Fix nbytes when no indexes

Author: benjeffery

python/tests/test_tables.py

@@ -3326,6 +3326,10 @@ def test_str(self):
         s = str(tables)
         assert len(s) > 0
 
+    def test_nbytes_empty_tables(self):
+        tables = tskit.TableCollection(1)
+        assert tables.nbytes == 119
+
     def test_nbytes(self, tmp_path, ts_fixture):
         tables = ts_fixture.dump_tables()
         tables.dump(tmp_path / "tables")

python/tskit/tables.py

@@ -309,7 +309,12 @@ def asdict(self):
 
     @property
     def nbytes(self):
-        return self.edge_insertion_order.nbytes + self.edge_removal_order.nbytes
+        total = 0
+        if self.edge_removal_order is not None:
+            total += self.edge_removal_order.nbytes
+        if self.edge_insertion_order is not None:
+            total += self.edge_insertion_order.nbytes
+        return total
 
 
 def keep_with_offset(keep, data, offset):