Add index_file config option

The option allows to use a different page for the section page instead
of the first child.

README.md

@@ -63,6 +63,21 @@ To make writing this kind of `nav` more natural ([in YAML there's no better opti
 
 [literate-nav]: https://oprypin.github.io/mkdocs-literate-nav/
 
+### Specifying the page to use for the section
+
+By default the first child is used as the section page, even if there is no
+`index.md`. If you want to change the page to use add the `index_file` config
+option:
+
+```yaml
+plugins:
+  - section-index:
+      index_file: index.md
+```
+
+The value is the name of the page to use, in this case `index.md`. If a child
+with that name does not exist no section page is generated.
+
 ## [Implementation](https://github.com/oprypin/mkdocs-section-index/blob/master/mkdocs_section_index/plugin.py)
 
 ### "Protocol"

example/docs/z_noindex/a.md

@@ -0,0 +1 @@
+# a

mkdocs_section_index/plugin.py

@@ -1,5 +1,6 @@
 import collections
 import logging
+import os
 
 import mkdocs.utils
 from jinja2 import Environment
@@ -15,7 +16,29 @@
 log.addFilter(mkdocs.utils.warning_filter)
 
 
+def move_second_before_first(first, second, before="previous_page", after="next_page"):
+    """
+    Move second element before first in a doubly linked list.
+    """
+    el_before_second = getattr(second, before)
+    el_after_second = getattr(second, after)
+    el_before_first = getattr(first, before)
+
+    # now fix links from left to right
+    if el_before_first:
+        setattr(el_before_first, after, second)
+    setattr(second, before, el_before_first)
+    setattr(second, after, first)
+    setattr(first, before, second)
+    if el_before_second:
+        setattr(el_before_second, after, el_after_second)
+    if el_after_second:
+        setattr(el_after_second, before, el_before_second)
+
+
 class SectionIndexPlugin(BasePlugin):
+    config_scheme = (("index_file", mkdocs.config.config_options.Type(str, default=None)),)
+
     def on_nav(self, nav: Navigation, config, files) -> Navigation:
         todo = collections.deque((nav.items,))
         while todo:
@@ -24,7 +47,17 @@ def on_nav(self, nav: Navigation, config, files) -> Navigation:
                 if not isinstance(section, Section) or not section.children:
                     continue
                 todo.append(section.children)
-                page = section.children[0]
+                index_file = self.config["index_file"]
+                if index_file is None:
+                    page_index = 0
+                    page = section.children[0]
+                else:
+                    for page_index, child in enumerate(section.children):
+                        if os.path.basename(child.file.src_path) == index_file:
+                            page = child
+                            break
+                    else:
+                        continue
                 if not isinstance(page, Page):
                     continue
                 assert not page.children
@@ -34,12 +67,16 @@ def on_nav(self, nav: Navigation, config, files) -> Navigation:
                     page.is_section = page.is_page = True
                     page.title = section.title
                     # The page leaves the section but takes over children that used to be its peers.
-                    section.children.pop(0)
+                    section.children.pop(page_index)
                     page.children = section.children
                     for child in page.children:
                         child.parent = page
+                    # Correct order if changed
+                    if page_index > 0:
+                        move_second_before_first(page.children[0], page)
                     # The page replaces the section; the section will be garbage-collected.
                     items[i] = page
+
         self._nav = nav
         return nav
 

