hdr: add DocumentPart.footer_part()

Author: scanny

docx/parts/document.py

@@ -58,7 +58,7 @@ def drop_header_part(self, rId):
 
     def footer_part(self, rId):
         """Return |FooterPart| related by *rId*."""
-        raise NotImplementedError
+        return self.related_parts[rId]
 
     def get_or_add_image(self, image_descriptor):
         """

tests/parts/test_document.py

@@ -60,6 +60,18 @@ def it_can_drop_a_specified_header_part(self, drop_rel_):
 
         drop_rel_.assert_called_once_with(document_part, "rId42")
 
+    def it_provides_access_to_a_footer_part_by_rId(
+        self, related_parts_prop_, related_parts_, footer_part_
+    ):
+        related_parts_prop_.return_value = related_parts_
+        related_parts_.__getitem__.return_value = footer_part_
+        document_part = DocumentPart(None, None, None, None)
+
+        footer_part = document_part.footer_part("rId9")
+
+        related_parts_.__getitem__.assert_called_once_with("rId9")
+        assert footer_part is footer_part_
+
     def it_provides_access_to_a_header_part_by_rId(
         self, related_parts_prop_, related_parts_, header_part_
     ):