) > 1:
         raise ValueError(
@@ -588,6 +583,15 @@ def write_dataframe(
 
         table = pa.Table.from_pandas(df, preserve_index=False)
 
+        # Null arrow columns are not supported by GDAL, so convert to string
+        for field_index, field in enumerate(table.schema):
+            if field.type == pa.null():
+                table = table.set_column(
+                    field_index,
+                    field.with_type(pa.string()),
+                    table[field_index].cast(pa.string()),
+                )
+
         if geometry_column is not None:
             # ensure that the geometry column is binary (for all-null geometries,
             # this could be a wrong type)

pyogrio/tests/test_geopandas_io.py

@@ -1157,6 +1157,11 @@ def test_write_dataframe_index(tmp_path, naturalearth_lowres, use_arrow):
 )
 @pytest.mark.requires_arrow_write_api
 def test_write_empty_dataframe(tmp_path, ext, columns, dtype, use_arrow):
+    """Test writing dataframe with no rows.
+
+    With use_arrow, object type columns with no rows are converted to null type columns
+    by pyarrow, but null columns are not supported by GDAL. Added to test fix for #513.
+    """
     expected = gp.GeoDataFrame(geometry=[], columns=columns, dtype=dtype, crs=4326)
     filename = tmp_path / f"test{ext}"
     write_dataframe(expected, filename, use_arrow=use_arrow)
@@ -1185,11 +1190,11 @@ def test_write_empty_geometry(tmp_path):
 
 @pytest.mark.requires_arrow_write_api
 def test_write_None_string_column(tmp_path, use_arrow):
-    if use_arrow:
-        pytest.xfail(
-            "error when an object column with only None values is written with arrow."
-        )
+    """Test pandas object columns with all None values.
 
+    With use_arrow, such columns are converted to null type columns by pyarrow, but null
+    columns are not supported by GDAL. Added to test fix for #513.
+    """
     gdf = gp.GeoDataFrame({"object_col": [None]}, geometry=[Point(0, 0)], crs=4326)
     filename = tmp_path / "test.gpkg"