Ignore style definitions using a style ID that has already been used

Author: mwilliamson

NEWS

@@ -1,3 +1,7 @@
+# 1.11.0
+
+* Ignore style definitions using a style ID that has already been used.
+
 # 1.10.0
 
 * Add "Heading" and "Body" styles, as found in documents created by Apple Pages,

mammoth/docx/styles_xml.py

@@ -67,7 +67,18 @@ def read_styles_xml_element(element):
             style = _read_style_element(style_element)
 
         style_set = styles.get(element_type)
-        if style_set is not None:
+
+        # Per 17.7.4.17 style (Style Definition) of ECMA-376 4th edition Part 1:
+        #
+        # > If multiple style definitions each declare the same value for their
+        # > styleId, then the first such instance shall keep its current
+        # > identifier with all other instances being reassigned in any manner
+        # > desired.
+        #
+        # For the purpose of conversion, there's no point holding onto styles
+        # with reassigned style IDs, so we ignore such style definitions.
+
+        if style_set is not None and style.style_id not in style_set:
             style_set[style.style_id] = style
 
     return Styles(

tests/docx/styles_xml_tests.py

@@ -101,6 +101,18 @@ def test_numbering_style_has_num_id_read_from_paragraph_properties():
     assert_equal("42", styles.find_numbering_style_by_id("List1").num_id)
 
 
+def test_when_multiple_style_elements_have_same_style_id_then_only_first_element_is_used():
+    element = xml_element("w:styles", {}, [
+        _table_style_element("TableNormal", "Normal Table"),
+        _table_style_element("TableNormal", "Table Normal"),
+    ])
+    styles = read_styles_xml_element(element)
+    assert_equal(
+        "Normal Table",
+        styles.find_table_style_by_id("TableNormal").name
+    )
+
+
 def _paragraph_style_element(style_id, name):
     return _style_element("paragraph", style_id, name)