Add _trial_type_to_metric_names to base Experiment (#5004)

Summary:
Pull Request resolved: #5004

This is Phase 1 of moving MultiTypeExperiment features into the base
Experiment class, enabling eventual deprecation of MultiTypeExperiment.

Adds `_trial_type_to_metric_names: dict[str, set[str]]` to Experiment —
a mapping from trial type to the set of metric names relevant to that
type. This is the natural complement to the existing
`_trial_type_to_runner` dict. Along with it, adds the following
properties and methods to Experiment:

- `trial_type_to_metric_names`: read-only property (shallow copy)
- `metric_to_trial_type`: computed inverse mapping, with optimization
  config metrics pinned to `default_trial_type`
- `metrics_for_trial_type(trial_type)`: returns Metric objects for a
  given trial type
- `default_trials`: returns trial indices matching the default type

MultiTypeExperiment is updated to populate `_trial_type_to_metric_names`
alongside `_metric_to_trial_type` in all mutation paths (init,
optimization_config setter, add/update/remove tracking metric). The
redundant MTE overrides for `metric_to_trial_type`,
`metrics_for_trial_type`, `default_trials`, and `default_trial_type`
are removed — they are now inherited from the base class.

The JSON decoder is updated to rebuild `_trial_type_to_metric_names`
from `_metric_to_trial_type` during deserialization for backward
compatibility.

Reviewed By: lena-kashtelyan

Differential Revision: D94970662

Author: mpolson64

ax/core/experiment.py

@@ -161,6 +161,11 @@ def __init__(
         self._trial_type_to_runner: dict[str | None, Runner | None] = {
             default_trial_type: runner
         }
+        # Maps each trial type to the set of metric names relevant to that type.
+        # This is the complement of _trial_type_to_runner and is used for
+        # multi-type experiments where different metrics apply to different
+        # trial types.
+        self._trial_type_to_metric_names: dict[str, set[str]] = {}
         # Used to keep track of whether any trials on the experiment
         # specify a TTL. Since trials need to be checked for their TTL's
         # expiration often, having this attribute helps avoid unnecessary
@@ -1928,6 +1933,57 @@ def default_trial_type(self) -> str | None:
         """
         return self._default_trial_type
 
+    @property
+    def trial_type_to_metric_names(self) -> dict[str, set[str]]:
+        """Map from trial type to the set of metric names relevant to that
+        type.
+
+        Returns a shallow copy of the internal mapping.
+        """
+        return dict(self._trial_type_to_metric_names)
+
+    @property
+    def metric_to_trial_type(self) -> dict[str, str]:
+        """Map each metric name to its associated trial type.
+
+        Computed from ``_trial_type_to_metric_names``. When a
+        ``default_trial_type`` is set and an ``optimization_config`` exists,
+        optimization config metrics are pinned to the default trial type.
+        """
+        result: dict[str, str] = {}
+        for trial_type, metric_names in self._trial_type_to_metric_names.items():
+            for name in metric_names:
+                result[name] = trial_type
+        opt_config = self._optimization_config
+        default_trial_type = self._default_trial_type
+        if default_trial_type is not None and opt_config is not None:
+            for metric_name in opt_config.metric_names:
+                result[metric_name] = default_trial_type
+        return result
+
+    def metrics_for_trial_type(self, trial_type: str) -> list[Metric]:
+        """Return the metrics associated with a given trial type.
+
+        Args:
+            trial_type: The trial type to look up metrics for.
+
+        Raises:
+            ValueError: If the trial type is not supported.
+        """
+        if not self.supports_trial_type(trial_type):
+            raise ValueError(f"Trial type `{trial_type}` is not supported.")
+        valid_names = self._trial_type_to_metric_names.get(trial_type, set())
+        return [self._metrics[name] for name in valid_names if name in self._metrics]
+
+    @property
+    def default_trials(self) -> set[int]:
+        """Return the indices for trials of the default type."""
+        return {
+            idx
+            for idx, trial in self.trials.items()
+            if trial.trial_type == self.default_trial_type
+        }
+
     def runner_for_trial_type(self, trial_type: str | None) -> Runner | None:
         """The default runner to use for a given trial type.
 

ax/core/multi_type_experiment.py

@@ -96,15 +96,16 @@ def __init__(
             default_data_type=default_data_type,
         )
 
-        # Ensure tracking metrics are registered in _metric_to_trial_type.
+        # Ensure tracking metrics are registered in _metric_to_trial_type
+        # and _trial_type_to_metric_names.
         # super().__init__ sets self._metrics directly, bypassing
         # add_tracking_metric, so tracking metrics won't be in
         # _metric_to_trial_type yet.
         for m in tracking_metrics or []:
             if m.name not in self._metric_to_trial_type:
-                self._metric_to_trial_type[m.name] = none_throws(
-                    self._default_trial_type
-                )
+                tt = none_throws(self._default_trial_type)
+                self._metric_to_trial_type[m.name] = tt
+                self._trial_type_to_metric_names.setdefault(tt, set()).add(m.name)
 
