ENH: Make datasets have subclass relationships

tests/pipeline/test_blaze.py

@@ -90,7 +90,7 @@ class BlazeToPipelineTestCase(TestCase):
         self.assertEqual(ds.__name__, name)
         self.assertTrue(issubclass(ds, DataSet))
         self.assertEqual(
-            {c.name: c.dtype for c in ds._columns},
+            {c.name: c.dtype for c in ds.columns},
             {'sid': np.int64, 'value': np.float64},
         )
 

tests/pipeline/test_term.py

@@ -28,6 +28,10 @@ class SomeDataSet(DataSet):
     buzz = Column(float64_dtype)
 
 
+class SubDataSet(SomeDataSet):
+    pass
+
+
 class SomeFactor(Factor):
     dtype = float64_dtype
     window_length = 5
@@ -321,3 +325,31 @@ class ObjectIdentityTestCase(TestCase):
 
         with self.assertRaises(InvalidDType):
             SomeFactor(dtype=1)
+
+
+class SubDataSetTestCase(TestCase):
+    def test_subdataset(self):
+        some_dataset_map = {
+            column.name: column for column in SomeDataSet.columns
+        }
+        sub_dataset_map = {
+            column.name: column for column in SubDataSet.columns
+        }
+        self.assertEqual(
+            set(some_dataset_map),
+            set(sub_dataset_map),
+        )
+        for k, some_dataset_column in some_dataset_map.items():
+            sub_dataset_column = sub_dataset_map[k]
+            self.assertIsNot(
+                some_dataset_column,
+                sub_dataset_column,
+                'subclass column %r should not have the same identity as'
+                ' the parent' % k,
+            )
+            self.assertEqual(
+                some_dataset_column.dtype,
+                sub_dataset_column.dtype,
+                'subclass column %r should have the same dtype as the parent' %
+                k,
+            )