Checks/errors related to ill-formed IOB input in CLI convert and debug-data (#4487)

* Error for ill-formed input to iob_to_biluo()

Check for empty label in iob_to_biluo(), which can result from
ill-formed input.

* Check for empty NER label in debug-data

spacy/cli/debug_data.py

@@ -206,6 +206,9 @@ def debug_data(
                 missing_values, "value" if missing_values == 1 else "values"
             )
         )
+        for label in new_labels:
+            if len(label) == 0:
+                msg.fail("Empty label found in new labels")
         if new_labels:
             labels_with_counts = [
                 (label, count)

spacy/errors.py

@@ -503,6 +503,7 @@ class Errors(object):
             "names: {names}")
     E175 = ("Can't remove rule for unknown match pattern ID: {key}")
     E176 = ("Alias '{alias}' is not defined in the Knowledge Base.")
+    E177 = ("Ill-formed IOB input detected: {tag}")
 
 
 @add_codes

spacy/gold.pyx

@@ -522,6 +522,8 @@ def _consume_ent(tags):
         tags.pop(0)
     label = tag[2:]
     if length == 1:
+        if len(label) == 0:
+            raise ValueError(Errors.E177.format(tag=tag))
         return ["U-" + label]
     else:
         start = "B-" + label

spacy/tests/test_gold.py

@@ -2,7 +2,7 @@
 from __future__ import unicode_literals
 
 from spacy.gold import biluo_tags_from_offsets, offsets_from_biluo_tags
-from spacy.gold import spans_from_biluo_tags, GoldParse
+from spacy.gold import spans_from_biluo_tags, GoldParse, iob_to_biluo
 from spacy.gold import GoldCorpus, docs_to_json
 from spacy.lang.en import English
 from spacy.tokens import Doc
@@ -87,6 +87,16 @@ def test_gold_ner_missing_tags(en_tokenizer):
     gold = GoldParse(doc, entities=biluo_tags)  # noqa: F841
 
 
+def test_iob_to_biluo():
+    good_iob = ["O", "O", "B-LOC", "I-LOC", "O", "B-PERSON"]
+    good_biluo = ["O", "O", "B-LOC", "L-LOC", "O", "U-PERSON"]
+    bad_iob = ["O", "O", "\"", "B-LOC", "I-LOC"]
+    converted_biluo = iob_to_biluo(good_iob)
+    assert good_biluo == converted_biluo
+    with pytest.raises(ValueError):
+        iob_to_biluo(bad_iob)
+
+
 def test_roundtrip_docs_to_json():
     text = "I flew to Silicon Valley via London."
     cats = {"TRAVEL": 1.0, "BAKING": 0.0}