feat(datasets): Extend preview mechanism

kedro-datasets/kedro_datasets/json/json_dataset.py

@@ -16,6 +16,8 @@
     get_protocol_and_path,
 )
 
+from kedro_datasets._typing import JSONPreview
+
 
 class JSONDataset(AbstractVersionedDataset[Any, Any]):
     """``JSONDataset`` loads/saves data from/to a JSON file using an underlying
@@ -160,3 +162,14 @@ def _invalidate_cache(self) -> None:
         """Invalidate underlying filesystem caches."""
         filepath = get_filepath_str(self._filepath, self._protocol)
         self._fs.invalidate_cache(filepath)
+
+    def preview(self) -> JSONPreview:
+        """
+        Generate a preview of the JSON dataset with a specified number of items.
+
+        Returns:
+            A string representing the JSON data for previewing.
+        """
+        data = self._load()
+
+        return json.dumps(data)

kedro-datasets/kedro_datasets/pandas/json_dataset.py

@@ -19,6 +19,8 @@
     get_protocol_and_path,
 )
 
+from kedro_datasets._typing import TablePreview
+
 logger = logging.getLogger(__name__)
 
 
@@ -188,3 +190,24 @@ def _invalidate_cache(self) -> None:
         """Invalidate underlying filesystem caches."""
         filepath = get_filepath_str(self._filepath, self._protocol)
         self._fs.invalidate_cache(filepath)
+
+    def preview(self, nrows: int = 5) -> TablePreview:
+        """
+        Generate a preview of the dataset with a specified number of rows,
+        including handling for both flat and nested JSON structures.
+
+        Args:
+            nrows: Number of rows to include in the preview. Defaults to 5.
+
+        Returns:
+            dict: A dictionary in a split format for preview, if possible.
+        """
+        # Create a copy, so it doesn't contaminate the original dataset
+        dataset_copy = self._copy()
+        dataset_copy._load_args.setdefault("lines", True)
+        dataset_copy._load_args["nrows"] = nrows
+        preview_df = dataset_copy._load()
+
+        preview_dict = preview_df.to_dict(orient="split")
+
+        return preview_dict

kedro-datasets/kedro_datasets/pandas/parquet_dataset.py

@@ -19,6 +19,8 @@
     get_protocol_and_path,
 )
 
+from kedro_datasets._typing import TablePreview
+
 logger = logging.getLogger(__name__)
 
 
@@ -214,3 +216,24 @@ def _invalidate_cache(self) -> None:
         """Invalidate underlying filesystem caches."""
         filepath = get_filepath_str(self._filepath, self._protocol)
         self._fs.invalidate_cache(filepath)
+
+    def preview(self, nrows: int = 5) -> TablePreview:
+        """
+        Generate a preview of the dataset with a specified number of rows.
+
+        Args:
+            nrows: The number of rows to include in the preview. Defaults to 5.
+
+        Returns:
+            dict: A dictionary containing the data in a split format.
+        """
+        import pyarrow.parquet as pq
+
+        load_path = str(self._get_load_path())
+
+        table = pq.read_table(
+            load_path, columns=self._load_args.get("columns"), use_threads=True
+        )[:nrows]
+        data_preview = table.to_pandas()
+
+        return data_preview.to_dict(orient="split")

kedro-datasets/kedro_datasets/pandas/sql_dataset.py

@@ -16,9 +16,11 @@
     get_filepath_str,
     get_protocol_and_path,
 )
-from sqlalchemy import create_engine, inspect
+from sqlalchemy import MetaData, Table, create_engine, inspect, select
 from sqlalchemy.exc import NoSuchModuleError
 
+from kedro_datasets._typing import TablePreview
+
 __all__ = ["SQLTableDataset", "SQLQueryDataset"]
 
 KNOWN_PIP_INSTALL = {
@@ -275,6 +277,31 @@ def _exists(self) -> bool:
         schema = self._load_args.get("schema", None)
         return insp.has_table(self._load_args["table_name"], schema)
 
