fix(docx): one integral silently drops every equation in the document

packages/markitdown/src/markitdown/converter_utils/docx/math/latex_dict.py

@@ -225,6 +225,8 @@
 CHR_DEFAULT = {
     "ACC_VAL": "\\hat{{{0}}}",
     "GROUP_CHR_VAL": "\\underbrace{{{0}}}",
+    # Omitting m:chr under m:naryPr means U+222B INTEGRAL (ISO/IEC 29500-1).
+    "NARY_VAL": "\\int",
 }
 
 POS = {

packages/markitdown/src/markitdown/converter_utils/docx/math/omml.py

@@ -363,10 +363,12 @@ def do_nary(self, elm):
         the n-ary object
         """
         res = []
-        bo = ""
+        # m:naryPr is itself optional, so an absent element means every property
+        # takes its default, the operator included.
+        bo = CHR_DEFAULT.get("NARY_VAL", "")
         for stag, t, e in self.process_children_list(elm):
             if stag == "naryPr":
-                bo = get_char(t.chr, store=CHR_BO)
+                bo = get_char(t.chr, default=CHR_DEFAULT.get("NARY_VAL"), store=CHR_BO)
             else:
                 res.append(t)
         return bo + BLANK.join(res)

packages/markitdown/tests/test_docx_math_nary.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3 -m pytest
+"""Tests for the default n-ary operator in DOCX math conversion.
+
+``m:chr`` under ``m:naryPr`` names the n-ary operator. ISO/IEC 29500-1 states
+that when the element is omitted the operator is U+222B INTEGRAL, so producers
+write ``m:chr`` only for the non-default operators such as the summation sign.
+``do_nary`` passed no default to ``get_char``, so an integral yielded ``None``
+and ``None + ""`` raised ``TypeError``.
+
+That failure is not local to the equation. ``pre_process_docx`` runs
+``_pre_process_math`` over the whole of ``word/document.xml`` inside a blanket
+``except Exception`` and, on error, writes the *original* unprocessed XML back.
+Mammoth does not render OMML, so one integral silently removes every equation
+in the document.
+"""
+
+from xml.etree import ElementTree as ET
+
+from markitdown.converter_utils.docx.math.omml import OMML_NS, oMath2Latex
+
+MATH_NS_DECL = f'xmlns:m="{OMML_NS[1:-1]}"'
+
+SUMMATION = "∑"  # N-ARY SUMMATION, written out by Word as it is not the default
+
+
+def _nary(nary_pr: str):
+    xml = (
+        f"<m:oMath {MATH_NS_DECL}><m:nary>"
+        f"{nary_pr}"
+        "<m:sub><m:r><m:t>0</m:t></m:r></m:sub>"
+        "<m:sup><m:r><m:t>1</m:t></m:r></m:sup>"
+        "<m:e><m:r><m:t>x</m:t></m:r></m:e>"
+        "</m:nary></m:oMath>"
+    )
+    return oMath2Latex(ET.fromstring(xml)).latex
+
+
+def test_nary_without_chr_defaults_to_integral():
+    # No m:chr, so the operator is U+222B. Previously raised TypeError.
+    latex = _nary('<m:naryPr><m:limLoc m:val="subSup"/><m:ctrlPr/></m:naryPr>')
+    assert latex == "\\int_{0}^{1}x"
+
+
+def test_nary_with_explicit_chr_is_unchanged():
+    latex = _nary(f'<m:naryPr><m:chr m:val="{SUMMATION}"/></m:naryPr>')
+    assert latex == "\\sum_{0}^{1}x"
+
+
+def test_nary_without_nary_pr_defaults_to_integral():
+    # m:naryPr is optional. Absent, every property takes its default, so the
+    # operator is still U+222B rather than nothing.
+    assert _nary("") == "\\int_{0}^{1}x"