Fix bug in metric tracker (#1306)

* fix + test

* changelog

CHANGELOG.md

@@ -52,6 +52,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed restrictive dtype checking in `spearman_corrcoef` when used with autocast ([#1303](https://github.com/Lightning-AI/metrics/pull/1303))
 
 
+- Fixed bug in `Metrictracker.best_metric` when `return_step=False` ([#1306](https://github.com/Lightning-AI/metrics/pull/1306))
+
+
 ## [0.10.1] - 2022-10-21
 
 ### Fixed

src/torchmetrics/wrappers/tracker.py

@@ -11,7 +11,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import warnings
 from copy import deepcopy
 from typing import Any, Dict, List, Tuple, Union
 
@@ -21,6 +20,7 @@
 
 from torchmetrics.collections import MetricCollection
 from torchmetrics.metric import Metric
+from torchmetrics.utilities.prints import rank_zero_warn
 
 
 class MetricTracker(ModuleList):
@@ -171,12 +171,12 @@ def best_metric(
         if isinstance(self._base_metric, Metric):
             fn = torch.max if self.maximize else torch.min
             try:
-                idx, best = fn(self.compute_all(), 0)
+                value, idx = fn(self.compute_all(), 0)
                 if return_step:
-                    return idx.item(), best.item()
-                return best.item()
+                    return value.item(), idx.item()
+                return value.item()
             except ValueError as error:
-                warnings.warn(
+                rank_zero_warn(
                     f"Encountered the following error when trying to get the best metric: {error}"
                     "this is probably due to the 'best' not being defined for this metric."
                     "Returning `None` instead.",
@@ -189,24 +189,24 @@ def best_metric(
         else:  # this is a metric collection
             res = self.compute_all()
             maximize = self.maximize if isinstance(self.maximize, list) else len(res) * [self.maximize]
-            idx, best = {}, {}
+            value, idx = {}, {}
             for i, (k, v) in enumerate(res.items()):
                 try:
                     fn = torch.max if maximize[i] else torch.min
                     out = fn(v, 0)
-                    idx[k], best[k] = out[0].item(), out[1].item()
+                    value[k], idx[k] = out[0].item(), out[1].item()
                 except ValueError as error:
-                    warnings.warn(
+                    rank_zero_warn(
                         f"Encountered the following error when trying to get the best metric for metric {k}:"
                         f"{error} this is probably due to the 'best' not being defined for this metric."
                         "Returning `None` instead.",
                         UserWarning,
                     )
-                    idx[k], best[k] = None, None
+                    value[k], idx[k] = None, None
 
             if return_step:
-                return idx, best
-            return best
+                return value, idx
+            return value
 
     def _check_for_increment(self, method: str) -> None:
         if not self._increment_called:

tests/unittests/wrappers/test_tracker.py

@@ -126,6 +126,9 @@ def test_tracker(base_metric, metric_input, maximize):
         assert val != 0.0
         assert idx in list(range(5))
 
+    val2 = tracker.best_metric(return_step=False)
+    assert val == val2
+
 
 @pytest.mark.parametrize(
     "base_metric",