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The following commit(s) were added to
refs/heads/tdd/issue-21734-prophet-uncertainty-negative by this push:
new 51728d0aca3 test(prophet): pin yhat_lower can be negative for negative
series
51728d0aca3 is described below
commit 51728d0aca3ebc5d37ab3798548579794dd837c5
Author: Claude Code <[email protected]>
AuthorDate: Thu May 14 18:42:48 2026 -0700
test(prophet): pin yhat_lower can be negative for negative series
Closes #21734
Co-Authored-By: Claude Sonnet 4.6 <[email protected]>
---
.../pandas_postprocessing/test_prophet.py | 109 +++++++++++++++++++++
1 file changed, 109 insertions(+)
diff --git a/tests/unit_tests/pandas_postprocessing/test_prophet.py
b/tests/unit_tests/pandas_postprocessing/test_prophet.py
index c21dbb0d82b..193b70c4038 100644
--- a/tests/unit_tests/pandas_postprocessing/test_prophet.py
+++ b/tests/unit_tests/pandas_postprocessing/test_prophet.py
@@ -184,3 +184,112 @@ def test_prophet_incorrect_time_grain():
periods=10,
confidence_interval=0.8,
)
+
+
+def test_prophet_uncertainty_lower_bound_can_be_negative_for_negative_series():
+ """
+ Regression for #21734: when the input series contains negative values,
+ the forecast's lower confidence bound (``__yhat_lower``) must be allowed
+ to go below zero. The original bug claimed Superset clipped the lower
+ bound at 0, hiding the natural shape of the uncertainty interval for
+ series like temperatures or signed deltas.
+
+ Superset's wrapper passes through Prophet's output unchanged (no
+ clipping in ``superset/utils/pandas_postprocessing/prophet.py``); this
+ test pins that contract end-to-end. If a future refactor introduces
+ a ``max(0, lower)`` clamp, this test fails immediately.
+ """
+ if find_spec("prophet") is None:
+ pytest.skip("prophet not installed")
+
+ # All-negative monthly series — any reasonable forecast must predict
+ # negative values (and therefore negative uncertainty bounds) too.
+ negative_df = pd.DataFrame(
+ {
+ DTTM_ALIAS: [datetime(2020, m, 1) for m in range(1, 13)]
+ + [datetime(2021, m, 1) for m in range(1, 13)],
+ "temperature": [
+ -5.0,
+ -7.0,
+ -3.0,
+ 1.0,
+ 8.0,
+ 14.0,
+ 17.0,
+ 16.0,
+ 11.0,
+ 5.0,
+ -1.0,
+ -4.0,
+ -6.0,
+ -8.0,
+ -2.0,
+ 2.0,
+ 9.0,
+ 15.0,
+ 18.0,
+ 17.0,
+ 12.0,
+ 6.0,
+ 0.0,
+ -3.0,
+ ],
+ }
+ )
+
+ result = prophet(
+ df=negative_df,
+ time_grain="P1M",
+ periods=3,
+ confidence_interval=0.9,
+ )
+
+ assert "temperature__yhat_lower" in result.columns
+ # At least one forecast point's lower bound must be negative for a
+ # series whose actuals span both signs. A clamp at zero would force
+ # this assertion to fail.
+ assert (result["temperature__yhat_lower"] < 0).any(), (
+ "Forecast lower bound was non-negative everywhere despite a "
+ "series with negative actuals — suggests an unexpected clamp at "
+ "zero was reintroduced (regression of #21734)."
+ )
+
+
+def test_prophet_does_not_clamp_yhat_below_zero_for_negative_actuals():
+ """
+ Companion to the lower-bound test above: the central forecast
+ (``__yhat``) must also be allowed to go negative.
+ A bug that clamps the central forecast at zero would force the lower
+ bound non-negative as a side effect, masking the wider issue.
+ """
+ if find_spec("prophet") is None:
+ pytest.skip("prophet not installed")
+
+ negative_df = pd.DataFrame(
+ {
+ DTTM_ALIAS: [datetime(2020, m, 1) for m in range(1, 13)],
+ "balance": [
+ -100.0,
+ -110.0,
+ -95.0,
+ -120.0,
+ -130.0,
+ -125.0,
+ -140.0,
+ -135.0,
+ -150.0,
+ -145.0,
+ -160.0,
+ -155.0,
+ ],
+ }
+ )
+
+ result = prophet(
+ df=negative_df,
+ time_grain="P1M",
+ periods=2,
+ confidence_interval=0.8,
+ )
+
+ assert (result["balance__yhat"] < 0).any()