     def add_trial_type(self, trial_type: str, runner: Runner) -> Self:
         """Add a new trial_type to be supported by this experiment.
@@ -129,9 +130,9 @@ def optimization_config(self, optimization_config: OptimizationConfig) -> None:
         for metric_name in optimization_config.metric_names:
             # Optimization config metrics are required to be the default trial type
             # currently. TODO: remove that restriction (T202797235)
-            self._metric_to_trial_type[metric_name] = none_throws(
-                self.default_trial_type
-            )
+            tt = none_throws(self.default_trial_type)
+            self._metric_to_trial_type[metric_name] = tt
+            self._trial_type_to_metric_names.setdefault(tt, set()).add(metric_name)
 
     def update_runner(self, trial_type: str, runner: Runner) -> Self:
         """Update the default runner for an existing trial_type.
@@ -166,7 +167,9 @@ def add_tracking_metric(
             raise ValueError(f"`{trial_type}` is not a supported trial type.")
 
         super().add_tracking_metric(metric)
-        self._metric_to_trial_type[metric.name] = none_throws(trial_type)
+        tt = none_throws(trial_type)
+        self._metric_to_trial_type[metric.name] = tt
+        self._trial_type_to_metric_names.setdefault(tt, set()).add(metric.name)
         if canonical_name is not None:
             self._metric_to_canonical_name[metric.name] = canonical_name
         return self
@@ -242,7 +245,14 @@ def update_tracking_metric(
             raise ValueError(f"`{trial_type}` is not a supported trial type.")
 
         super().update_tracking_metric(metric)
-        self._metric_to_trial_type[metric.name] = none_throws(trial_type)
+        # Remove from old trial type set
+        old_tt = self._metric_to_trial_type.get(metric.name)
+        if old_tt is not None and old_tt in self._trial_type_to_metric_names:
+            self._trial_type_to_metric_names[old_tt].discard(metric.name)
+        # Add to new trial type set
+        tt = none_throws(trial_type)
+        self._metric_to_trial_type[metric.name] = tt
+        self._trial_type_to_metric_names.setdefault(tt, set()).add(metric.name)
         if canonical_name is not None:
             self._metric_to_canonical_name[metric.name] = canonical_name
         return self
@@ -252,6 +262,11 @@ def remove_tracking_metric(self, metric_name: str) -> Self:
         if metric_name not in self._metrics:
             raise ValueError(f"Metric `{metric_name}` doesn't exist on experiment.")
 
+        # Clean up _trial_type_to_metric_names
+        old_tt = self._metric_to_trial_type.get(metric_name)
+        if old_tt is not None and old_tt in self._trial_type_to_metric_names:
+            self._trial_type_to_metric_names[old_tt].discard(metric_name)
+
         # Required fields
         del self._metrics[metric_name]
         del self._metric_to_trial_type[metric_name]
@@ -295,46 +310,6 @@ def _fetch_trial_data(
         # Invoke parent's fetch method using only metrics for this trial_type
         return super()._fetch_trial_data(trial.index, metrics=metrics, **kwargs)
 
-    @property
-    def default_trials(self) -> set[int]:
-        """Return the indicies for trials of the default type."""
-        return {
-            idx
-            for idx, trial in self.trials.items()
-            if trial.trial_type == self.default_trial_type
-        }
-
-    @property
-    def metric_to_trial_type(self) -> dict[str, str]:
-        """Map metrics to trial types.
-
-        Adds in default trial type for OC metrics to custom defined trial types..
-        """
-        opt_config_types = {
-            metric_name: self.default_trial_type
-            for metric_name in self.optimization_config.metric_names
-        }
-        return {**opt_config_types, **self._metric_to_trial_type}
-
-    # -- Overridden functions from Base Experiment Class --
-    @property
-    def default_trial_type(self) -> str | None:
-        """Default trial type assigned to trials in this experiment."""
-        return self._default_trial_type
-
-    def metrics_for_trial_type(self, trial_type: str) -> list[Metric]:
-        """The default runner to use for a given trial type.
-
-        Looks up the appropriate runner for this trial type in the trial_type_to_runner.
-        """
-        if not self.supports_trial_type(trial_type):
-            raise ValueError(f"Trial type `{trial_type}` is not supported.")
-        return [
-            self.metrics[metric_name]
-            for metric_name, metric_trial_type in self._metric_to_trial_type.items()
-            if metric_trial_type == trial_type
-        ]
-
     def supports_trial_type(self, trial_type: str | None) -> bool:
         """Whether this experiment allows trials of the given type.
 

ax/storage/json_store/decoder.py

@@ -720,6 +720,12 @@ def multi_type_experiment_from_json(
     experiment._metric_to_trial_type = _metric_to_trial_type
     experiment._trial_type_to_runner = _trial_type_to_runner
 
+    # Rebuild _trial_type_to_metric_names from _metric_to_trial_type
+    trial_type_to_metric_names: dict[str, set[str]] = {}
+    for metric_name, trial_type in _metric_to_trial_type.items():
+        trial_type_to_metric_names.setdefault(trial_type, set()).add(metric_name)
+    experiment._trial_type_to_metric_names = trial_type_to_metric_names
+
     _load_experiment_info(
         exp=experiment,
         exp_info=experiment_info,