tests/test_plugin.py

@@ -15,39 +15,81 @@
 
 
 @pytest.mark.parametrize("directory_urls", ["use_directory_urls", "no_directory_urls"])
-@pytest.mark.parametrize("nav", ["explicit_nav", "derived_nav"])
-def test_real_example(tmpdir, directory_urls, nav):
+@pytest.mark.parametrize("nav_src", ["explicit_nav", "derived_nav"])
+@pytest.mark.parametrize("index_file", ["default_index_file", "index.md", "foo.md"])
+def test_real_example(tmpdir, directory_urls, nav_src, index_file):
     config = dict(
         docs_dir=str(example_dir / "docs"),
         site_dir=tmpdir,
         use_directory_urls=(directory_urls == "use_directory_urls"),
-        nav=load_config(str(example_dir / "mkdocs.yml"))["nav"] if nav == "explicit_nav" else None,
+        nav=load_config(str(example_dir / "mkdocs.yml"))["nav"]
+        if nav_src == "explicit_nav"
+        else None,
     )
+    if nav_src == "derived_nav" and index_file != "default_index_file":
+        # only test index_file for derived nav
+        # if index_file is None we test the default value (which is first child)
+        config["index_file"] = index_file
+    else:
+        index_file = None
     files = get_files(config)
     nav = get_navigation(files, config)
-    nav = plugin.SectionIndexPlugin().on_nav(nav, config, files)
+    instance = plugin.SectionIndexPlugin()
+    instance.load_config(config)
+    if index_file is None:
+        assert instance.config["index_file"] is None
+    nav = instance.on_nav(nav, instance.config, files)
 
-    assert len(nav.pages) == 5
-    assert len(nav.items) == 3
+    assert len(nav.pages) == (7 if nav_src == "derived_nav" else 5)
+    assert len(nav.items) == (4 if nav_src == "derived_nav" else 3)
+
+    # items = index.md, baz.md, borgs/, z_noindex/
 
     assert nav.items[1].is_page
     assert nav.items[1].file.name == "baz"
     assert not nav.items[1].is_section
 
-    sec = nav.items[2]
-    assert isinstance(sec, SectionPage)
-    assert sec.is_section
-    assert sec.is_page
-    assert sec.title == "Borgs"
-    assert sec.url in ("borgs/", "borgs/index.html")
-    assert sec.file.name == "index"
-
-    assert len(sec.children) == 2
-    assert sec.children[0].is_page
-    assert sec.children[0].file.name == "bar"
-
-    assert nav.items[1].next_page == sec
-    assert sec.children[1].parent == sec
+    assert nav.items[0].file.name == "index"
+    assert not nav.items[0].is_section
+
+    borgs_sec = nav.items[2]
+    assert isinstance(borgs_sec, SectionPage)
+    assert borgs_sec.is_section
+    assert borgs_sec.is_page
+    assert borgs_sec.title == "Borgs"
+    if index_file == "foo.md":
+        assert borgs_sec.url in ("borgs/foo/", "borgs/foo.html")
+        assert borgs_sec.file.name == "foo"
+    else:
+        assert borgs_sec.url in ("borgs/", "borgs/index.html")
+        assert borgs_sec.file.name == "index"
+
+    assert len(borgs_sec.children) == 2
+    assert borgs_sec.children[0].is_page
+    if index_file == "foo.md":
+        assert borgs_sec.children[0].file.name == "index"
+    else:
+        assert borgs_sec.children[0].file.name == "bar"
+
+    assert nav.items[1].next_page == borgs_sec
+    assert borgs_sec.children[1].parent == borgs_sec
+
+    # check order
+    if index_file == "foo.md":
+        # new section page
+        assert nav.items[3].previous_page == nav.items[2].children[-1]
+        assert nav.items[3].next_page == nav.items[3].children[0]
+
+        # page previously before new section page
+        assert nav.items[2].children[0].previous_page == nav.items[2]
+        # page previously after new section page
+        assert nav.items[2].children[1].previous_page == nav.items[2].children[0]
+
+        # first child
+        assert nav.items[2].children[0].next_page == nav.items[2].children[1]
+
+        # previous page before child[0]
+        assert nav.items[1].next_page == nav.items[2]
 
 
 @dataclasses.dataclass
@@ -74,7 +116,9 @@ def test_nav_repr(golden, tmpdir):
         config = dict(nav=golden["input"], use_directory_urls=use_directory_urls)
         files = FakeFiles(config)
         nav = get_navigation(files, config)
-        nav = plugin.SectionIndexPlugin().on_nav(nav, config, files)
+        instance = plugin.SectionIndexPlugin()
+        instance.load_config(config)
+        nav = instance.on_nav(nav, instance.config, files)
         assert str(nav) == golden.out[use_directory_urls]