mezzanine的head 导航条、左侧tree、footer是由page_menu产生的。page_menu的算法，先计算出每一页的孩子，然后再逐页去page_menu。
@register.render_tag
def page_menu(context, token):
"""
Return a list of child pages for the given parent, storing all
pages in a dict in the context when first called using parents as keys
for retrieval on subsequent recursive calls from the menu template.
"""
# First arg could be the menu template file name, or the parent page.
# Also allow for both to be used.
template_name = None
parent_page = None
parts = token.split_contents()[1:]#对page_menu菜单分离，取得参数
for part in parts:
part = Variable(part).resolve(context)
if isinstance(part, str):
template_name = part#文本为模板名
elif isinstance(part, Page):
parent_page = part#父页
if template_name is None:#没有提供模板的话，从上下文中获取
try:
template_name = context["menu_template_name"]
except KeyError:
error = "No template found for page_menu in: %s" % parts
raise TemplateSyntaxError(error)
context["menu_template_name"] = template_name#设置模板
if "menu_pages" not in context:#没有菜单页，取得当前用户有权访问的页
try:
user = context["request"].user
slug = context["request"].path
except KeyError:
user = None
slug = ""
num_children = lambda id: lambda: len(context["menu_pages"][id])#匿名函数，函数中包含函数
has_children = lambda id: lambda: num_children(id)() > 0
rel = [m.__name__.lower()#取得Page的子类，但不包括Page自身
for m in Page.get_content_models()
if not m._meta.proxy]
published = Page.objects.published(for_user=user).select_related(*rel)#把相关联的model也进行查询
# Store the current page being viewed in the context. Used
# for comparisons in page.set_menu_helpers.
if "page" not in context:
try:
context.dicts[0]["_current_page"] = published.exclude(
content_model="link").get(slug=slug)
except Page.DoesNotExist:
context.dicts[0]["_current_page"] = None
elif slug:
context.dicts[0]["_current_page"] = context["page"]#记录当前页
# Some homepage related context flags. on_home is just a helper
# indicated we're on the homepage. has_home indicates an actual
# page object exists for the homepage, which can be used to
# determine whether or not to show a hard-coded homepage link
# in the page menu.
home = home_slug()
context.dicts[0]["on_home"] = slug == home
context.dicts[0]["has_home"] = False
# Maintain a dict of page IDs -> parent IDs for fast
# lookup in setting page.is_current_or_ascendant in
# page.set_menu_helpers.
context.dicts[0]["_parent_page_ids"] = {}#保存父页id的词典
pages = defaultdict(list)#pages[]不存在时，不会出错，返回[]
for page in published.order_by("_order"):
page.set_helpers(context)#判断是否是当前页，有无孩子
context["_parent_page_ids"][page.id] = page.parent_id#各页的父
setattr(page, "num_children", num_children(page.id))
setattr(page, "has_children", has_children(page.id))
pages[page.parent_id].append(page)#取得各页的子页list
if page.slug == home:
context.dicts[0]["has_home"] = True
# Include menu_pages in all contexts, not only in the
# block being rendered.
context.dicts[0]["menu_pages"] = pages
# ``branch_level`` must be stored against each page so that the
# calculation of it is correctly applied. This looks weird but if we do
# the ``branch_level`` as a separate arg to the template tag with the
# addition performed on it, the addition occurs each time the template
# tag is called rather than once per level.
context["branch_level"] = 0#默认层次为0
parent_page_id = None
if parent_page is not None:
context["branch_level"] = getattr(parent_page, "branch_level", 0) + 1
parent_page_id = parent_page.id # Build the ``page_branch`` template variable, which is the list of
# pages for the current parent. Here we also assign the attributes
# to the page object that determines whether it belongs in the
# current menu template being rendered.
context["page_branch"] = context["menu_pages"].get(parent_page_id, [])#取得该页的子页
context["page_branch_in_menu"] = False
for page in context["page_branch"]:#处理该分支
page.in_menu = page.in_menu_template(template_name)#判断是否在该菜单
page.num_children_in_menu = 0
if page.in_menu:
context["page_branch_in_menu"] = True
for child in context["menu_pages"].get(page.id, []):
if child.in_menu_template(template_name):
page.num_children_in_menu += 1
page.has_children_in_menu = page.num_children_in_menu > 0
page.branch_level = context["branch_level"]
page.parent = parent_page
context["parent_page"] = page.parent # Prior to pages having the ``in_menus`` field, pages had two
# boolean fields ``in_navigation`` and ``in_footer`` for
# controlling menu inclusion. Attributes and variables
# simulating these are maintained here for backwards
# compatibility in templates, but will be removed eventually.
page.in_navigation = page.in_menu
page.in_footer = not (not page.in_menu and "footer" in template_name)
if page.in_navigation:
context["page_branch_in_navigation"] = True
if page.in_footer:
context["page_branch_in_footer"] = True t = get_template(template_name)
return t.render(Context(context))