Reject out-of-range column index in Dataset.add_formatter

src/tablib/core.py

@@ -638,7 +638,7 @@ def add_formatter(self, col, handler):
             else:
                 raise KeyError
 
-        if col is None or col <= self.width:
+        if col is None or col < self.width:
             self._formatters.append((col, handler))
         else:
             raise InvalidDatasetIndex

tests/test_tablib.py

@@ -19,7 +19,7 @@
 
 import tablib
 from tablib.core import Row, detect_format
-from tablib.exceptions import UnsupportedFormat
+from tablib.exceptions import InvalidDatasetIndex, UnsupportedFormat
 from tablib.formats import registry
 
 try:
@@ -643,6 +643,21 @@ def _formatter(cell_value):
             {'first_name': 'THOMAS', 'last_name': 'JEFFERSON', 'gpa': '50'},
         ])
 
+    def test_add_formatter_out_of_range_column(self):
+        """A column index past the last column must be rejected, not silently
+        accepted (which then crashed on export)."""
+
+        def _formatter(cell_value):
+            return cell_value
+
+        # width is 3 -> valid indices are 0, 1, 2
+        self.assertEqual(self.founders.width, 3)
+        # the last valid index is accepted
+        self.assertTrue(self.founders.add_formatter(2, _formatter))
+        # one past the last column raises instead of being silently accepted
+        with self.assertRaises(InvalidDatasetIndex):
+            self.founders.add_formatter(3, _formatter)
+
     def test_unicode_renders_markdown_table(self):
         # add another entry to test right field width for
         # integer