Databook.add_sheet accepts Dataset subclasses

tablib/core.py

@@ -955,7 +955,7 @@ class Databook(object):
 
     def add_sheet(self, dataset):
         """Adds given :class:`Dataset` to the :class:`Databook`."""
-        if type(dataset) is Dataset:
+        if isinstance(dataset, Dataset):
             self._datasets.append(dataset)
         else:
             raise InvalidDatasetType

test_tablib.py

@@ -669,5 +669,20 @@ Old       |Man       |100500
         )
 
 
+    def test_databook_add_sheet_accepts_dataset_subclasses(self):
+        class DatasetSubclass(tablib.Dataset):
+            pass
+
+        # just checking if subclass of tablib.Dataset can be added to Databook
+        dataset = DatasetSubclass()
+        dataset.append(self.john)
+        dataset.append(self.tom)
+
+        try:
+            book.add_sheet(dataset)
+        except tablib.InvalidDatasetType:
+            self.fail("Subclass of tablib.Dataset should be accepted by Databook.add_sheet")
+
+
 if __name__ == '__main__':
     unittest.main()