hdr: add _Header._prior_header

Author: scanny

docx/oxml/section.py

@@ -250,6 +250,13 @@ def page_width(self, value):
         pgSz = self.get_or_add_pgSz()
         pgSz.w = value
 
+    @property
+    def preceding_sectPr(self):
+        """sectPr immediately preceding this one or None if this is the first."""
+        # ---[1] predicate returns list of zero or one value---
+        preceding_sectPrs = self.xpath("./preceding::w:sectPr[1]")
+        return preceding_sectPrs[0] if len(preceding_sectPrs) > 0 else None
+
     @property
     def start_type(self):
         """

docx/section.py

@@ -300,4 +300,8 @@ def _header_part(self):
     @property
     def _prior_header(self):
         """|_Header| proxy on prior sectPr element or None if this is first section."""
-        raise NotImplementedError
+        preceding_sectPr = self._sectPr.preceding_sectPr
+        return (
+            None if preceding_sectPr is None
+            else _Header(preceding_sectPr, self._document_part)
+        )

tests/test_section.py

@@ -471,6 +471,29 @@ def it_knows_when_it_has_a_header_part_to_help(self, has_header_part_fixture):
 
         assert has_header_part is expected_value
 
+    def it_provides_access_to_the_prior_Header_to_help(
+        self, request, document_part_, header_
+    ):
+        doc_elm = element("w:document/(w:sectPr,w:sectPr)")
+        prior_sectPr, sectPr = doc_elm[0], doc_elm[1]
+        header = _Header(sectPr, document_part_)
+        # ---mock must occur after construction of "real" header---
+        _Header_ = class_mock(request, "docx.section._Header", return_value=header_)
+
+        prior_header = header._prior_header
+
+        _Header_.assert_called_once_with(prior_sectPr, document_part_)
+        assert prior_header is header_
+
+    def but_it_returns_None_when_its_the_first_header(self):
+        doc_elm = element("w:document/w:sectPr")
+        sectPr = doc_elm[0]
+        header = _Header(sectPr, None)
+
+        prior_header = header._prior_header
+
+        assert prior_header is None
+
     # fixtures -------------------------------------------------------
 
     @pytest.fixture(
@@ -522,6 +545,10 @@ def _get_or_add_header_part_(self, request):
     def _has_header_part_prop_(self, request):
         return property_mock(request, _Header, "_has_header_part")
 
+    @pytest.fixture
+    def header_(self, request):
+        return instance_mock(request, _Header)
+
     @pytest.fixture
     def header_part_(self, request):
         return instance_mock(request, HeaderPart)