A lightweight Python asyncio HTTP/1.1 client. No additional tasks are created; all code is in a single module; and other than the standard library only a single dependency is required, aiodnsresolver

Lowhaio has a deliberately limited scope: it includes just enough code to be a useful HTTP client and allow more complex behaviour to be added on top if required.

Connections are DNS-aware, in that they are only re-used if they match a current A record for the domain.

pip install lowhaio

The API is streaming-first: for both request and response bodies, asynchronous iterators are used.

import asyncio
from lowhaio import Pool

async def main():
    request, close = Pool()

    async def request_body():
        yield b'a'
        yield b'bc'

    code, headers, response_body = await request(
        b'POST', 'https://postman-echo.com/post',
        headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
        body=request_body,
    )
    async for chunk in response_body:
        print(chunk)

    await close()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

However, there are helper functions streamed and buffered when this isn't required or possible.

import asyncio
from lowhaio import Pool, streamed, buffered

async def main():
    request, close = Pool()

    request_body = streamed(b'abc')

    code, headers, response_body = await request(
        b'POST', 'https://postman-echo.com/post',
        headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
        body=request_body,
    )
    print(await buffered(response_body))

    await close()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

The only header automatically added to requests is the host header, which is the idna/punycode-encoded domain name from the requested URL.

Exceptions are usually subclasses of HttpError. If a lower-level exception caused this, it is set in the __cause__ attribute of the HttpError. Specifically, before any data is sent HttpConnectionError is raised, and after data is sent, HttpDataError is raised. This difference is to make it possible to know if non-idempotent requests can be retried.

However, asyncio.CancelledError and exceptions that do not directive from Exception, such as SystemExit, are allowed to bubble up.

Lowhaio can be used with an custom SSL context through through the get_ssl_context parameter to Pool. For example, to use the certifi CA bundle, you can install it by

pip install certifi

and use it as below.

import asyncio
import ssl

import certifi
from lowhaio import Pool, buffered, streamed

async def main():
    request, close = Pool(
        get_ssl_context=lambda: ssl.create_default_context(cafile=certifi.where()),
    )

    request_body = streamed(b'abc')

    code, headers, response_body = await request(
        b'POST', 'https://postman-echo.com/post',
        headers=((b'content-length', b'3'), (b'content-type', b'text/plain'),),
        body=request_body,
    )
    print(await buffered(response_body))

    await close()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

The scope of the core functions is restricted to:

• (TLS) connection opening, closing and pooling;
• passing and receiving HTTP headers and streaming bodies;
• decoding chunked responses;
• raising exceptions on timeouts.

This is to make the core behaviour useful to a reasonable range of uses, but to not include what can be added by layer(s) on top. Specifically not included:

• following redirects, implemented by lowhaio-redirect;
• retrying failed requests, implemented by lowhaio-retry;
• encoding chunked requests, implemented by lowhaio-chunked;
• authentication, such as AWS Signature Version 4 implemented by lowhaio-aws-sigv4, or AWS Signature Version 4 with unsigned payload implemented by lowhaio-aws-sigv4-unsigned-payload;
• compressing/decompressing requests/responses;
• cookies.