Tolerate empty samples for textual/uuid PKs

Author: Sergey Vasilyev

data_diff/databases/base.py

@@ -1093,11 +1093,7 @@ def _refine_coltypes(
             list,
             log_message=table_path,
         )
-        if not samples_by_row:
-            raise ValueError(f"Table {table_path} is empty.")
-
-        samples_by_col = list(zip(*samples_by_row))
-
+        samples_by_col = list(zip(*samples_by_row)) if samples_by_row else [[]] * len(text_columns)
         for col_name, samples in safezip(text_columns, samples_by_col):
             uuid_samples = [s for s in samples if s and is_uuid(s)]
 

tests/test_diff_tables.py

@@ -696,10 +696,12 @@ def setUp(self):
         self.differ = HashDiffer(bisection_factor=2)
 
     def test_right_table_empty(self):
-        self.assertRaises(ValueError, list, self.differ.diff_tables(self.a, self.b))
+        # NotImplementedError: Cannot use a column of type Text(_notes=[]) as a key
+        self.assertRaises(NotImplementedError, list, self.differ.diff_tables(self.a, self.b))
 
     def test_left_table_empty(self):
-        self.assertRaises(ValueError, list, self.differ.diff_tables(self.a, self.b))
+        # NotImplementedError: Cannot use a column of type Text(_notes=[]) as a key
+        self.assertRaises(NotImplementedError, list, self.differ.diff_tables(self.a, self.b))
 
 
 class TestInfoTree(DiffTestCase):