Allow JSON score formatting without a signature

sacrebleu/metrics/base.py

@@ -83,9 +83,10 @@ def format(
             return sc
 
         if is_json:
-            for param in signature.split("|"):
-                key, value = param.split(":")
-                d[key] = value
+            if signature:
+                for param in signature.split("|"):
+                    key, value = param.split(":")
+                    d[key] = value
             return json.dumps(d, indent=1, ensure_ascii=False)
 
         return full_score

test/test_metrics_base.py

@@ -1,10 +1,25 @@
+import json
 from contextlib import nullcontext as does_not_raise
 
 import pytest
 
+from sacrebleu.metrics import BLEU, CHRF, TER
 from sacrebleu.metrics.base import Metric
 
 
+@pytest.mark.parametrize("metric_class", [BLEU, CHRF, TER])
+@pytest.mark.parametrize("kwargs", [{}, {"signature": ""}])
+def test_score_format_json_without_signature(metric_class, kwargs):
+    metric = metric_class()
+    score = metric.corpus_score(["The model learns from data."], [["The model learns from text."]])
+
+    result = json.loads(score.format(width=3, is_json=True, **kwargs))
+
+    assert result["name"] == score.name
+    assert result["score"] == round(score.score, 3)
+    assert result["signature"] == ""
+
+
 class ConcreteTestMetric(Metric):
     """Concrete test metric so the ABC can be instantiated."""