Correctly calculate number of csv rows to delete

CSV Editor would delete too many rows if multiple columns where
selected.  This fix correctly calculates how many rows are selected
when deleting rows.  Fixes #13

Author: alandmoore

Chapter05/csv_editor.py

@@ -152,8 +152,17 @@ def insert_below(self):
 
     def remove_rows(self):
         selected = self.tableview.selectedIndexes()
+        # Errata:  The book contains the following code:
+        #if selected:
+        #    self.model.removeRows(selected[0].row(), len(selected), None)
+
+        # This is incorrect, as len(selected) is the number of *cells* selected,
+        # not the number of *rows* selected.
+
+        # correct approach would look like this:
+        num_rows = len(set(index.row() for index in selected))
         if selected:
-            self.model.removeRows(selected[0].row(), len(selected), None)
+            self.model.removeRows(selected[0].row(), num_rows, None)
 
 
 if __name__ == '__main__':