I can apply formulas

src/myfasthtml/controls/DataGrid.py

@@ -233,7 +233,7 @@ class DataGrid(MultipleInstance):
     # add columns manager
     self._columns_manager = DataGridColumnsManager(self)
     self._columns_manager.bind_command("ToggleColumn", self.commands.on_column_changed())
-    self._columns_manager.bind_command("UpdateColumn", self.commands.on_column_changed())
+    self._columns_manager.bind_command("SaveColumnDetails", self.commands.on_column_changed())
     
     if self._settings.enable_formatting:
       completion_engine = FormattingCompletionEngine(self._parent, self.get_table_name())
@@ -418,8 +418,7 @@ class DataGrid(MultipleInstance):
       self._df_store.ns_fast_access = _init_fast_access(self._df)
       self._df_store.ns_row_data = _init_row_data(self._df)
       self._df_store.ns_total_rows = len(self._df) if self._df is not None else 0
-      if init_state:
+      self._register_existing_formulas()
-        self._register_existing_formulas()
     
     return self
   
@@ -677,8 +676,27 @@ class DataGrid(MultipleInstance):
       return Span(*res, cls=f"{css_class} truncate", style=style) if len(res) > 1 else res[0]
     
     column_type = col_def.type
-    value = self._df_store.ns_fast_access[col_def.col_id][row_index]
+
-    
+    # Formula column: safe read — ns_fast_access entry may not exist yet if the
+    # engine hasn't run its first recalculation pass.
+    if column_type == ColumnType.Formula:
+      col_array = self._df_store.ns_fast_access.get(col_def.col_id)
+      if col_array is None or row_index >= len(col_array):
+        return NotStr('<span class="dt2-cell-content-text truncate">—</span>')
+      value = col_array[row_index]
+      # Display error strings produced by the evaluator (#ERR, #DIV/0!, …)
+      if isinstance(value, str) and value.startswith("#"):
+        return NotStr(
+          f'<span class="dt2-cell-content-error truncate">{html.escape(value)}</span>'
+        )
+      # Infer number vs text style from the actual computed value type
+      if isinstance(value, (int, float)) and not isinstance(value, bool):
+        column_type = ColumnType.Number
+      else:
+        column_type = ColumnType.Text
+    else:
+      value = self._df_store.ns_fast_access[col_def.col_id][row_index]
+
     # Boolean type - uses cached HTML (only 2 possible values)
     if column_type == ColumnType.Bool:
       return _mk_bool_cached(value)
@@ -722,8 +740,9 @@ class DataGrid(MultipleInstance):
     """
     if not col_def.visible:
       return None
-    
+
-    value = self._df_store.ns_fast_access[col_def.col_id][row_index]
+    col_array = self._df_store.ns_fast_access.get(col_def.col_id)
+    value = col_array[row_index] if col_array is not None and row_index < len(col_array) else None
     content = self.mk_body_cell_content(col_pos, row_index, col_def, filter_keyword_lower)
     
     return OptimizedDiv(content,

src/myfasthtml/controls/DataGridColumnsManager.py

@@ -41,8 +41,8 @@ class Commands(BaseCommands):
                    self._owner.show_all_columns).htmx(target=f"#{self._id}", swap="innerHTML")
   
   def save_column_details(self, col_id):
-    return Command(f"UpdateColumn",
+    return Command(f"SaveColumnDetails",
-                   f"Update column {col_id}",
+                   f"Save column {col_id}",
                    self._owner,
                    self._owner.save_column_details,
                    kwargs={"col_id": col_id}
@@ -71,7 +71,7 @@ class DataGridColumnsManager(MultipleInstance):
       self._parent._parent,
       self._parent.get_table_name(),
     )
-    conf = DslEditorConf(save_button=False, line_numbers=False, engine_id=completion_engine.get_id())
+    conf = DslEditorConf(name="formula", save_button=False, line_numbers=False, engine_id=completion_engine.get_id())
     self._formula_editor = DataGridFormulaEditor(self, conf=conf, _id=f"{self._id}-formula-editor")
     DslsManager.register(completion_engine, FormulaParser())
   