+    def preview(self, nrows: int = 5) -> TablePreview:
+        """
+        Generate a preview of the dataset with a specified number of rows.
+
+        Args:
+            nrows: The number of rows to include in the preview. Defaults to 5.
+
+        Returns:
+            dict: A dictionary containing the data in a split format.
+        """
+
+        table_name = self._load_args["table_name"]
+
+        metadata = MetaData()
+        table_ref = Table(table_name, metadata, autoload_with=self.engine)
+
+        query = select(table_ref).limit(nrows)
+
+        with self.engine.connect() as conn:
+            result = conn.execute(query)
+            data_preview = pd.DataFrame(result.fetchall(), columns=result.keys())
+
+        preview_data = data_preview.to_dict(orient="split")
+        return preview_data
+
 
 class SQLQueryDataset(AbstractDataset[None, pd.DataFrame]):
     """``SQLQueryDataset`` loads data from a provided SQL query. It

kedro-datasets/tests/json/test_json_dataset.py

@@ -1,3 +1,5 @@
+import inspect
+import json
 from pathlib import Path, PurePosixPath
 
 import pytest
@@ -197,3 +199,22 @@ def test_versioning_existing_dataset(
         Path(json_dataset._filepath.as_posix()).unlink()
         versioned_json_dataset.save(dummy_data)
         assert versioned_json_dataset.exists()
+
+    def test_preview(self, json_dataset, dummy_data):
+        """Test the preview method."""
+        json_dataset.save(dummy_data)
+        preview_data = json_dataset.preview()
+
+        # Load the data directly for comparison
+        with json_dataset._fs.open(json_dataset._get_load_path(), mode="r") as fs_file:
+            full_data = json.load(fs_file)
+
+        expected_data = json.dumps(full_data)
+
+        assert (
+            preview_data == expected_data
+        ), "The preview data does not match the expected data."
+        assert (
+            inspect.signature(json_dataset.preview).return_annotation.__name__
+            == "JSONPreview"
+        )

kedro-datasets/tests/pandas/test_json_dataset.py

@@ -1,3 +1,5 @@
+import inspect
+import json
 from pathlib import Path, PurePosixPath
 
 import pandas as pd
@@ -40,6 +42,17 @@ def dummy_dataframe():
     return pd.DataFrame({"col1": [1, 2], "col2": [4, 5], "col3": [5, 6]})
 
 
+@pytest.fixture
+def json_lines_data(tmp_path):
+    data = [
+        {"name": "Alice", "age": 30, "city": "New York"},
+        {"name": "Bob", "age": 25, "city": "Los Angeles"},
+    ]
+    filepath = tmp_path / "lines_test.json"
+    filepath.write_text("\n".join(json.dumps(item) for item in data))
+    return filepath.as_posix()
+
+
 class TestJSONDataset:
     def test_save_and_load(self, json_dataset, dummy_dataframe):
         """Test saving and reloading the data set."""
@@ -142,6 +155,30 @@ def test_catalog_release(self, mocker):
         dataset.release()
         fs_mock.invalidate_cache.assert_called_once_with(filepath)
 
+    def test_preview_json(self, json_lines_data):
+        dataset = JSONDataset(filepath=json_lines_data, load_args={"lines": True})
+        preview_data = dataset.preview(nrows=2)
+        expected_columns = ["name", "age", "city"]
+        expected_data = [["Alice", 30, "New York"], ["Bob", 25, "Los Angeles"]]
+
+        assert preview_data["columns"] == expected_columns
+        assert preview_data["data"] == expected_data
+        assert len(preview_data["data"]) == 2
+        assert (
+            inspect.signature(dataset.preview).return_annotation.__name__
+            == "TablePreview"
+        )
+
+    def test_preview_json_lines(self, json_dataset, json_lines_data):
+        json_dataset._filepath = json_lines_data
+        json_dataset._load_args = {"lines": True}
+        preview_data = json_dataset.preview()
+        assert len(preview_data["data"]) == 2
+        assert (
+            inspect.signature(json_dataset.preview).return_annotation.__name__
+            == "TablePreview"
+        )
+
 
 class TestJSONDatasetVersioned:
     def test_version_str_repr(self, load_version, save_version):

kedro-datasets/tests/pandas/test_parquet_dataset.py

@@ -1,3 +1,4 @@
+import inspect
 from pathlib import Path, PurePosixPath
 
 import pandas as pd
@@ -42,6 +43,17 @@ def dummy_dataframe():
     return pd.DataFrame({"col1": [1, 2], "col2": [4, 5], "col3": [5, 6]})
 
