Preserve zero-mean confidence intervals in score output

sacrebleu/metrics/base.py

@@ -45,6 +45,9 @@ def format(
         is_json: bool = False,
     ) -> str:
         """Returns a pretty representation of the score.
+
+        Includes a computed confidence interval even when its mean is zero.
+
         :param width: Floating point decimal precision width.
         :param score_only: If `True`, and the format is not `json`,
         returns a single score string.
@@ -61,7 +64,7 @@ def format(
 
         sc = f"{self.score:.{width}f}"
 
-        if self._mean > 0:
+        if self._mean >= 0:
             confidence_mean = f"{self._mean:.{width}f}"
             confidence_var = f"{self._ci:.{width}f}"
             confidence_str = f"μ = {confidence_mean} ± {confidence_var}"

test/test_metrics_base.py

@@ -1,8 +1,9 @@
+import json
 from contextlib import nullcontext as does_not_raise
 
 import pytest
 
-from sacrebleu.metrics.base import Metric
+from sacrebleu.metrics.base import Metric, Score
 
 
 class ConcreteTestMetric(Metric):
@@ -353,3 +354,35 @@ def test_check_corpus_score_args(hyps, refs, expected_context):
     metric = ConcreteTestMetric()
     with expected_context:
         metric._check_corpus_score_args(hyps=hyps, refs=refs)
+
+
+@pytest.mark.parametrize("mean", [0.0, 1.0])
+def test_score_format_confidence_interval(mean):
+    score = Score("TEST", mean)
+    score.estimate_ci([score] * 40)
+    confidence = f"\u03bc = {mean:.2f} \u00b1 0.00"
+
+    assert score.format() == f"TEST = {mean:.2f} ({confidence})"
+    assert score.format(score_only=True) == f"{mean:.2f} ({confidence})"
+    assert json.loads(score.format(signature="version:test", is_json=True)) == {
+        "name": "TEST",
+        "score": mean,
+        "signature": "version:test",
+        "version": "test",
+        "confidence_mean": mean,
+        "confidence_var": 0.0,
+        "confidence": confidence,
+    }
+
+
+def test_score_format_without_confidence_interval():
+    score = Score("TEST", 0.0)
+
+    assert score.format() == "TEST = 0.00"
+    assert score.format(score_only=True) == "0.00"
+    assert json.loads(score.format(signature="version:test", is_json=True)) == {
+        "name": "TEST",
+        "score": 0.0,
+        "signature": "version:test",
+        "version": "test",
+    }