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Streaming of Contents

Internally, htpy is built with generators. Most of the time, you would render the full page with str(), but htpy can also incrementally generate pages which can then be streamed to the browser. If your page uses a database or other services to retrieve data, you can sending the first part of the page to the client while the page is being generated.


Streaming requires a bit of discipline and care to get right. You need to ensure to avoid doing too much work up front and use lazy constructs such as generators and callables. Most of the time, rendering the page without streaming will be the easiest way to get going. Streaming can give you improved user experience from faster pages/rendering.

This video shows what it looks like in the browser to generate a HTML table with Django StreamingHttpResponse (source code):

This example simulates a (very) slow fetch of data and shows the power of streaming: The browser loads CSS and gradually shows the contents. By loading CSS files in the <head> tag before dynamic content, the browser can start working on loading the CSS and styling the page while the server keeps generating the rest of the page.

Using Generators and Callables as Children

Django's querysets are lazily evaluated. They will not execute a database query before their value is actually needed.

This example shows how this property of Django querysets can be used to create a page that streams objects:

from django.http import StreamingHttpResponse
from htpy import ul, li

from myapp.models import Article

def article_list(request):
    return StreamingHttpResponse(ul[
        (li[article.title] for article in Article.objects.all())

Using Callables to Delay Evalutation

Pass a callable that does not accept any arguements as child to delay the evaluation.

This example shows how the page starts rendering and outputs the <h1> tag and then calls calculate_magic_number.

import time
from htpy import div, h1

def calculate_magic_number() -> str:
    print("  (running the complex calculation...)")
    return "42"

element = div[
    h1["Welcome to my page"],
    "The magic number is ",

for chunk in element:


Welcome to my page
The magic number is
42    # <-- Appears after 3 seconds

You may use lambda to create a function without arguments to make a an expression lazy:

from htpy import div, h1

def fib(n: int) -> int:
    if n == 0:
        return 0
    elif n == 1:
        return 1
        return fib(n - 1) + fib(n - 2)

        lambda: str(fib(20)),
# output: <div><h1>Fibonacci!</h1>fib(12)=6765</div>