 
+@pytest.fixture
+def dummy_dataframe_preview():
+    return pd.DataFrame(
+        {
+            "col1": [1, 2, 3, 4, 5, 6],
+            "col2": [4, 5, 6, 7, 8, 9],
+            "col3": [5, 6, 7, 8, 9, 10],
+        }
+    )
+
+
 class TestParquetDataset:
     def test_credentials_propagated(self, mocker):
         """Test propagating credentials for connecting to GCS"""
@@ -213,6 +225,32 @@ def test_arg_partition_cols(self, dummy_dataframe, tmp_path):
         with pytest.raises(DatasetError, match=pattern):
             dataset.save(dummy_dataframe)
 
+    @pytest.mark.parametrize(
+        "nrows,expected_rows",
+        [
+            (5, 5),
+            (
+                10,
+                6,
+            ),  # Test with specified rows more than total, assuming 6 rows in dummy data
+        ],
+    )
+    def test_preview(
+        self, parquet_dataset, dummy_dataframe_preview, nrows, expected_rows
+    ):
+        """Test the preview functionality for ParquetDataset."""
+        parquet_dataset.save(dummy_dataframe_preview)
+        previewed_data = parquet_dataset.preview(nrows=nrows)
+
+        # Assert preview data matches expected rows
+        assert len(previewed_data["data"]) == expected_rows
+        # Assert columns match
+        assert previewed_data["columns"] == list(dummy_dataframe_preview.columns)
+        assert (
+            inspect.signature(parquet_dataset.preview).return_annotation.__name__
+            == "TablePreview"
+        )
+
 
 class TestParquetDatasetVersioned:
     def test_version_str_repr(self, load_version, save_version):

kedro-datasets/tests/pandas/test_sql_dataset.py

@@ -1,10 +1,12 @@
+import inspect
 from pathlib import PosixPath
 from unittest.mock import ANY
 
 import pandas as pd
 import pytest
 import sqlalchemy
 from kedro.io.core import DatasetError
+from sqlalchemy.exc import SQLAlchemyError
 
 import kedro_datasets
 from kedro_datasets.pandas import SQLQueryDataset, SQLTableDataset
@@ -60,6 +62,19 @@ def query_file_dataset(request, sql_file):
     return SQLQueryDataset(**kwargs)
 
 
+@pytest.fixture
+def sql_dataset(tmp_path):
+    connection_string = "sqlite:///:memory:"
+    table_name = "test_table"
+
+    engine = sqlalchemy.create_engine(connection_string)
+    test_data = pd.DataFrame({"col1": [1, 2, 3], "col2": ["a", "b", "c"]})
+    test_data.to_sql(table_name, engine, index=False)
+
+    credentials = {"con": connection_string}
+    return SQLTableDataset(table_name=table_name, credentials=credentials)
+
+
 class TestSQLTableDataset:
     _unknown_conn = "mysql+unknown_module://scott:tiger@localhost/foo"
 
@@ -211,6 +226,28 @@ def test_additional_params(self, mocker):
             CONNECTION, **additional_params
         )
 
+    def test_preview_normal_scenario(self, sql_dataset):
+        engine = sql_dataset.engine
+        expected_df = pd.DataFrame({"col1": [1, 2, 3], "col2": ["a", "b", "c"]})
+        expected_df.to_sql(sql_dataset._load_args["table_name"], engine, index=False)
+        preview = sql_dataset.preview(nrows=3)
+
+        assert "columns" in preview
+        assert "data" in preview
+        assert len(preview["data"]) == len(expected_df)
+
+        return_annotation = inspect.signature(sql_dataset.preview).return_annotation
+        assert return_annotation == "TablePreview"
+
+    def test_preview_sql_error(self, table_dataset, mocker):
+        mocker.patch(
+            "pandas.read_sql_query",
+            side_effect=SQLAlchemyError("Mocked SQL error", "", ""),
+        )
+
+        with pytest.raises(SQLAlchemyError):
+            table_dataset.preview(nrows=3)
+
 
 class TestSQLTableDatasetSingleConnection:
     def test_single_connection(self, dummy_dataframe, mocker):