class BaseHandler:
  ...
  def _get_response(self, request):
    ...
    if hasattr(request, 'urlconf'):
      urlconf = request.urlconf
      set_urlconf(urlconf)
      resolver = get_resolver(urlconf)
    else:
      resolver = get_resolver()

urlpatterns = [
  path('index/', views.index), # 普通路径
  re_path(r'^articles/([0-9]{4})/$', views.articles), # 正则路径
  path("app01/", include("app01.urls")),
]

def include(arg, namespace=None):
  ...
  return (urlconf_module, app_name, namespace)


def _path(route, view, kwargs=None, name=None, Pattern=None):
  if isinstance(view, (list, tuple)):
    # For include(...) processing.
    pattern = Pattern(route, is_endpoint=False)
    urlconf_module, app_name, namespace = view
    return URLResolver(
      pattern,
      urlconf_module,
      kwargs,
      app_name=app_name,
      namespace=namespace,
    )
  elif callable(view):
    pattern = Pattern(route, name=name, is_endpoint=True)
    return URLPattern(pattern, view, kwargs, name)
  else:
    raise TypeError('view must be a callable or a list/tuple in the case of include().')


path = partial(_path, Pattern=RoutePattern)
re_path = partial(_path, Pattern=RegexPattern)

def include(arg, namespace=None):
  ...
  if isinstance(urlconf_module, str):
    urlconf_module = import_module(urlconf_module)
  patterns = getattr(urlconf_module, 'urlpatterns', urlconf_module)
  app_name = getattr(urlconf_module, 'app_name', app_name)
  ...
  return (urlconf_module, app_name, namespace)

class URLPattern:
  def __init__(self, pattern, callback, default_args=None, name=None):
    # 需要匹配的 urlpattern，这里根据是path还是re_path 分别是 RoutePattern或RegexPattern的实例
    self.pattern = pattern
    self.callback = callback # the view
    self.default_args = default_args or {}
    self.name = name
  ...
  def resolve(self, path):
    调用 RoutePattern 或 RegexPattern 的实例中的 match 方法进行匹配（注意这里不是 re 模块里面的 match）
    match = self.pattern.match(path)
    if match:
      new_path, args, kwargs = match
      # Pass any extra_kwargs as **kwargs.
      kwargs.update(self.default_args)
      # 匹配成功返回 `ResolverMatch`
      return ResolverMatch(self.callback, args, kwargs, self.pattern.name, route=str(self.pattern))
  ...

class URLResolver:
  def resolve(self, path):
    path = str(path) # path may be a reverse_lazy object
    tried = []
    # 匹配 path
    match = self.pattern.match(path)
    if match:
      new_path, args, kwargs = match
      # 如果匹配成功，则继续匹配它的url_patterns
      for pattern in self.url_patterns:
        try:
          # 这个pattern可能是URLPattern，也可能是URLResolver；如果是URLPattern，匹配成功则返回ResolverMatch；如果是URLResolver，则会递归调用下去。
          sub_match = pattern.resolve(new_path)
        ...
        else:
          if sub_match:
            ...
            # 匹配成功返回ResolverMatch
            return ResolverMatch(
              sub_match.func,
              sub_match_args,
              sub_match_dict,
              sub_match.url_name,
              [self.app_name] + sub_match.app_names,
              [self.namespace] + sub_match.namespaces,
              self._join_route(current_route, sub_match.route),
            )
          tried.append([pattern])
      raise Resolver404({'tried': tried, 'path': new_path})
    raise Resolver404({'path': path})