feat: use the variant as the exposure event value

When `get_experiment_flag` records a `$flag_exposure` event, send the
selected variant as the event value when present (the variant's key, or
"control" for the control bucket), falling back to the flag value for
standard flags. This identifies the experiment cohort without changing
the event shape.

Author: gagantrivedi

flagsmith/flagsmith.py

@@ -362,11 +362,13 @@ def get_experiment_flag(
         if not self._event_processor:
             raise ValueError("Events must be enabled to use experiment flags.")
         flag = self.get_identity_flags(identifier, traits).get_flag(feature_name)
-        if not flag.is_default:
+        if isinstance(flag, Flag):
             self.track_exposure_event(
                 feature_name=feature_name,
                 identifier=identifier,
-                value=flag.value,
+                # The variant identifies the cohort for multivariate experiments;
+                # fall back to the value for standard flags.
+                value=flag.variant if flag.variant is not None else flag.value,
                 traits=traits,
             )
         return flag

tests/test_flagsmith.py

@@ -1116,3 +1116,73 @@ def default_flag_handler(feature_name: str) -> DefaultFlag:
     assert result.is_default is True
     assert result.value == "default-variant"
     mock_track.assert_not_called()
+
+
+def test_get_experiment_flag_uses_variant_as_exposure_value(
+    mocker: MockerFixture, api_key: str
+) -> None:
+    # Given - a resolved flag carrying a variant
+    config = EventProcessorConfig(events_api_url="http://test/")
+    flagsmith = Flagsmith(
+        environment_key=api_key, enable_events=True, event_processor_config=config
+    )
+    flag = Flag(
+        enabled=True,
+        value="blue",
+        feature_name="checkout_v2",
+        feature_id=1,
+        variant="control",
+    )
+    mocker.patch.object(
+        flagsmith,
+        "get_identity_flags",
+        return_value=Flags(flags={"checkout_v2": flag}),
+    )
+    mock_track = mocker.patch.object(flagsmith._event_processor, "track_exposure_event")
+
+    # When
+    flagsmith.get_experiment_flag(feature_name="checkout_v2", identifier="user1")
+
+    # Then - the exposure value is the variant, not the flag value
+    mock_track.assert_called_once_with(
+        feature_name="checkout_v2",
+        identifier="user1",
+        value="control",
+        traits=None,
+        metadata=None,
+    )
+
+
+def test_get_experiment_flag_falls_back_to_value_without_variant(
+    mocker: MockerFixture, api_key: str
+) -> None:
+    # Given - a resolved flag with no variant
+    config = EventProcessorConfig(events_api_url="http://test/")
+    flagsmith = Flagsmith(
+        environment_key=api_key, enable_events=True, event_processor_config=config
+    )
+    flag = Flag(
+        enabled=True,
+        value="blue",
+        feature_name="checkout_v2",
+        feature_id=1,
+        variant=None,
+    )
+    mocker.patch.object(
+        flagsmith,
+        "get_identity_flags",
+        return_value=Flags(flags={"checkout_v2": flag}),
+    )
+    mock_track = mocker.patch.object(flagsmith._event_processor, "track_exposure_event")
+
+    # When
+    flagsmith.get_experiment_flag(feature_name="checkout_v2", identifier="user1")
+
+    # Then - the exposure value falls back to the flag value
+    mock_track.assert_called_once_with(
+        feature_name="checkout_v2",
+        identifier="user1",
+        value="blue",
+        traits=None,
+        metadata=None,
+    )