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Tremolo is a stream-oriented, asynchronous, programmable HTTP server written in pure Python. It can also serve as an ASGI server.

Tremolo provides a common routing functionality to some unique features such as download/upload speed limiters, etc. While maintaining its simplicity and performance.

Being built with a stream in mind, Tremolo tends to use yield instead of return in route handlers.

async def hello_world(**server):
    yield b'Hello '
    yield b'world!'

You can take advantage of this to serve big files efficiently:

async def my_big_data(content_type='application/octet-stream', **server):
    # buffer_size = 16384
    buffer_size = server['context'].options['buffer_size']

    with open('/my/folder/', 'rb') as f:
        chunk = True

        while chunk:
            chunk =
            yield chunk

And other use cases…


Tremolo is only suitable for those who value minimalism and stability over features.

With only 3k lines of code, with no dependencies other than the Python Standard Library, it gives you:


Here is a complete hello world example in case you missed the usual return.

from tremolo import Tremolo

app = Tremolo()

async def hello_world(**server):
    return 'Hello world!', 'latin-1'

if __name__ == '__main__':'', 8000, debug=True)

Well, latin-1 on the right side is not required. The default is utf-8.

You can save it as and just run it with python3

Your first hello world page with Tremolo will be at http://localhost:8000/hello.

ASGI Server

Tremolo is an HTTP Server framework. You can build abstractions on top of it, say an ASGI server.

In fact, Tremolo already has ASGI server (plus WebSocket) implementation.

So you can immediately use existing ASGI applications / frameworks, behind Tremolo (ASGI server).

For example, If a minimal ASGI application with the name

async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            (b'content-type', b'text/plain')

    await send({
        'type': 'http.response.body',
        'body': b'Hello world!'

Then you can run as follows:

python3 -m tremolo --debug --bind example:app

To see more available options:

python3 -m tremolo --help

It’s also possible to run the ASGI server programmatically (example with uvloop):



The first thing to note is that Tremolo is a pure Python server framework.

As a pure Python server framework, it is hard to find a comparison. Because most servers/frameworks today are full of steroids like httptools, uvloop, Rust, etc.

You can try comparing with Uvicorn with the following option (disabling steroids to be fair):

uvicorn --loop asyncio --http h11 --log-level error example:app


python3 -m tremolo --log-level ERROR example:app

You will find that Tremolo is reasonably fast.

However, it should be noted that bottlenecks often occur on the application side. Which means that in real-world usage, throughput reflects more on the application than the server.


Tremolo utilizes SO_REUSEPORT (Linux 3.9+) to load balance worker processes.'', 8000, worker_num=2)

Tremolo can also listen to multiple ports in case you are using an external load balancer like Nginx / HAProxy.

app.listen(8002)'', 8000)

You can even get higher concurrency with PyPy or uvloop:

import asyncio
import uvloop


python3 -m pip install --upgrade tremolo


Just run python3 for all tests. Or individual test_*.py in the tests/ folder, for example python3 tests/

If you also want measurements with coverage:

coverage run
coverage combine
coverage report
coverage html # to generate html reports