fix(markdown): fix empty block handling (#843)

Signed-off-by: Panos Vagenas <35837085+vagenas@users.noreply.github.com>

docling/backend/md_backend.py

@@ -6,6 +6,7 @@ from pathlib import Path
 from typing import List, Optional, Set, Union
 
 import marko
+import marko.element
 import marko.ext
 import marko.ext.gfm
 import marko.inline
@@ -163,14 +164,14 @@ class MarkdownDocumentBackend(DeclarativeDocumentBackend):
 
     def iterate_elements(
         self,
-        element: marko.block.Element,
+        element: marko.element.Element,
         depth: int,
         doc: DoclingDocument,
         parent_element: Optional[NodeItem] = None,
     ):
         # Iterates over all elements in the AST
         # Check for different element types and process relevant details
-        if isinstance(element, marko.block.Heading):
+        if isinstance(element, marko.block.Heading) and len(element.children) > 0:
             self.close_table(doc)
             self.process_inline_text(parent_element, doc)
             _log.debug(
@@ -205,17 +206,22 @@ class MarkdownDocumentBackend(DeclarativeDocumentBackend):
                 )
 
         elif isinstance(element, marko.block.List):
+            has_non_empty_list_items = False
+            for child in element.children:
+                if isinstance(child, marko.block.ListItem) and len(child.children) > 0:
+                    has_non_empty_list_items = True
+                    break
+
             self.close_table(doc)
             self.process_inline_text(parent_element, doc)
             _log.debug(f" - List {'ordered' if element.ordered else 'unordered'}")
-            list_label = GroupLabel.LIST
+            if has_non_empty_list_items:
-            if element.ordered:
+                label = GroupLabel.ORDERED_LIST if element.ordered else GroupLabel.LIST
-                list_label = GroupLabel.ORDERED_LIST
+                parent_element = doc.add_group(
-            parent_element = doc.add_group(
+                    label=label, name=f"list", parent=parent_element
-                label=list_label, name=f"list", parent=parent_element
+                )
-            )
 
-        elif isinstance(element, marko.block.ListItem):
+        elif isinstance(element, marko.block.ListItem) and len(element.children) > 0:
             self.close_table(doc)
             self.process_inline_text(parent_element, doc)
             _log.debug(" - List item")
@@ -245,20 +251,18 @@ class MarkdownDocumentBackend(DeclarativeDocumentBackend):
 
             doc.add_picture(parent=parent_element, caption=fig_caption)
 
-        elif isinstance(element, marko.block.Paragraph):
+        elif isinstance(element, marko.block.Paragraph) and len(element.children) > 0:
             self.process_inline_text(parent_element, doc)
 
         elif isinstance(element, marko.inline.RawText):
             _log.debug(f" - Paragraph (raw text): {element.children}")
-            snippet_text = str(element.children).strip()
+            snippet_text = element.children.strip()
             # Detect start of the table:
             if "|" in snippet_text:
                 # most likely part of the markdown table
                 self.in_table = True
                 if len(self.md_table_buffer) > 0:
-                    self.md_table_buffer[len(self.md_table_buffer) - 1] += str(
+                    self.md_table_buffer[len(self.md_table_buffer) - 1] += snippet_text
-                        snippet_text
-                    )
                 else:
                     self.md_table_buffer.append(snippet_text)
             else:
@@ -274,18 +278,15 @@ class MarkdownDocumentBackend(DeclarativeDocumentBackend):
             snippet_text = str(element.children).strip()
             doc.add_code(parent=parent_element, text=snippet_text)
 
-        elif isinstance(element, marko.block.CodeBlock):
+        elif (
+            isinstance(element, (marko.block.CodeBlock, marko.block.FencedCode))
+            and len(element.children) > 0
+            and isinstance((first_child := element.children[0]), marko.inline.RawText)
+            and len(snippet_text := (first_child.children.strip())) > 0
+        ):
             self.close_table(doc)
             self.process_inline_text(parent_element, doc)
             _log.debug(f" - Code Block: {element.children}")
-            snippet_text = str(element.children[0].children).strip()  # type: ignore
-            doc.add_code(parent=parent_element, text=snippet_text)
-
-        elif isinstance(element, marko.block.FencedCode):
-            self.close_table(doc)
-            self.process_inline_text(parent_element, doc)
-            _log.debug(f" - Code Block: {element.children}")
-            snippet_text = str(element.children[0].children).strip()  # type: ignore
             doc.add_code(parent=parent_element, text=snippet_text)
 
         elif isinstance(element, marko.inline.LineBreak):
@@ -309,14 +310,21 @@ class MarkdownDocumentBackend(DeclarativeDocumentBackend):
                 self.close_table(doc)
                 _log.debug("Some other element: {}".format(element))
 
+        processed_block_types = (
+            marko.block.ListItem,
+            marko.block.Heading,
+            marko.block.CodeBlock,
+            marko.block.FencedCode,
+            # marko.block.Paragraph,
+            marko.inline.RawText,
+        )
+
         # Iterate through the element's children (if any)
-        if not isinstance(element, marko.block.ListItem):
+        if hasattr(element, "children") and not isinstance(
-            if not isinstance(element, marko.block.Heading):
+            element, processed_block_types
-                if not isinstance(element, marko.block.FencedCode):
+        ):
-                    # if not isinstance(element, marko.block.Paragraph):
+            for child in element.children:
-                    if hasattr(element, "children"):
+                self.iterate_elements(child, depth + 1, doc, parent_element)
-                        for child in element.children:
-                            self.iterate_elements(child, depth + 1, doc, parent_element)
 
     def is_valid(self) -> bool:
         return self.valid

tests/data/groundtruth/docling_v2/blocks.md.md

@@ -0,0 +1,33 @@
+Unordered list:
+
+- foo
+
+Empty unordered list:
+
+Ordered list:
+
+- bar
+
+Empty ordered list:
+
+Heading:
+
+# my heading
+
+Empty heading:
+
+Indented code block:
+
+```
+print("Hi!")
+```
+
+Empty indented code block:
+
+Fenced code block:
+
+```
+print("Hello world!")
+```
+
+Empty fenced code block:

tests/data/md/blocks.md

@@ -0,0 +1,43 @@
+Unordered list:
+
+- foo
+
+Empty unordered list:
+
+-
+
+Ordered list:
+
+1. bar
+
+Empty ordered list:
+
+1.
+
+Heading:
+
+# my heading
+
+Empty heading:
+
+#
+
+Indented code block:
+
+    print("Hi!")
+
+Empty indented code block:
+
+    
+
+Fenced code block:
+
+```python
+print("Hello world!")
+```
+
+Empty fenced code block:
+
+```
+
+```