@@ -102,7 +102,7 @@ class DataGridColumnsManager(MultipleInstance):
           col_def.width = int(v)
         elif k == "formula":
           col_def.formula = v or ""
-          self._register_formula(col_def)
+          # self._register_formula(col_def), Will be done in save_column_details()
         else:
           setattr(col_def, k, v)
     
@@ -133,7 +133,8 @@ class DataGridColumnsManager(MultipleInstance):
   
   def save_column_details(self, col_id, client_response):
     logger.debug(f"save_column_details {col_id=}, {client_response=}")
-    self._get_updated_col_def_from_col_id(col_id, client_response, copy=False)
+    col_def = self._get_updated_col_def_from_col_id(col_id, client_response, copy=False)
+    self._register_formula(col_def)
     self._parent.save_state()
     
     return self._mk_inner_content()
@@ -149,12 +150,17 @@ class DataGridColumnsManager(MultipleInstance):
     return self.mk_column_details(col_def)
   
   def _register_formula(self, col_def) -> None:
-    """Register or remove a formula column with the FormulaEngine."""
+    """Register or remove a formula column with the FormulaEngine.
+
+    Registers only when col_def.type is Formula and the formula text is
+    non-empty.  Removes the formula in all other cases so the engine stays
+    consistent with the column definition.
+    """
     engine = self._parent.get_formula_engine()
     if engine is None:
       return
     table = self._parent.get_table_name()
-    if col_def.formula:
+    if col_def.type == ColumnType.Formula and col_def.formula:
       try:
         engine.set_formula(table, col_def.col_id, col_def.formula)
         logger.debug("Registered formula for %s.%s", table, col_def.col_id)

src/myfasthtml/controls/DslEditor.py

@@ -167,7 +167,7 @@ class DslEditor(MultipleInstance):
     return Textarea(
       self._state.content,
       id=f"ta_{self._id}",
-      name=f"ta_{self._id}",
+      name=self.conf.name if (self.conf and self.conf.name) else f"ta_{self._id}",
       cls="hidden",
     )
   

src/myfasthtml/controls/helpers.py

@@ -7,7 +7,7 @@ from myfasthtml.core.utils import merge_classes
 from myfasthtml.icons.fluent import question20_regular, brain_circuit20_regular, number_row20_regular, \
   number_symbol20_regular
 from myfasthtml.icons.fluent_p1 import checkbox_checked20_regular, checkbox_unchecked20_regular, \
-  checkbox_checked20_filled
+  checkbox_checked20_filled, math_formula16_regular
 from myfasthtml.icons.fluent_p2 import text_bullet_list_square20_regular, text_field20_regular
 from myfasthtml.icons.fluent_p3 import calendar_ltr20_regular
 
@@ -160,4 +160,5 @@ icons = {
     ColumnType.Datetime: calendar_ltr20_regular,
     ColumnType.Bool: checkbox_checked20_filled,
     ColumnType.Enum: text_bullet_list_square20_regular,
+    ColumnType.Formula: math_formula16_regular,
 }

src/myfasthtml/core/formula/engine.py

@@ -96,7 +96,9 @@ class FormulaEngine:
     # Registers in DAG and raises FormulaCycleError if cycle detected
     self._graph.add_formula(table, col, formula)
     self._formulas[(table, col)] = formula
-    
+    # Mark dirty so the column is computed on the next render
+    self._graph.mark_dirty(table, col)
+
     logger.debug("Formula set for %s.%s: %s", table, col, formula_text)
   
   def remove_formula(self, table: str, col: str) -> None:

tests/controls/test_datagrid_columns_manager.py

@@ -37,6 +37,9 @@ class MockDataGrid(MultipleInstance):
   def get_table_name(self):
     return "mock_table"
 
+  def get_formula_engine(self):
+    return None
+
 
 @pytest.fixture
 def mock_datagrid(root_instance):