fix(metrics): stop spurious overwrite warnings from set_default_dimensions

Metrics.set_default_dimensions called provider.set_default_dimensions
and then re-added every dimension through add_dimension, so the second
pass always found the keys already registered and warned even on the
first call. Remove the redundant loop and delegate to the provider.

The provider also re-registers default dimensions internally, in
clear_metrics after every flush and on repeated set_default_dimensions
calls, which triggered the same warning on every warm invocation. Warn
only when a dimension is overwritten with a different value, matching
the warning message and the intent of #5653.

Closes #8402

Author: vishwakt

aws_lambda_powertools/metrics/metrics.py

@@ -174,7 +174,6 @@ def log_metrics(
         )
 
     def set_default_dimensions(self, **dimensions) -> None:
-        self.provider.set_default_dimensions(**dimensions)
         """Persist dimensions across Lambda invocations
 
         Parameters
@@ -195,9 +194,7 @@ def set_default_dimensions(self, **dimensions) -> None:
             def lambda_handler():
                 return True
         """
-        for name, value in dimensions.items():
-            self.add_dimension(name, value)
-
+        self.provider.set_default_dimensions(**dimensions)
         self.default_dimensions.update(**dimensions)
 
     def clear_default_dimensions(self) -> None:

aws_lambda_powertools/metrics/provider/cloudwatch_emf/cloudwatch.py

@@ -317,7 +317,7 @@ def add_dimension(self, name: str, value: str) -> None:
             )
             return
 
-        if name in self.dimension_set or name in self.default_dimensions:
+        if name in self.dimension_set and self.dimension_set[name] != value:
             warnings.warn(
                 f"Dimension '{name}' has already been added. The previous value will be overwritten.",
                 category=PowertoolsUserWarning,

tests/functional/metrics/required_dependencies/test_metrics_cloudwatch_emf.py

@@ -1133,6 +1133,80 @@ def test_clear_default_dimensions(namespace):
     assert not my_metrics.default_dimensions
 
 
+def test_set_default_dimensions_no_warning_on_first_call(namespace):
+    # GIVEN a Metrics instance with no dimensions set
+    my_metrics = Metrics(namespace=namespace)
+
+    # WHEN we persist default dimensions for the first time
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("default")
+        my_metrics.set_default_dimensions(environment="test", log_group="/lambda/test")
+
+    # THEN no overwrite warning should be emitted
+    assert not [warning for warning in w if "has already been added" in str(warning.message)]
+
+
+def test_set_default_dimensions_no_warning_when_unchanged(namespace):
+    # GIVEN a Metrics instance with default dimensions persisted
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.set_default_dimensions(environment="test", log_group="/lambda/test")
+
+    # WHEN we persist the same default dimensions again e.g., on a warm invocation
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("default")
+        my_metrics.set_default_dimensions(environment="test", log_group="/lambda/test")
+
+    # THEN no overwrite warning should be emitted
+    assert not [warning for warning in w if "has already been added" in str(warning.message)]
+
+
+def test_set_default_dimensions_warns_when_value_changes(namespace):
+    # GIVEN a Metrics instance with a default dimension persisted
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.set_default_dimensions(environment="test")
+
+    # WHEN we persist the same default dimension with a different value
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("default")
+        my_metrics.set_default_dimensions(environment="prod")
+
+    # THEN a single overwrite warning should be emitted
+    assert len([warning for warning in w if "has already been added" in str(warning.message)]) == 1
+
+
+def test_log_metrics_with_default_dimensions_no_warning_across_invocations(namespace, metric, capsys):
+    # GIVEN a Metrics instance with default dimensions persisted
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.set_default_dimensions(environment="test", log_group="/lambda/test")
+
+    @my_metrics.log_metrics
+    def lambda_handler(evt, ctx):
+        my_metrics.add_metric(**metric)
+
+    # WHEN metrics are flushed across multiple invocations
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("default")
+        lambda_handler({}, {})
+        lambda_handler({}, {})
+
+    # THEN no overwrite warning should be emitted
+    assert not [warning for warning in w if "has already been added" in str(warning.message)]
+
+
+def test_add_dimension_no_warning_when_value_unchanged(namespace):
+    # GIVEN a Metrics instance with a dimension added
+    my_metrics = Metrics(namespace=namespace)
+    my_metrics.add_dimension("environment", "test")
+
+    # WHEN the same dimension is added again with the same value
+    with warnings.catch_warnings(record=True) as w:
+        warnings.simplefilter("default")
+        my_metrics.add_dimension("environment", "test")
+
+    # THEN no overwrite warning should be emitted
+    assert not [warning for warning in w if "has already been added" in str(warning.message)]
+
+
 def test_add_dimensions_with_empty_value(namespace, capsys, metric):
     # GIVEN Metrics is initialized
     my_metrics = Metrics(namespace=namespace)