[io] Use the artefact confidence as the objectness score of a block without words

doctr/io/elements.py

@@ -401,9 +401,11 @@ def __init__(
             lines = []
         if artefacts is None:
             artefacts = []
-        # Compute the objectness score of the line
+        # Compute the objectness score of the block
         if objectness_score is None:
-            objectness_score = float(np.mean([w.objectness_score for line in lines for w in line.words]))
+            # A block without any word (only artefacts) takes the confidence of its artefacts, instead of NaN
+            scores = [w.objectness_score for line in lines for w in line.words] or [a.confidence for a in artefacts]
+            objectness_score = float(np.mean(scores))
         # Resolve the geometry using the smallest enclosing bounding box
         if geometry is None:
             line_boxes = [word.geometry for line in lines for word in line.words]

tests/common/test_io_elements.py

@@ -448,6 +448,9 @@ def test_block_artefacts_only():
     assert block.lines == []
     assert block.geometry == ((0, 0), (1, 1))
     assert block.render() == ""
+    # The objectness score is the confidence of the artefacts, which keeps the export valid JSON
+    assert block.objectness_score == 0.8
+    json.loads(json.dumps(block.export(), allow_nan=False))
 
 
 def test_block_rotated_lines_from_dict():