Cap max values for choice parameters (#1686)

Summary:
Pull Request resolved: #1686

See title

Reviewed By: esantorella

Differential Revision: D47006938

fbshipit-source-id: 8e291bf743ed22a6985ecef6b0da53200baf842c

ax/core/parameter.py

@@ -22,7 +22,7 @@
 # TODO: Do a more comprehensive audit of how floating point precision issues
 # may creep up and implement a more principled fix
 EPS = 1.5e-7
-
+MAX_VALUES_CHOICE_PARAM = 1000
 FIXED_CHOICE_PARAM_ERROR = (
     "ChoiceParameters require multiple feasible values. "
     "Please use FixedParameter instead when setting a single possible value."
@@ -474,6 +474,12 @@ def __init__(
         # A choice parameter with only one value is a FixedParameter.
         if not len(values) > 1:
             raise UserInputError(f"{self._name}({values}): {FIXED_CHOICE_PARAM_ERROR}")
+        # Cap the number of possible values
+        if len(values) > MAX_VALUES_CHOICE_PARAM:
+            raise UserInputError(
+                f"`ChoiceParameter` with more than {MAX_VALUES_CHOICE_PARAM} values "
+                "is not supported! Use a `RangeParameter` instead."
+            )
         # pyre-fixme[4]: Attribute must be annotated.
         self._values = self._cast_values(values)
         # pyre-fixme[4]: Attribute must be annotated.

ax/core/tests/test_parameter.py

@@ -315,6 +315,23 @@ def testHierarchicalValidation(self) -> None:
                 dependents={"not_a_value": "other_param"},
             )
 
+    def testMaxValuesValidation(self) -> None:
+        ChoiceParameter(
+            name="x",
+            parameter_type=ParameterType.INT,
+            values=list(range(999)),  # pyre-ignore
+        )
+        with self.assertRaisesRegex(
+            UserInputError,
+            "`ChoiceParameter` with more than 1000 values is not supported! Use a "
+            "`RangeParameter` instead.",
+        ):
+            ChoiceParameter(
+                name="x",
+                parameter_type=ParameterType.INT,
+                values=list(range(1001)),  # pyre-ignore
+            )
+
     def testHierarchical(self) -> None:
         # Test case where only some of the values entail dependents.
         hierarchical_param = ChoiceParameter(