Implement NumberingPart.new() to auto-create an empty numbering part

src/docx/parts/document.py

@@ -104,7 +104,8 @@ def numbering_part(self) -> NumberingPart:
         try:
             return cast(NumberingPart, self.part_related_by(RT.NUMBERING))
         except KeyError:
-            numbering_part = NumberingPart.new()
+            assert self.package is not None
+            numbering_part = NumberingPart.new(self.package)
             self.relate_to(numbering_part, RT.NUMBERING)
             return numbering_part
 

src/docx/parts/numbering.py

@@ -1,17 +1,41 @@
 """|NumberingPart| and closely related objects."""
 
-from ..opc.part import XmlPart
-from ..shared import lazyproperty
+from __future__ import annotations
+
+import os
+from typing import TYPE_CHECKING, cast
+
+from docx.opc.constants import CONTENT_TYPE as CT
+from docx.opc.packuri import PackURI
+from docx.opc.part import XmlPart
+from docx.oxml.parser import parse_xml
+from docx.shared import lazyproperty
+
+if TYPE_CHECKING:
+    from docx.oxml.numbering import CT_Numbering
+    from docx.package import Package
 
 
 class NumberingPart(XmlPart):
     """Proxy for the numbering.xml part containing numbering definitions for a document
     or glossary."""
 
     @classmethod
-    def new(cls) -> "NumberingPart":
-        """Newly created numbering part, containing only the root ``<w:numbering>`` element."""
-        raise NotImplementedError
+    def new(cls, package: Package) -> "NumberingPart":
+        """Return a newly created numbering part, containing only the root ``<w:numbering>``
+        element."""
+        partname = PackURI("/word/numbering.xml")
+        content_type = CT.WML_NUMBERING
+        element = cast("CT_Numbering", parse_xml(cls._default_numbering_xml()))
+        return cls(partname, content_type, element, package)
+
+    @classmethod
+    def _default_numbering_xml(cls) -> bytes:
+        """Return a bytestream containing XML for a default numbering part."""
+        path = os.path.join(os.path.split(__file__)[0], "..", "templates", "default-numbering.xml")
+        with open(path, "rb") as f:
+            xml_bytes = f.read()
+        return xml_bytes
 
     @lazyproperty
     def numbering_definitions(self):

src/docx/templates/default-numbering.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<w:numbering
+  xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"
+  xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main"
+/>

tests/parts/test_document.py

@@ -195,7 +195,7 @@ def and_it_creates_a_numbering_part_if_not_present(
 
         numbering_part = document_part.numbering_part
 
-        NumberingPart_.new.assert_called_once_with()
+        NumberingPart_.new.assert_called_once_with(package_)
         relate_to_.assert_called_once_with(document_part, numbering_part_, RT.NUMBERING)
         assert numbering_part is numbering_part_
 

tests/parts/test_numbering.py

@@ -2,7 +2,9 @@
 
 import pytest
 
+from docx.opc.constants import CONTENT_TYPE as CT
 from docx.oxml.numbering import CT_Numbering
+from docx.package import Package
 from docx.parts.numbering import NumberingPart, _NumberingDefinitions
 
 from ..oxml.unitdata.numbering import a_num, a_numbering
@@ -21,6 +23,20 @@ def it_provides_access_to_the_numbering_definitions(self, num_defs_fixture):
         _NumberingDefinitions_.assert_called_once_with(numbering_elm_)
         assert numbering_definitions is numbering_definitions_
 
+    def it_constructs_a_default_numbering_part_to_help(self):
+        package = Package()
+
+        numbering_part = NumberingPart.new(package)
+
+        assert isinstance(numbering_part, NumberingPart)
+        assert numbering_part.partname == "/word/numbering.xml"
+        assert numbering_part.content_type == CT.WML_NUMBERING
+        assert numbering_part.package is package
+        assert numbering_part.element.tag == (
+            "{http://schemas.openxmlformats.org/wordprocessingml/2006/main}numbering"
+        )
+        assert len(numbering_part.element) == 0
+
     # fixtures -------------------------------------------------------
 
     @pytest.fixture