Fix delete_topics when custom labels are set

bertopic/_bertopic.py

@@ -2193,6 +2193,10 @@ def delete_topics(
         # Check if -1 exists in the current topics
         had_outliers = -1 in set(self.topics_)
 
+        # `custom_labels_` is a list ordered by topic rather than a mapping, so keep
+        # track of the topics it currently refers to in order to remap it below
+        custom_labels_topics = sorted(set(self.topics_))
+
         # If adding -1 for the first time, initialize its attributes
         if not had_outliers and any(topic in topics_to_delete for topic in self.topics_):
             # Initialize c-TF-IDF for -1 topic (zeros)
@@ -2215,9 +2219,11 @@ def delete_topics(
                 outlier_image = np.zeros((1, self.representative_images_.shape[1]))
                 self.representative_images_ = np.vstack([outlier_image, self.representative_images_])
 
-            # Initialize custom labels for -1 topic if they exist
+            # Initialize custom labels for -1 topic if they exist. Topic -1 sorts
+            # first, so its label is prepended rather than assigned to index -1.
             if hasattr(self, "custom_labels_") and self.custom_labels_ is not None:
-                self.custom_labels_[-1] = ""
+                self.custom_labels_.insert(0, "")
+                custom_labels_topics.insert(0, -1)
 
             # Initialize ctfidf model diagonal for -1 topic (ones) if it exists
             if hasattr(self, "ctfidf_model") and self.ctfidf_model is not None:
@@ -2270,12 +2276,13 @@ def delete_topics(
 
         # Update custom labels if they exist
         if hasattr(self, "custom_labels_") and self.custom_labels_ is not None:
-            new_labels = {
-                (final_mapping[old_topic] if old_topic != -1 else -1): label
-                for old_topic, label in self.custom_labels_.items()
-                if old_topic not in topics_to_delete
-            }
-            self.custom_labels_ = new_labels
+            if len(self.custom_labels_) == len(custom_labels_topics):
+                new_labels = {
+                    (final_mapping[old_topic] if old_topic != -1 else -1): label
+                    for old_topic, label in zip(custom_labels_topics, self.custom_labels_)
+                    if old_topic not in topics_to_delete
+                }
+                self.custom_labels_ = [new_labels[topic] for topic in sorted(new_labels)]
 
         # Update topic representations
         new_representations = {

tests/test_reduction/test_delete.py

@@ -57,3 +57,43 @@ def test_delete(model, request):
         assert mapped_labels == topic_model.topics_[950:]
     else:
         assert mapped_labels == topic_model.topics_
+
+
+@pytest.mark.parametrize(
+    "model",
+    [
+        ("kmeans_pca_topic_model"),
+        ("base_topic_model"),
+        ("custom_topic_model"),
+        ("merged_topic_model"),
+        ("reduced_topic_model"),
+        ("online_topic_model"),
+    ],
+)
+def test_delete_with_custom_labels(model, request):
+    """Custom labels are a list, so deleting topics must remap them positionally."""
+    topic_model = copy.deepcopy(request.getfixturevalue(model))
+
+    # Label every topic after itself so misalignment is detectable
+    original_topics = sorted(set(topic_model.topics_))
+    topic_model.set_topic_labels([f"label of topic {topic}" for topic in original_topics])
+    labels_before = dict(zip(original_topics, topic_model.custom_labels_))
+
+    topics_to_delete = [topic for topic in original_topics if topic != -1][:2]
+    topic_model.delete_topics(topics_to_delete)
+
+    remaining_topics = sorted(set(topic_model.topics_))
+    assert isinstance(topic_model.custom_labels_, list)
+    assert len(topic_model.custom_labels_) == len(remaining_topics)
+
+    # Every surviving topic keeps its own label, even though topics are renumbered
+    labels_after = dict(zip(remaining_topics, topic_model.custom_labels_))
+    mappings = topic_model.topic_mapper_.get_mappings(original_topics=False)
+    for topic in original_topics:
+        if topic in topics_to_delete or topic == -1:
+            continue
+        assert labels_after[mappings[topic]] == labels_before[topic]
+
+    # A newly created outlier topic gets an empty label rather than stealing one
+    if -1 not in original_topics:
+        assert labels_after[-1] == ""