deferred.py

class DeferredHTMLMiddleware):
    """Adds `deferred_html` to request.

    Use with the defer and render_deferred template tags.
    """
    
    def __init__(self, get_response: Callable):
        self.get_response = get_resposne

    def __call__(self, request: HttpRequest) -> HttpResponse:
        """Middleware implementation."""
        request.deferred_html = collections.defaultdict(set)
        return self.get_response(request)
      
      
@register.simple_block_tag(takes_context=True)
def defer(context: RequestContext, content: str, tag: str) -> str:
    """Instead of rendering the content immediately, render using the `render_deferred` tag.
    This is useful for rendering content we don't want to include immediately e.g. JS or CSS tags.

    Use with DeferredHTMLMiddleware.

    Example:
        {% defer "js" %}
        <script src="https://example.com/script.js"></script>
        {% enddefer %}
    at end of page:
        {% render_deferred "js" %}
    """
    context.request.deferred_html[tag].add(content)
    return ""


@register.simple_tag(takes_context=True)
def render_deferred(context: RequestContext, tag: str) -> str:
    """Renders all deferred content, cleares the list and returns the result."""
    if deferred_html := context.request.deferred_html.pop(tag, None):
        return format_html_join("\n", "{}", ((block,) for block in deferred_html))
    return ""