added unit test

tests/test_utils.py

@@ -0,0 +1,39 @@
+import pytest
+from nucleus import DatasetItem
+from nucleus import utils
+
+import io
+
+
+class TestNonSerializableObject:
+    def weird_function():
+        print("can't touch this. Dun dun dun dun.")
+
+
+def test_serialize():
+
+    test_items = [
+        DatasetItem("fake_url1", "fake_id1"),
+        DatasetItem(
+            "fake_url2",
+            "fake_id2",
+            metadata={
+                "ok": "field",
+                "bad": TestNonSerializableObject(),
+            },
+        ),
+    ]
+
+    with io.StringIO() as in_memory_filelike:
+        with pytest.raises(ValueError) as error:
+            utils.serialize_and_write(
+                test_items,
+                in_memory_filelike,
+            )
+        assert "DatasetItem" in str(error.value)
+        assert "fake_id2" in str(error.value)
+        assert "fake_id1" not in str(error.value)
+
+        test_items[1].metadata["bad"] = "fixed"
+
+        utils.serialize_and_write(test_items, in_memory_filelike)