Add support for parsing JSON files in array form (#4997)

* Support parsing JSON lists

* Add error handling

* Minor improvements

* Add tests

* Comment

src/datasets/packaged_modules/json/json.py

@@ -15,6 +15,25 @@
 logger = datasets.utils.logging.get_logger(__name__)
 
 
+if datasets.config.PYARROW_VERSION.major >= 7:
+
+    def pa_table_from_pylist(mapping):
+        return pa.Table.from_pylist(mapping)
+
+else:
+
+    def pa_table_from_pylist(mapping):
+        # Copied from: https://github.com/apache/arrow/blob/master/python/pyarrow/table.pxi#L5193
+        arrays = []
+        names = []
+        if mapping:
+            names = list(mapping[0].keys())
+        for n in names:
+            v = [row[n] if n in row else None for row in mapping]
+            arrays.append(v)
+        return pa.Table.from_arrays(arrays, names)
+
+
 @dataclass
 class JsonConfig(datasets.BuilderConfig):
     """BuilderConfig for JSON."""
@@ -125,18 +144,29 @@ def _generate_tables(self, files):
                                         )
                                         block_size *= 2
                         except pa.ArrowInvalid as e:
-                            logger.error(f"Failed to read file '{file}' with error {type(e)}: {e}")
                             try:
                                 with open(file, encoding="utf-8") as f:
                                     dataset = json.load(f)
                             except json.JSONDecodeError:
+                                logger.error(f"Failed to read file '{file}' with error {type(e)}: {e}")
                                 raise e
-                            raise ValueError(
-                                f"Not able to read records in the JSON file at {file}. "
-                                f"You should probably indicate the field of the JSON file containing your records. "
-                                f"This JSON file contain the following fields: {str(list(dataset.keys()))}. "
-                                f"Select the correct one and provide it as `field='XXX'` to the dataset loading method. "
-                            ) from None
+                            # If possible, parse the file as a list of json objects and exit the loop
+                            if isinstance(dataset, list):  # list is the only sequence type supported in JSON
+                                try:
+                                    pa_table = pa_table_from_pylist(dataset)
+                                except (pa.ArrowInvalid, AttributeError) as e:
+                                    logger.error(f"Failed to read file '{file}' with error {type(e)}: {e}")
+                                    raise ValueError(f"Not able to read records in the JSON file at {file}.") from None
+                                yield file_idx, self._cast_table(pa_table)
+                                break
+                            else:
+                                logger.error(f"Failed to read file '{file}' with error {type(e)}: {e}")
+                                raise ValueError(
+                                    f"Not able to read records in the JSON file at {file}. "
+                                    f"You should probably indicate the field of the JSON file containing your records. "
+                                    f"This JSON file contain the following fields: {str(list(dataset.keys()))}. "
+                                    f"Select the correct one and provide it as `field='XXX'` to the dataset loading method. "
+                                ) from None
                         # Uncomment for debugging (will print the Arrow table size and elements)
                         # logger.warning(f"pa_table: {pa_table} num rows: {pa_table.num_rows}")
                         # logger.warning('\n'.join(str(pa_table.slice(i, 1).to_pydict()) for i in range(pa_table.num_rows)))

tests/packaged_modules/test_csv.py

@@ -12,7 +12,7 @@
 
 @pytest.fixture
 def csv_file(tmp_path):
-    filename = tmp_path / "malformed_file.csv"
+    filename = tmp_path / "file.csv"
     data = textwrap.dedent(
         """\
         header1,header2

tests/packaged_modules/test_json.py

@@ -0,0 +1,71 @@
+import textwrap
+
+import pyarrow as pa
+import pytest
+
+from datasets.packaged_modules.json.json import Json
+
+
+@pytest.fixture
+def jsonl_file(tmp_path):
+    filename = tmp_path / "file.jsonl"
+    data = textwrap.dedent(
+        """\
+        {"col_1": 1, "col_2": 2}
+        {"col_1": 10, "col_2": 20}
+        """
+    )
+    with open(filename, "w") as f:
+        f.write(data)
+    return str(filename)
+
+
+@pytest.fixture
+def json_file_with_list_of_dicts(tmp_path):
+    filename = tmp_path / "file_with_list_of_dicts.json"
+    data = textwrap.dedent(
+        """\
+        [
+            {"col_1": 1, "col_2": 2},
+            {"col_1": 10, "col_2": 20}
+        ]
+        """
+    )
+    with open(filename, "w") as f:
+        f.write(data)
+    return str(filename)
+
+
+@pytest.fixture
+def json_file_with_list_of_dicts_field(tmp_path):
+    filename = tmp_path / "file_with_list_of_dicts_field.json"
+    data = textwrap.dedent(
+        """\
+        {
+            "field1": 1,
+            "field2": "aabb",
+            "field3": [
+                {"col_1": 1, "col_2": 2},
+                {"col_1": 10, "col_2": 20}
+            ]
+        }
+        """
+    )
+    with open(filename, "w") as f:
+        f.write(data)
+    return str(filename)
+
+
+@pytest.mark.parametrize(
+    "file_fixture, config_kwargs",
+    [
+        ("jsonl_file", {}),
+        ("json_file_with_list_of_dicts", {}),
+        ("json_file_with_list_of_dicts_field", {"field": "field3"}),
+    ],
+)
+def test_json_generate_tables(file_fixture, config_kwargs, request):
+    json = Json(**config_kwargs)
+    generator = json._generate_tables([[request.getfixturevalue(file_fixture)]])
+    pa_table = pa.concat_tables([table for _, table in generator])
+    assert pa_table.to_pydict() == {"col_1": [1, 10], "col_2": [2, 20]}