Testing case where you try to update cells which dont exist

src/textual/widgets/_data_table.py

@@ -523,8 +523,17 @@ class DataTable(ScrollView, Generic[CellType], can_focus=True):
             value: The new value to put inside the cell.
             update_width: Whether to resize the column width to accommodate
                 for the new cell content.
+
+        Raises:
+            CellDoesNotExist: When the supplied `row_key` and `column_key`
+                cannot be found in the table.
         """
-        self.data[row_key][column_key] = value
+        try:
+            self.data[row_key][column_key] = value
+        except KeyError:
+            raise CellDoesNotExist(
+                f"No cell exists for row_key={row_key!r}, column_key={column_key!r}."
+            ) from None
         self._update_count += 1
 
         # Recalculate widths if necessary
@@ -545,6 +554,7 @@ class DataTable(ScrollView, Generic[CellType], can_focus=True):
             update_width: Whether to resize the column width to accommodate
                 for the new cell content.
         """
+        # TODO: Validate coordinate and raise exception
         row_key, column_key = self.coordinate_to_cell_key(coordinate)
         self.update_cell(row_key, column_key, value, update_width=update_width)
 

tests/test_data_table.py

@@ -303,6 +303,16 @@ async def test_update_cell_cell_exists():
         assert table.get_cell_value("1", "A") == "NEW_VALUE"
 
 
+async def test_update_cell_cell_doesnt_exist():
+    app = DataTableApp()
+    async with app.run_test():
+        table = app.query_one(DataTable)
+        table.add_column("A", key="A")
+        table.add_row("1", key="1")
+        with pytest.raises(CellDoesNotExist):
+            table.update_cell("INVALID", "CELL", "Value")
+
+
 # TODO: Test update coordinate