fix(datatable): raise error for update_cell with invalid column key

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## Unreleased
+
+### Fixed
+
+- Fixed `DataTable.update_cell` not raising an error with an invalid column key https://github.com/Textualize/textual/issues/3335
+
 ## [0.38.1] - 2023-09-21
 
 ### Fixed

src/textual/widgets/_data_table.py

@@ -745,12 +745,15 @@ def update_cell(
         if isinstance(column_key, str):
             column_key = ColumnKey(column_key)
 
-        try:
-            self._data[row_key][column_key] = value
-        except KeyError:
+        if (
+            row_key not in self._row_locations
+            or column_key not in self._column_locations
+        ):
             raise CellDoesNotExist(
                 f"No cell exists for row_key={row_key!r}, column_key={column_key!r}."
-            ) from None
+            )
+
+        self._data[row_key][column_key] = value
         self._update_count += 1
 
         # Recalculate widths if necessary

tests/test_data_table.py

@@ -641,6 +641,17 @@ async def test_update_cell_cell_doesnt_exist():
             table.update_cell("INVALID", "CELL", "Value")
 
 
+async def test_update_cell_invalid_column_key():
+    """Regression test for https://github.com/Textualize/textual/issues/3335"""
+    app = DataTableApp()
+    async with app.run_test():
+        table = app.query_one(DataTable)
+        table.add_column("Column1", key="C1")
+        table.add_row("TargetValue", key="R1")
+        with pytest.raises(CellDoesNotExist):
+            table.update_cell("R1", "INVALID_COLUMN", "New Value")
+
+
 async def test_update_cell_at_coordinate_exists():
     app = DataTableApp()
     async with app.run_test():