Engine: Save Model asks before it writes over a Python cell

Save Model (new on main) asks before it replaces data in the column it writes
to, but it looked for a value or a formula only, so a Python cell whose cached
value is empty was taken for an empty cell and its code was replaced without
the question. Python code now counts as content there, as it does elsewhere.

crates/engine/src/cmd/solver/model_range.rs

@@ -107,12 +107,12 @@ fn check_column_spills(sheet: &Sheet, column: RangeRef, ours: &[CellRef]) -> Res
     Ok(())
 }
 
-/// Refused, unless `overwrite`, when a cell of the model's column holds a value or a formula
-/// (a model saved there before included): Save Model then asks before it replaces data, as
-/// Excel does. The message carries [`OVERWRITE_ERROR`], which the dialog turns into its
+/// Refused, unless `overwrite`, when a cell of the model's column holds a value, a formula or
+/// Python code (a model saved there before included): Save Model then asks before it replaces
+/// data, as Excel does. The message carries [`OVERWRITE_ERROR`], which the dialog turns into its
 /// question.
 fn check_column_empty(sheet: &Sheet, column: RangeRef, overwrite: bool) -> Result<()> {
-    let taken = column.iter().find(|c| sheet.cell(*c).is_some_and(|x| !x.value.is_empty() || x.formula.is_some()));
+    let taken = column.iter().find(|c| sheet.cell(*c).is_some_and(Cell::has_content));
     match taken {
         Some(c) if !overwrite => Err(EngineError::Other(format!(
             "The range {} {OVERWRITE_ERROR} ({} isn't empty); pass overwrite: true to replace it.",

crates/engine/src/tests/python_cells.rs

@@ -349,3 +349,25 @@ fn unmerging_lets_a_blocked_python_result_spill_again() {
     s.execute("home.unmergeCells", json!({"range": "A2:B2"})).unwrap();
     assert_eq!((super::v(&s, "A1"), super::v(&s, "A2")), (Value::Number(5.0), Value::Number(6.0)), "it spills again");
 }
+
+#[test]
+fn save_model_asks_before_it_writes_over_a_python_cell_that_shows_nothing() {
+    let mut s = s();
+    for (cell, input) in [("B1", "=5*A1+4*A2"), ("B2", "=6*A1+4*A2")] {
+        s.execute("cell.set", json!({"cell": cell, "input": input})).unwrap();
+    }
+    // H2 is a Python cell whose cached value is empty: its code is still content.
+    {
+        let d = s.doc_mut().unwrap();
+        let wb = Arc::make_mut(&mut d.wb);
+        let sh = wb.sheet_mut(0).unwrap();
+        sh.set_cell(
+            CellRef::parse("H2").unwrap(),
+            Cell { value: Value::Empty, formula: None, style: Default::default(), py: Some(Arc::new(PyCode::new("None"))) },
+        );
+    }
+    let model = json!({"objective": "B1", "variables": "A1:A2", "constraints": ["B2 <= 24"]});
+    let e = s.execute("data.solverSaveModel", json!({"to": "H1", "model": model})).unwrap_err().to_string();
+    assert!(e.contains(crate::cmd::analysis::OVERWRITE_ERROR) && e.contains("H2"), "{e}");
+    assert_eq!(code(&s, 0, "H2").as_deref(), Some("None"), "nothing was written");
+}