Validate all sympy-parsed inputs early with clear error messages (#5196)

Summary:

1. **`ax/core/parameter.py` (`DerivedParameter._parse_expression_str`)**: Wrapped bare `sympify()` call in try/except to convert `SympifyError` to `UserInputError` with a descriptive message.

2. **`ax/utils/common/sympy.py` (`parse_objective_expression`)**: Wrapped bare `sympify()` call in try/except to convert `SympifyError` to `UserInputError`.

3. **`ax_core_instantiation_utils.py` (`_make_objectives`)**: Migrated from `MultiObjective(objectives=[...])` to the new expression-based `Objective(expression=..., metric_name_to_signature=...)` API. This is a behavioral change: objectives are now constructed using a single `Objective` with a comma-separated expression string instead of wrapping individual `Objective` instances in a `MultiObjective`. The corresponding test in `base_utils_test.py` is updated to match.

Differential Revision: D100058027

Author: saitcakmak

ax/core/parameter.py

@@ -31,7 +31,7 @@
 from sympy.core.mul import Mul
 from sympy.core.numbers import Float, Integer
 from sympy.core.symbol import Symbol
-from sympy.core.sympify import sympify
+from sympy.core.sympify import sympify, SympifyError
 
 logger: Logger = get_logger(__name__)
 
@@ -1342,7 +1342,13 @@ def _parse_expression_str(self, expression_str: str) -> None:
 
         Currently only linear functions are supported.
         """
-        expression = sympify(sanitize_name(expression_str))
+        try:
+            expression = sympify(sanitize_name(expression_str))
+        except SympifyError as e:
+            raise UserInputError(
+                f"Unable to parse derived parameter expression: "
+                f"{expression_str}. Error: {e}"
+            ) from e
         if isinstance(expression, (Float, Integer)):
             raise UserInputError(
                 "Derived parameters must have at least one parameter in "

ax/core/tests/test_objective.py

@@ -412,6 +412,14 @@ def test_SpecialCharMetricNames(self) -> None:
             parsed = parse_objective_expression(names[0])
             self.assertNotEqual(str(parsed), names[0])
 
+    def test_parse_objective_expression_sympify_error(self) -> None:
+        """Test that unparseable expressions raise UserInputError."""
+        with self.assertRaisesRegex(
+            UserInputError,
+            "Unable to parse objective expression",
+        ):
+            parse_objective_expression("m1 +* m2")
+
     def test_UniqueId(self) -> None:
         """Test _unique_id used for sorting."""
         obj = Objective(expression="m1", metric_name_to_signature={"m1": "m1"})

ax/core/tests/test_parameter.py

@@ -1128,6 +1128,17 @@ def test_invalid_inputs(self) -> None:
                 name="x", parameter_type=ParameterType.FLOAT, expression_str="y ** 2"
             )
 
+        # test unparseable expression
+        with self.assertRaisesRegex(
+            UserInputError,
+            "Unable to parse derived parameter expression",
+        ):
+            DerivedParameter(
+                name="x",
+                parameter_type=ParameterType.FLOAT,
+                expression_str="a +* b",
+            )
+
     def test_eq(self) -> None:
         param2 = DerivedParameter(
             name="x", parameter_type=ParameterType.FLOAT, expression_str="2.0 * a + 1.0"

ax/utils/common/sympy.py

@@ -100,7 +100,12 @@ def parse_objective_expression(expression_str: str) -> Expr | tuple[Expr, ...]:
         raise UserInputError("Objective expression string must not be empty.")
 
     sanitized = sanitize_name(expression_str, sanitize_parens=True)
-    parsed = sympify(sanitized)
+    try:
+        parsed = sympify(sanitized)
+    except SympifyError as e:
+        raise UserInputError(
+            f"Unable to parse objective expression: {expression_str}. Error: {e}"
+        ) from e
 
     if isinstance(parsed, tuple):
         if any(not isinstance(p, Expr) for p in parsed):